tag:theconversation.com,2011:/fr/topics/jupiter-2940/articlesJupiter – The Conversation2024-03-04T16:15:33Ztag:theconversation.com,2011:article/2250022024-03-04T16:15:33Z2024-03-04T16:15:33ZJupiter’s moon Europa produces less oxygen than we thought – it may affect our chances of finding life there<figure><img src="https://images.theconversation.com/files/579561/original/file-20240304-24-hmeyeg.jpeg?ixlib=rb-1.1.0&rect=20%2C0%2C1976%2C1000&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Europa seen in true colour (left) and false colour (right).</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p>Jupiter’s icy moon Europa has long been thought of as one of the most habitable worlds in the Solar System. Now the Juno mission to Jupiter has directly sampled its atmosphere in detail for the first time. The results, <a href="https://www.nature.com/articles/s41550-024-02206-x">published in Nature Astronomy</a>, show that Europa’s icy surface produces less oxygen than we thought.</p>
<p>There are plenty of reasons to be excited about the possibility of finding microbial life on Europa. Evidence from the Galileo mission has shown that the moon <a href="https://pubmed.ncbi.nlm.nih.gov/9450749/">has an ocean</a> below its icy surface containing about twice the amount of water as Earth’s oceans. Also, models derived from Europa data show that its ocean floor is in contact with rock, enabling chemical water-rock interactions that <a href="https://theconversation.com/nasa-considers-sending-swimming-robots-to-habitable-ocean-worlds-of-the-solar-system-186228">produce energy</a>, making it the prime candidate for life.</p>
<p>Telescope observations, meanwhile, reveal a weak, <a href="https://europa.nasa.gov/news/18/hubble-finds-oxygen-atmosphere-on-jupiters-moon-europa/">oxygen-rich atmosphere</a>. It also looks as though <a href="https://www.nasa.gov/news-release/nasas-hubble-spots-possible-water-plumes-erupting-on-jupiters-moon-europa/">plumes of water erupt</a> intermittently from the ocean. And there is some evidence of the presence of <a href="https://europa.nasa.gov/why-europa/ingredients-for-life/#:%7E:text=NASA%2FJPL%2DCaltech-,Europa's%20surface%20is%20blasted%20by%20radiation%20from%20Jupiter.,in%20Europa's%20extremely%20tenuous%20atmosphere.">basic chemical elements</a> on the surface – including carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur – used by life on Earth. Some of these could seep down into the water from the atmosphere and surface.</p>
<p>The heating of Europa and its ocean is partly thanks to the moon’s orbit around Jupiter, which produces tidal forces to heat an otherwise frigid environment. </p>
<p>Although Europa boasts three basic ingredients for life – water, the right chemical elements and a source of heat – we don’t yet know if there has been enough time for life to develop.</p>
<figure class="align-center ">
<img alt="Plumes seen on Europa." src="https://images.theconversation.com/files/579569/original/file-20240304-16-u47ybe.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/579569/original/file-20240304-16-u47ybe.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=506&fit=crop&dpr=1 600w, https://images.theconversation.com/files/579569/original/file-20240304-16-u47ybe.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=506&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/579569/original/file-20240304-16-u47ybe.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=506&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/579569/original/file-20240304-16-u47ybe.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=636&fit=crop&dpr=1 754w, https://images.theconversation.com/files/579569/original/file-20240304-16-u47ybe.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=636&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/579569/original/file-20240304-16-u47ybe.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=636&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Plumes seen on Europa.</span>
<span class="attribution"><span class="source">Nasa</span></span>
</figcaption>
</figure>
<p>The other prime candidate in our solar system is Mars, the Rosalind Franklin rover’s target in 2028. Life <a href="https://theconversation.com/perseverance-mars-rover-how-to-prove-whether-theres-life-on-the-red-planet-154982">might have started on Mars</a> at the same time as it did on Earth, but then probably stopped due to climate change. </p>
<p>A third candidate is Saturn’s moon Enceladus where the Cassini-Huygens mission discovered plumes of water from a sub-surface salty ocean, <a href="https://www.nature.com/articles/s41586-023-05987-9">also in contact with rock</a> at the ocean’s floor. </p>
<p>Titan is the closest runner up in fourth place, <a href="https://iopscience.iop.org/article/10.3847/2041-8213/aa7851">with its thick atmosphere</a> of organic compounds including hydrocarbon and tholins, born in the high atmosphere. These then float down to the surface coating it with ingredients for life.</p>
<h2>Losing oxygen</h2>
<p>The Juno mission boasts <a href="https://ui.adsabs.harvard.edu/abs/2017SSRv..213..547M/abstract">the best charged particle instruments</a> sent to Jupiter so far. It can measure the energy, direction and composition of charged particles on the surface. Similar instruments at Saturn and Titan <a href="https://uwaterloo.ca/chem13-news-magazine/february-2017/chemistry/what-earth-are-tholins">found tholins</a> (a type of organic substance) there. But they also measured particles that suggested atmospheres at Saturn’s moons Rhea and Dione, in addition to those at Titan and Enceladus.</p>
<p>These particles are known as <a href="https://www.sciencedirect.com/topics/physics-and-astronomy/pickup-ions">pickup ions</a>. Planetary atmospheres consist of neutral particles, but the top of an atmosphere becomes “ionised” (meaning it loses electrons) in sunlight and via collisions with other particles, forming ions (charged atoms that have lost electrons) and free electrons. </p>
<p>When a plasma – a charged gas making up the fourth state of matter beyond solid, liquid and gas – flows past an atmosphere with newly formed ions, it disturbs the atmosphere with electric fields which can accelerate the new ions – the first part of an ion pickup process. </p>
<p>These pickup ions then spiral around the planet’s magnetic field and are usually lost from the atmosphere, while some hit the surface and are absorbed. The pickup process has rid the Martian atmosphere of particles after the red planet’s magnetic field was lost 3.8 billion years ago.</p>
<p>Europa also has a pickup process. The new measurements show the telltale signs of pickup molecular oxygen and hydrogen ions from the surface and atmosphere. Some of these escape from Europa, whereas some hit the icy surface enhancing the amount of oxygen at and under the surface. </p>
<p>This confirms that oxygen and hydrogen are indeed the main constituents of Europa’s atmosphere – in agreement with remote observations. However, the measurements imply that the amount of oxygen being produced – released by the surface to the atmosphere – is only about 12kg per second, at the lower end of earlier estimates from about 5kg to 1,100 kg per second. </p>
<p>This would indicate that the surface suffers very little erosion. The measurements indicate that this may amount to only 1.5cm of Europa’s surface per million years, which is less than we had thought. So Europa is constantly losing oxygen due to pickup processes, with only a small amount of additional oxygen being released from the surface to replenish it and ending up back on the surface.</p>
<p>So what does that mean for its chances of hosting life? Some of the oxygen trapped in the surface may find its way to the subsurface ocean to nourish any life there. But based on the study’s estimate of the overall loss of oxygen, this should be less than the 0.3kg-300kg per second estimated earlier. </p>
<p>It remains to be seen whether this rate, recorded on 29, September 2022, is usual. Perhaps it is not representative of the overall oxygen on the moon. It may be that the eruption of plumes, orbital position and upstream conditions increase and decrease the rate at certain times, respectively.</p>
<p>Nasa’s <a href="https://www.jpl.nasa.gov/missions/europa-clipper">Europa Clipper mission</a>, to be launched later this year, and the Juice mission which will make two flybys of Europa on its way to orbit Ganymede, will be able to follow up these measurements, and provide much more information on Europa’s habitability.</p><img src="https://counter.theconversation.com/content/225002/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Coates receives funding from STFC and UKSA (UK). </span></em></p>Only about 12kg of oxygen is produced per second on Europa, which is on the lower side of previous estimates from about 5kg to 1,100 kg per second.Andrew Coates, Professor of Physics, Deputy Director (Solar System) at the Mullard Space Science Laboratory, UCLLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2194402023-12-29T11:39:42Z2023-12-29T11:39:42ZSix space missions to look forward to in 2024<figure><img src="https://images.theconversation.com/files/565441/original/file-20231213-29-hd06pq.png?ixlib=rb-1.1.0&rect=27%2C21%2C1367%2C1253&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Nasa</span></span></figcaption></figure><p>It’s going to be a bumper time for space missions in 2024 – especially to the Moon, our nearest neighbour. And that’s following on from an already epic 2023. </p>
<p>I’m a laboratory scientist, so I always like to have a “proper” sample to analyse. Rather than peering through telescopes to look at the stars, I prefer to see them in a vial in my lab. My technique of choice is to burn the material to ashes while measuring the organic compounds and other species that are liberated in the process. </p>
<p>So it was a great delight to see the safe return of <a href="https://theconversation.com/osiris-rex-nasa-reveals-evidence-of-water-and-carbon-in-sample-delivered-to-earth-from-an-asteroid-215484">Nasa’s Osiris-Rex</a> mission from asteroid (101955) Bennu in September 2023. It was an even greater pleasure to receive a few precious crumbs of Bennu to study. </p>
<h2>CLPS missions</h2>
<p>Nasa’s series of <a href="https://www.nasa.gov/commercial-lunar-payload-services/">Commercial Lunar Payload Service (CLPS) missions</a>, many of which will launch in 2024, are set to bring a variety of instruments to the Moon. These missions are built and launched by different private companies under contract from Nasa. </p>
<p>The CLPS programme is part of Nasa’s <a href="https://theconversation.com/artemis-why-it-may-be-the-last-mission-for-nasa-astronauts-195065">Artemis initiative</a> to continue human exploration of the Moon. One of the main aims of the programme is to investigate the possibilities of using lunar resources as fuel – hence, some of the instruments on CLPS-1, aka <a href="https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=PEREGRN-1">Peregrine</a>, are designed to assess the amount of hydrogen on the lunar surface. In fact, my colleagues at the Open University have <a href="https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/PITMS_a_mini_mass_spectrometer_for_the_Moon">built an instrument</a> for doing so.</p>
<p>CLPS-2 is timetabled to launch in early January 2024, and there are four other CLPS missions planned for launch throughout the year. That is the good thing about the Moon – it’s so close that there aren’t many worries about launch windows (no complicated orbits to compute) or distance to travel.</p>
<p>Indeed, it is hoped that human exploration of the Moon will take a small step forward, possibly as early as November 2024, when <a href="https://www.nasa.gov/mission/artemis-ii/">Artemis II</a> orbits the Moon for several days. One of the astronauts on-board will be female – definitely a giant leap in what has, until now, been a solely masculine exploration of our nearest neighbour.</p>
<h2>Trailblazer</h2>
<p>Continuing the lunar theme, Nasa’s <a href="https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=L-TRLBLZR">Trailblazer mission</a> travels to the Moon to understand where any water is situated. Is it locked inside rock as part of the mineral structure, or is it deposited as ice on the rocky surface? </p>
<p>Trailblazer is currently scheduled for launch in the first quarter of 2024. However, no precise date has been confirmed. It’s a small mission, part of the <a href="https://www.nasa.gov/specials/artemis/">Artemis</a> human lunar exploration programme.</p>
<h2>Chang'e 6</h2>
<p>The launch of <a href="https://en.wikipedia.org/wiki/Chang%27e_6">Chang’e 6</a>, the latest Chinese mission to the Moon, is planned for May 2024 and is intended to bring material back to Earth. This is particularly significant because the spacecraft will collect material from the lunar farside – the South Pole Aitkin Basin. </p>
<p>This is a region where it is believed there is abundant frozen water. We do not have any samples of material from this part of the Moon – and although any ice will be long gone by the time the samples are back on Earth, it is anticipated we will learn a lot about this unexplored region and its potential as a source of water for human visitors. </p>
<h2>Hera</h2>
<p>In September 2022, Nasa’s <a href="https://science.nasa.gov/mission/dart/">Dart mission</a> encountered a system consisting of two asteroids called Didymos and Dimorphos, and crashed into Dimorphos (the junior partner). The impact had a purpose: to see if such a collision could divert the asteroid in its path – a necessary goal if ever Earth were to be the target of a direct hit by an incoming asteroid. </p>
<figure class="align-center ">
<img alt="Artist's impression of Hera mission." src="https://images.theconversation.com/files/564917/original/file-20231211-27-3yuer.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/564917/original/file-20231211-27-3yuer.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=337&fit=crop&dpr=1 600w, https://images.theconversation.com/files/564917/original/file-20231211-27-3yuer.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=337&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/564917/original/file-20231211-27-3yuer.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=337&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/564917/original/file-20231211-27-3yuer.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/564917/original/file-20231211-27-3yuer.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/564917/original/file-20231211-27-3yuer.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Artist’s impression of Hera mission.</span>
<span class="attribution"><span class="source">Esa/wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Two years later, the European Space Agency’s <a href="https://www.esa.int/Space_Safety/Hera">Hera</a> mission will launch to visit the same pair of asteroids. It is not designed to hit either body, but to measure the effect of Dart’s earlier impact. At the time of the collision, the orbit of Dimorphos around Didymos got faster by 33 minutes – a significant movement that showed the path of an asteroid could be deflected. </p>
<p>But what we don’t know (and won’t until Hera arrives in 2026) is how effective the impact was. Has Dimorphos remained in its new orbit, bounced back into its old orbit, or continued to speed up? Hera will investigate in detail – and its results will help to define Earth’s planetary defence protocol. Assuming, that is, <a href="https://theconversation.com/dont-look-up-several-asteroids-are-heading-towards-earth-heres-how-we-deal-with-threats-in-real-life-174512">we take notice</a>.</p>
<h2>Europa Clipper</h2>
<p>Launching almost at the same time as Hera is a Nasa flagship mission: the <a href="https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=EUROPA-CL">Europa Clipper</a> to Jupiter’s icy moon, Europa.</p>
<p>This mission has been long-awaited, ever since the Galileo mission first showed us views of Europa’s icy surface in the late 1990s. Since then, we have learnt about the ocean that lurks beneath the icy shell. Excitingly, Europa <a href="https://theconversation.com/europa-there-may-be-life-on-jupiters-moon-and-two-new-missions-will-pave-the-way-for-finding-it-122551">may host life</a> in the form of a substantial fauna analogous to the animals that live on the deep ocean floor around hydrothermal vents. </p>
<figure class="align-center ">
<img alt="Artist's impression of Europa Clipper." src="https://images.theconversation.com/files/564913/original/file-20231211-27-kjjpd2.jpg?ixlib=rb-1.1.0&rect=9%2C9%2C1573%2C1360&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/564913/original/file-20231211-27-kjjpd2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=528&fit=crop&dpr=1 600w, https://images.theconversation.com/files/564913/original/file-20231211-27-kjjpd2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=528&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/564913/original/file-20231211-27-kjjpd2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=528&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/564913/original/file-20231211-27-kjjpd2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=664&fit=crop&dpr=1 754w, https://images.theconversation.com/files/564913/original/file-20231211-27-kjjpd2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=664&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/564913/original/file-20231211-27-kjjpd2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=664&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Artist’s impression of Europa Clipper.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<p>Europa Clipper will fly past Europa between 40 and 50 times, taking detailed images of the surface, monitoring the satellite for icy plumes – and, most importantly, looking to see whether this moon has the conditions suitable to support life. The mission will also investigate whether Europa’s ocean is salty, and whether the essential building blocks of life (carbon, nitrogen and sulphur) are present.</p>
<p>Sadly though, it is not until 2030 that any of these observations will be transmitted back to us, so we will have to wait patiently until then. The investigation will be complemented by observations from Esa’s Juice mission, which is currently on its way to Jupiter.</p>
<h2>MMX</h2>
<p>I began this article with mention of my delight at the return of material from Bennu. I will finish it with my anticipation of further delights to come. I know I have mentioned return of material from the Moon – but in fact, I am much more excited by the prospect of material returning from another moon. The moon in question is Phobos, one of the satellites of Mars. </p>
<p>The launch of the Japanese Space Agency’s <a href="https://theconversation.com/the-longstanding-mystery-of-mars-moons-and-the-mission-that-could-solve-it-219161">Martian Moon Exploration (MMX)</a> mission to Phobos is currently scheduled for September 2024, and designed to return material to Earth in 2029.</p>
<p>I’ll be 70 by the time the material comes back – but, I hope, not too decrepit to take pleasure in analysis of a unique sample from an enigmatic body.</p><img src="https://counter.theconversation.com/content/219440/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Monica Grady works for The Open University. She receives funding from The UKRI-Science and Technology Facilities Council. She is a Senior Research Fellow at the Natural History Museum, London and Chancellor of Liverpool Hope University. She tweets (X's?) as @MonicaGrady</span></em></p>Moons and asteroids will be visited by spacecraft from Earth next year.Monica Grady, Professor of Planetary and Space Sciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2114672023-09-18T21:55:08Z2023-09-18T21:55:08ZDiscovering the universe from our own backyards<figure><img src="https://images.theconversation.com/files/542371/original/file-20230809-19-r1poh9.jpg?ixlib=rb-1.1.0&rect=5%2C4%2C988%2C661&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Is there life beyond our world?</span> <span class="attribution"><span class="source">(Shutterstock)</span></span></figcaption></figure><p>When I was a college student, I worked at the Charlevoix Astronomical Observatory in Québec. </p>
<p>It was a pretty decent summer job, as I got to observe celestial bodies until the dead of night, talk to astronomy buffs about space exploration and watch children be amazed by Saturn’s rings. </p>
<p>Over the dozens of astronomy nights I’ve hosted, one question has consistently come up:</p>
<p>“Does life exist anywhere else?”</p>
<p>Answering this fundamental question, articulated by the first philosophers, which has transcended time and eras and <a href="https://theconversation.com/are-we-alone-in-the-universe-4-essential-reads-on-potential-contact-with-aliens-210955">still remains at the heart of our rational thinking</a>, was a big assignment for me as a CEGEP student at the time. </p>
<p>I merely offered a simple “most likely,” before adding a surprising “and if that’s the case, the answer lies here, on Earth, in places called ‘planetary analogues.’”</p>
<p>Planetary analogues are locations on Earth that replicate one or more extreme conditions found on another celestial body. For example, temperature, pressure and solar radiation.</p>
<p>Both for technical and financial reasons, carrying out several space missions per year, manned or unmanned, is simply not realistic, especially as these missions take several years to complete.</p>
<p>Yet the Earth, our magnificent blue planet where life thrives, has some extreme, dangerous and cruel places. These places can reproduce certain conditions found in the arid deserts of Mars or the suffocating atmosphere of Venus. </p>
<p>What if these places were, in fact, habitats where life has developed?</p>
<h2>Lakes under ice</h2>
<p>For example, consider Europa, one of the moons of Jupiter, which, along with Mars, is one of the top contenders in our quest for extraterrestrial life. Its surface is covered in a dense layer of ice about ten kilometres thick, beneath which lies… an ocean. An ocean <a href="https://doi.org/10.1038/34857">of… liquid water</a>! </p>
<p>It turns out that in Antarctica, almost 400 lakes exist in similar conditions, that is to say that they lie below a permanent ice blanket, protected from everything that happens on the surface. These are known as “subglacial” lakes.</p>
<p>Such is the case of <a href="https://doi.org/10.1038/414603a">Lake Vostok</a>, the largest and deepest lake in Antarctica. It was in the 1960s that scientists first suspected the presence of a lake beneath a four-kilometre thick layer of ice. </p>
<p>This icy barrier deprives the lake of gaseous exchanges with the atmosphere or exposure to solar radiation, making it a permanently dark place that is poor in nutrients and subject to enormous pressure — not very hospitable.</p>
<p>However, the water at the surface of the lake is concentrated in oxygen, the key chemical element for living metabolism. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/542010/original/file-20230809-24-a15igl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/542010/original/file-20230809-24-a15igl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/542010/original/file-20230809-24-a15igl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1067&fit=crop&dpr=1 600w, https://images.theconversation.com/files/542010/original/file-20230809-24-a15igl.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1067&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/542010/original/file-20230809-24-a15igl.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1067&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/542010/original/file-20230809-24-a15igl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1340&fit=crop&dpr=1 754w, https://images.theconversation.com/files/542010/original/file-20230809-24-a15igl.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1340&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/542010/original/file-20230809-24-a15igl.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1340&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Lake Vostok (Antarctica) lies under four kilometres of ice.</span>
<span class="attribution"><span class="source">Wikimedia</span></span>
</figcaption>
</figure>
<h2>A love for extreme conditions</h2>
<p>In 2008, <a href="https://doi.org/10.1126/science.286.5447.2144">analyses of the ice covering Lake Vostok</a> revealed the presence of micro-organisms! This essentially means that life can indeed adapt to hostile environments that would otherwise be fatal for most organisms. These super-organisms, or “extremophile,” are able to tolerate these extreme conditions. </p>
<p>As a result, the waters of Lake Vostok, isolated from the Earth’s surface for millions of years, could well contain life too — an ideal planetary analogue.</p>
<p>Studying Lake Vostok, and its possible extremophile life forms, is almost like being on Jupiter’s moon Europa. And it’s almost like studying its ocean. Were Lake Vostok able to develop life, why not the ocean on Europa as well?</p>
<p>Subglacial lakes such as Vostok are just one example of the dozens of planetary analogue sites that have been identified. For example, in order to study certain Martian craters, the <a href="https://doi.org/10.1017/S1473550413000396">Earth’s deserts are the perfect playgrounds</a>. Scientists are exploring the Mojave (United States), <a href="https://doi.org/10.1038/s41467-023-36172-1">Atacama (Chile)</a> and Namib (Africa) deserts, which are dry and arid. Their soil also contains extremophiles, the study of which tells us about the development of life in hot environments where water is limited.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/542017/original/file-20230809-17-hzx8dz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Mojave desert" src="https://images.theconversation.com/files/542017/original/file-20230809-17-hzx8dz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/542017/original/file-20230809-17-hzx8dz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=382&fit=crop&dpr=1 600w, https://images.theconversation.com/files/542017/original/file-20230809-17-hzx8dz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=382&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/542017/original/file-20230809-17-hzx8dz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=382&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/542017/original/file-20230809-17-hzx8dz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=480&fit=crop&dpr=1 754w, https://images.theconversation.com/files/542017/original/file-20230809-17-hzx8dz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=480&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/542017/original/file-20230809-17-hzx8dz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=480&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The soil of the Mojave Desert contains extremophilic organisms.</span>
<span class="attribution"><span class="source">(Shutterstock)</span></span>
</figcaption>
</figure>
<h2>Preparing for space missions on Earth</h2>
<p>As well as providing a better understanding of life and its emergence, investigating planetary analogues has another advantage: preparing and simulating space missions.</p>
<p>Just think — if we’re developing a new technology to sample a rock on Mars, it would be wise to try it out first, wouldn’t it? And not just inside NASA studios, where the parameters are controlled. We must step out and go to remote, uncomfortable regions. </p>
<p>That’s what the <a href="https://doi.org/10.1130/SPE483">Apollo astronauts of the 50s and 60s</a> did (those who aimed for the moon). They went to meteorite impact craters, volcanoes, deserts, all over the Earth, for months on end. All so they could practice their techniques with a variety of adapted tools, all slowed down by their space suits. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/542009/original/file-20230809-27-z4e7sr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/542009/original/file-20230809-27-z4e7sr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/542009/original/file-20230809-27-z4e7sr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/542009/original/file-20230809-27-z4e7sr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/542009/original/file-20230809-27-z4e7sr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/542009/original/file-20230809-27-z4e7sr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/542009/original/file-20230809-27-z4e7sr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/542009/original/file-20230809-27-z4e7sr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Astronauts Dave Scott (left) and Jim Irwin (right) sample rocks for a possible mission to the moon in 1971.</span>
<span class="attribution"><span class="source">(Analogs for Planetary Exploration [2011])</span></span>
</figcaption>
</figure>
<h2>It all begins on Earth</h2>
<p>Space exploration and the understanding of our solar system begin on Earth. At first glance, this idea may seem counter-intuitive, but it actually makes a lot of sense when you consider the remote, almost inaccessible and extreme environments our planet contains. </p>
<p>Astrochemistry and astrobiology have emerged in this same way, as multidisciplinary fields that equip us for our research into the evolution of Earth and life.</p>
<p>Now, if I were asked the question — “Does life exist anywhere else?” — I, still naive, but starting my PhD in the chemistry of extreme polar environments, would answer:</p>
<blockquote>
<p>Ask me again in five years!</p>
</blockquote>
<p>Joking aside, analogues have their limitations in that the conditions can never be recreated in their entirety. As a result, scientists need to be cautious in their approach and avoid jumping to hasty conclusions. </p>
<p>Life in Lake Vostok is not synonymous with life on Europa, far from it. But let’s just say that it’s an excellent first step that will guide us considerably in our future missions.</p><img src="https://counter.theconversation.com/content/211467/count.gif" alt="La Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Daniel Fillion ne travaille pas, ne conseille pas, ne possède pas de parts, ne reçoit pas de fonds d'une organisation qui pourrait tirer profit de cet article, et n'a déclaré aucune autre affiliation que son organisme de recherche.</span></em></p>Planetary analogues are sites on Earth that are so extreme that they replicate those of celestial bodies in our solar system.Daniel Fillion, Candidat au doctorat en océanographie, Université du Québec à Rimouski (UQAR)Licensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2011882023-06-05T01:08:59Z2023-06-05T01:08:59Z5 incredible craters that will make you fall in love with the grandeur of our Solar System<figure><img src="https://images.theconversation.com/files/525076/original/file-20230509-23-xn5f6x.jpeg?ixlib=rb-1.1.0&rect=120%2C63%2C1644%2C1014&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Occator crater, Ceres.</span> <span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/images/pia20180-occator-in-false-color">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA</a></span></figcaption></figure><p>Impact cratering happens on every solid body in the Solar System. In fact, it is the dominant process affecting the surfaces on most extraterrestrial bodies today. </p>
<p>On Earth, however, such craters are often lost over time by active geological processes, but elsewhere in the Solar System there are some truly majestic examples of impact craters preserved for all to see.</p>
<p>Here, we pick our highlights of what the Solar System has to offer.</p>
<h2>1. South Pole–Aitken basin, the Moon</h2>
<p>Our first crater is a big one: the biggest, deepest and oldest impact crater on the Moon. It is 2,500km diameter, 6.2 to 8.2km deep and formed roughly 4.2 billion years ago. As the name suggests, it is at the south pole on the far side of the Moon, although the crater rim can be seen from Earth as a dark mountain range, just on the border between the light and dark side of the moon.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/525071/original/file-20230509-15-2ly6rr.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A rainbow coloured image of a textured circle with red around the edges and blue in the middle" src="https://images.theconversation.com/files/525071/original/file-20230509-15-2ly6rr.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/525071/original/file-20230509-15-2ly6rr.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=587&fit=crop&dpr=1 600w, https://images.theconversation.com/files/525071/original/file-20230509-15-2ly6rr.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=587&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/525071/original/file-20230509-15-2ly6rr.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=587&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/525071/original/file-20230509-15-2ly6rr.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=738&fit=crop&dpr=1 754w, https://images.theconversation.com/files/525071/original/file-20230509-15-2ly6rr.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=738&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/525071/original/file-20230509-15-2ly6rr.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=738&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A colour-coded topographical image taken by NASA’s Lunar Orbiter Laser Altimeter, showing the South Pole–Aitken basin in blue.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/LRO/multimedia/lroimages/lola-20100409-aitken.html">NASA/Goddard</a></span>
</figcaption>
</figure>
<p>It is a prime site favoured by lunar scientists to visit and learn about our Moon’s geology. The depth excavated by the crater is almost as deep as the deepest ocean trenches on Earth. It gives us a unique view of the interior of the Moon’s crust, with 4.2 billion years of history exposed. </p>
<p>In 2019, a rover from the Chinese space agency, Chang'e 4, <a href="https://www.planetary.org/space-missions/change-4">touched down in the basin</a> and carried out the first scientific experiments there. One of the most interesting of these was the <a href="https://www.universetoday.com/141229/theres-life-on-the-moon-chinas-lander-just-sprouted-the-first-plants/">Lunar Micro Ecosystem</a>, a collection of seeds and insect eggs designed to see if life could flourish in a tiny biosphere on the surface.</p>
<h2>2. Unnamed Crater (S1094b), Mars</h2>
<p>There are many famous craters on Mars, from the homes of Mars rovers (<a href="https://mars.nasa.gov/msl/home/">Gale Crater for Curiosity</a> or <a href="https://mars.nasa.gov/mars2020/">Jezero for Perseverance</a>) to the hypothesised source regions of Mars meteorites (<a href="https://www.higp.hawaii.edu/%7Epmm/Tooting_Crater.html">Tooting</a> or <a href="https://www.jpl.nasa.gov/images/pia12840-terrain-model-of-mars-mojave-crater">Mojave</a>). But one of the newest craters on the red planet is actually quite a dramatic one. </p>
<figure class="align-center ">
<img alt="An animation of a grey landscape where a dark splash mark suddenly appears" src="https://images.theconversation.com/files/526069/original/file-20230515-124665-1wa14a.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/526069/original/file-20230515-124665-1wa14a.gif?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/526069/original/file-20230515-124665-1wa14a.gif?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/526069/original/file-20230515-124665-1wa14a.gif?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/526069/original/file-20230515-124665-1wa14a.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/526069/original/file-20230515-124665-1wa14a.gif?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/526069/original/file-20230515-124665-1wa14a.gif?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The impact event on Mars on Christmas Eve 2021.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/Malin Space Science Systems/Peter Grindrod</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>While Mars rovers claim all the glory for exploring the Martian surface, the satellites orbiting Mars have been making discoveries of their own for decades. NASA’s Mars Reconnaissance Orbiter (MRO) was launched in 2005 but is still operational, and its 16+ years of Mars’s surface images allow us to make comparisons year on year, highlighting differences between data sets. </p>
<p>On Christmas Eve 2021, NASA’s InSight mission detected a large “Marsquake” on the red planet, which MRO data later helped to identify as <a href="https://doi.org/10.1126/science.abq7704">a new impact on the other side of Mars</a>.</p>
<p>The vibrant, fresh impact ejecta (“blankets” of material thrown aside by the impact) can be seen clearly from space using the context camera data aboard the orbiter, and thanks to InSight we even know what it <a href="https://youtu.be/17hsIedHKx8">sounded like</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/525068/original/file-20230509-23-bnnzdk.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Two greyscale images of a textured surface, on the right one there are dark streaks visible in a concentric pattern" src="https://images.theconversation.com/files/525068/original/file-20230509-23-bnnzdk.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/525068/original/file-20230509-23-bnnzdk.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=301&fit=crop&dpr=1 600w, https://images.theconversation.com/files/525068/original/file-20230509-23-bnnzdk.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=301&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/525068/original/file-20230509-23-bnnzdk.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=301&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/525068/original/file-20230509-23-bnnzdk.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=378&fit=crop&dpr=1 754w, https://images.theconversation.com/files/525068/original/file-20230509-23-bnnzdk.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=378&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/525068/original/file-20230509-23-bnnzdk.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=378&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A before-and-after comparison of the location on Mars’s Amazonis Planitia where a meteoroid impacted on December 24 2021.</span>
<span class="attribution"><a class="source" href="https://mars.nasa.gov/resources/27087/context-camera-views-an-impact-crater-in-amazonis-planitia/">NASA/JPL-Caltech/MSSS</a></span>
</figcaption>
</figure>
<h2>3. Enki Catena, Ganymede</h2>
<p>Enki Catena is a chain crater on Ganymede, one of the Galilean satellites of Jupiter. At latest count, <a href="https://astronomy.com/news/2023/02/jupiter-now-has-92-moons">Jupiter has more than 90 moons</a>, a mini planetary system of its own.</p>
<p>Jupiter’s gravity creates tidal forces which shape the moons and give us some of the most interesting geological features we have yet found, from the volcanoes of Io to the subsurface ocean of Europa. There are also strings of craters found on two of the moons, Callisto and Ganymede.</p>
<p>These crater chains were first spotted when the Voyager 1 spacecraft gave us some of the first pictures of the surface of these moons in 1979. They were thought to potentially be collapsed lava tubes, features that have been observed on Mars and the Moon.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/521137/original/file-20230416-24-ynnjdo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A textured grey surface with a line-shaped scoop taken out of it" src="https://images.theconversation.com/files/521137/original/file-20230416-24-ynnjdo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/521137/original/file-20230416-24-ynnjdo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=609&fit=crop&dpr=1 600w, https://images.theconversation.com/files/521137/original/file-20230416-24-ynnjdo.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=609&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/521137/original/file-20230416-24-ynnjdo.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=609&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/521137/original/file-20230416-24-ynnjdo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=766&fit=crop&dpr=1 754w, https://images.theconversation.com/files/521137/original/file-20230416-24-ynnjdo.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=766&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/521137/original/file-20230416-24-ynnjdo.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=766&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Chain of impact craters Enki Catena on Ganymede.</span>
<span class="attribution"><a class="source" href="https://photojournal.jpl.nasa.gov/catalog/pia01610">NASA/JPL/Brown University</a></span>
</figcaption>
</figure>
<p>However, their origin remained under debate until the Shoemaker-Levy 9 comet was observed as it smashed into Jupiter. The comet was seen <a href="https://www.nature.com/articles/365731a0">breaking into multiple pieces</a> and this gave an idea as to how these chains might form – the gravity from Jupiter pulls apart objects into many pieces that all impact close together.</p>
<p>Enki Catena is a chain of 13 craters which crosses from an area of dark to bright terrain on Ganymede. It is 162km in length and about 10km wide. </p>
<p>The <a href="https://sci.esa.int/web/juice">European Space Agency’s Juice mission</a> will visit the Jovian system in the 2030s and allow us to see the surfaces in greater detail than ever before. We might even find more of these crater chains.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/the-much-anticipated-juice-mission-to-jupiter-launches-today-heres-what-it-might-discover-203669">The much-anticipated JUICE mission to Jupiter launches today. Here's what it might discover</a>
</strong>
</em>
</p>
<hr>
<h2>4. Occator Crater, Ceres</h2>
<p>Ceres is the largest body in the main asteroid belt between Mars and Jupiter. It is large and round enough to be considered a “dwarf planet” (along with Pluto and three less famous examples, Eris, Makemake and Haumea).</p>
<p><iframe id="tc-infographic-852" class="tc-infographic" height="400px" src="https://cdn.theconversation.com/infographics/852/1843fe3dc0b49b387d1fc0ea9b551efa73cdb7cb/site/index.html" width="100%" style="border: none" frameborder="0"></iframe></p>
<p>The Occator crater on Ceres is impressive because it contains a bright spot in the centre that has been observed both from space, and from Earth at Mauna Kea Observatory, Hawaii. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/525555/original/file-20230511-19-lkluwg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A grey image with a round circle in the middle, dotted with several white spots" src="https://images.theconversation.com/files/525555/original/file-20230511-19-lkluwg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/525555/original/file-20230511-19-lkluwg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/525555/original/file-20230511-19-lkluwg.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/525555/original/file-20230511-19-lkluwg.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/525555/original/file-20230511-19-lkluwg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/525555/original/file-20230511-19-lkluwg.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/525555/original/file-20230511-19-lkluwg.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Occator crater with its bright spots as imaged by the Dawn mission.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/jpl/occator-crater-and-ceres-brightest-spots">NASA</a></span>
</figcaption>
</figure>
<p>NASA’s Dawn mission entered an orbit around Ceres in 2015, and imaged the bright spot in Occator crater known as “Spot 5”. It’s a three kilometre wide dome covered in bright salts on the crater floor, likely resulting from hydrothermal activity.</p>
<p>Occator crater itself is 92km in diameter and 3km deep. Simulations indicate that the impactor (the space rock that created the crater) was rougly 5km across, striking Ceres between 20–25 million years ago.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/525556/original/file-20230511-11888-acngdy.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A dark bowl-shaped depression with what looks like white ice spread in the middle" src="https://images.theconversation.com/files/525556/original/file-20230511-11888-acngdy.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/525556/original/file-20230511-11888-acngdy.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=349&fit=crop&dpr=1 600w, https://images.theconversation.com/files/525556/original/file-20230511-11888-acngdy.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=349&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/525556/original/file-20230511-11888-acngdy.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=349&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/525556/original/file-20230511-11888-acngdy.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=439&fit=crop&dpr=1 754w, https://images.theconversation.com/files/525556/original/file-20230511-11888-acngdy.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=439&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/525556/original/file-20230511-11888-acngdy.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=439&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">This mosaic from NASA’s Dawn spacecraft combines images obtained from altitudes as low as 22 miles (35 km) above Ceres’ surface.</span>
<span class="attribution"><a class="source" href="https://solarsystem.nasa.gov/resources/1074/mosaic-of-cerealia-facula/">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI</a></span>
</figcaption>
</figure>
<h2>5. Aurelia, Venus</h2>
<p>Venus is <a href="https://www.nasa.gov/feature/why-is-venus-called-earths-evil-twin-we-asked-a-nasa-scientist-episode-32">sometimes called Earth’s twin</a>. It is when it comes to size, but the surface images we have of Venus show the planets have very different features.</p>
<p>The best such images were taken in the 1990s by NASA’s Magellan spacecraft. Venus has a thick cloudy atmosphere, and visible-light cameras can’t see through to the surface. Magellan was equipped with a radar which can “see” the surface – but the images can be harder to interpret. </p>
<p>In radar, dark terrain is very smooth and bright terrain is very rough. This makes impact craters stand out really well in radar images. The ejecta are very rough, especially against the surrounding volcanic plains, so they appear bright in the images.</p>
<p>This is Aurelia, a 32km impact crater on Venus. </p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/521138/original/file-20230416-18-3lnwor.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A greyscale image with a bright white semicircular shape standing out against a dark background" src="https://images.theconversation.com/files/521138/original/file-20230416-18-3lnwor.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/521138/original/file-20230416-18-3lnwor.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/521138/original/file-20230416-18-3lnwor.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/521138/original/file-20230416-18-3lnwor.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/521138/original/file-20230416-18-3lnwor.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/521138/original/file-20230416-18-3lnwor.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/521138/original/file-20230416-18-3lnwor.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Aurelia crater on Venus, imaged by Magellan in 1996.</span>
<span class="attribution"><a class="source" href="https://photojournal.jpl.nasa.gov/catalog/?IDNumber=PIA00239">NASA/JPL</a></span>
</figcaption>
</figure>
<p>You can see it stands out against the grey plains that surround it. The black terrain on the edges of the bright white ejecta are smooth flows of rock that melted when the impact hit.</p>
<p>Speaking of volcanoes on Venus, recently a group from the University of Alaska Fairbanks used this Magellan data to find the first active volcano on <a href="https://www.nasa.gov/feature/jpl/nasa-s-magellan-data-reveals-volcanic-activity-on-venus/">Venus</a></p>
<p>NASA has <a href="https://solarsystem.nasa.gov/planets/venus/exploration/#otp_future_missions">three Venus missions in development</a> over the next 10 years, so hopefully soon we will know much more about our enigmatic twin.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/these-5-spectacular-impact-craters-on-earth-highlight-our-planets-wild-history-197618">These 5 spectacular impact craters on Earth highlight our planet's wild history</a>
</strong>
</em>
</p>
<hr>
<img src="https://counter.theconversation.com/content/201188/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>If we could go sightseeing across our cosmic neighbourhood, these would be some of the best highlights.Helen Brand, Senior Beamline Scientist - Powder Diffraction, Australian Nuclear Science and Technology OrganisationNatasha Stephen, Director of Science & Engagement, The Geological Society of London, and Honorary Lecturer, Imperial College LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2036692023-04-13T05:40:16Z2023-04-13T05:40:16ZThe much-anticipated JUICE mission to Jupiter launches today. Here’s what it might discover<figure><img src="https://images.theconversation.com/files/520681/original/file-20230413-20-ybr84v.jpeg?ixlib=rb-1.1.0&rect=717%2C0%2C2284%2C1281&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Enhanced image by Kevin M. Gill (CC-BY) based on images provided courtesy of NASA/JPL-Caltech/SwRI/MSSS.Media</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p><strong>Editor’s note (April 14 2023):</strong> <em>The 13 April launch was postponed due to weather conditions, but the team will attempt another launch on 14 April at 10:14pm AEST. You can follow the launch live via <a href="https://www.esa.int/ESA_Multimedia/ESA_Web_TV">ESA Web TV</a>.</em> </p>
<p>The European Space Agency’s JUICE mission (Jupiter Icy Moons Explorer) <a href="https://www.esa.int/Science_Exploration/Space_Science/Juice/How_to_follow_the_Juice_launch_live">is launching today</a> at 10:15pm AEST from Europe’s spaceport in French Guiana.</p>
<p>JUICE will be targeting three water-rich worlds – Jupiter’s moons Ganymede, Europa and Callisto – to check out potential habitats and evidence of past alien life, both on and below the surface. There’s an excellent reason why these worlds in particular are the mission target – they might be habitable for life as we know it.</p>
<h2>The moons of Jupiter</h2>
<p>Although we have just one moon lighting up our night skies, <a href="https://coolcosmos.ipac.caltech.edu/ask/99-How-many-moons-does-Jupiter-have-">Jupiter has at least 92</a>. Some, including the four <a href="https://lasp.colorado.edu/outerplanets/moons_galilean.php">Galilean moons</a> (the largest Jovian moons) formed alongside Jupiter nearly 4.5 billion years ago in the early Solar System. Others have been drawn in and captured by this massive planet, adding to the collection over time.</p>
<p>These moons are made of hugely diverse materials, and some are thought to have conditions favourable for life, or could have in the past.</p>
<p>Fewer than ten interplanetary missions have ever flown past Jupiter, with only two NASA missions stopping to orbit the planet and investigate further: <a href="https://www.jpl.nasa.gov/missions/galileo">the Galileo mission</a> between 1995 and 2003, and the <a href="https://www.jpl.nasa.gov/missions/juno">current Juno mission</a>, launched in 2011. These are the only two to have also made dedicated passes of the moons, gathering valuable information for upcoming missions. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/520665/original/file-20230413-24-d6rig7.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram showing three missions to Jupiter" src="https://images.theconversation.com/files/520665/original/file-20230413-24-d6rig7.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/520665/original/file-20230413-24-d6rig7.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/520665/original/file-20230413-24-d6rig7.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/520665/original/file-20230413-24-d6rig7.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/520665/original/file-20230413-24-d6rig7.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/520665/original/file-20230413-24-d6rig7.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/520665/original/file-20230413-24-d6rig7.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">NASA’s Juno mission laid the groundwork for both Juice and the upcoming Europa Clipper mission.</span>
<span class="attribution"><a class="source" href="https://www.esa.int/ESA_Multimedia/Images/2022/12/A_trio_of_missions_to_Jupiter">ESA</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>Life as we know it</h2>
<p>The Galilean moons are of particular interest. The second smallest, Io, may not be habitable, but has <a href="https://theconversation.com/four-volcanic-hotspots-in-the-solar-system-202383">some of the largest active volcanoes</a> in the Solar System (with eruptions that can be seen from Earth!).</p>
<p>The other three, Ganymede, Europa and Callisto, are all thought to have large bodies of liquid water under their icy surfaces, and maybe even thin atmospheres. </p>
<p>Ganymede’s liquid iron core also <a href="https://solarsystem.nasa.gov/moons/jupiter-moons/ganymede/in-depth/">gives it a magnetic field</a>, the only known moon in the Solar System to have one. Our own magnetic field protects Earth’s atmosphere from the harsh solar winds, shielding us from solar radiation. These are factors we associate with fostering and protecting life on Earth.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/520683/original/file-20230413-24-mm3ijm.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An image of a blue circle on a black background with concentric ellipses extending to both sides" src="https://images.theconversation.com/files/520683/original/file-20230413-24-mm3ijm.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/520683/original/file-20230413-24-mm3ijm.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=367&fit=crop&dpr=1 600w, https://images.theconversation.com/files/520683/original/file-20230413-24-mm3ijm.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=367&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/520683/original/file-20230413-24-mm3ijm.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=367&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/520683/original/file-20230413-24-mm3ijm.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=461&fit=crop&dpr=1 754w, https://images.theconversation.com/files/520683/original/file-20230413-24-mm3ijm.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=461&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/520683/original/file-20230413-24-mm3ijm.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=461&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A sketch of the magnetic field lines around Ganymede, which are generated in the moon’s iron core. Hubble Space Telescope measurements of Ganymede’s aurorae, which follow magnetic field lines, suggest that a subsurface saline ocean also influences the behaviour of the moon’s auroras.</span>
<span class="attribution"><span class="source">NASA, ESA, and A. Feild (STScI)</span></span>
</figcaption>
</figure>
<p>We only know of life on Earth, so when we go looking for where life might exist (or once existed) elsewhere, we’re looking for factors we consider essential to life as we know it.</p>
<p>Watery or icy worlds are the first targets, as we know life on Earth originated in and around water. A rocky surface with warmish temperatures would be even more ideal. Jupiter itself is a complete write-off: the crushing pressures, toxic gases, freezing temperatures and lack of a stable surface would never support life as we know it. But the big, icy moons have good protection deep under the ice, potentially liquid water, and elements like carbon and oxygen.</p>
<p>JUICE will use its suite of science instruments to check out the thicknesses of the moons’ icy crusts, what they’re made of, and look for subsurface liquid water. On Europa in particular, it will look for evidence of organic molecules.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/the-search-for-life-beneath-the-ice-why-were-going-back-to-europa-43846">The search for life beneath the ice: why we're going back to Europa</a>
</strong>
</em>
</p>
<hr>
<h2>An extremely efficient journey</h2>
<p>After its launch, the solar-powered JUICE will take nearly eight years to get to Jupiter. The spacecraft will use minimal propulsion, instead using other planets to give it speed and set its course.</p>
<p>These manoeuvres are called “<a href="https://www.esa.int/Science_Exploration/Space_Science/Exploring_space/Let_gravity_assist_you">gravity assists</a>”. This essentially means JUICE will fly purposefully toward a planet, just missing it, in order to get pulled in by its gravity and “slingshot” past the other side. It may take time, but it is extremely efficient.</p>
<p>Juice’s first gravity assist in 2024 will go around <em>both</em> Earth and our Moon – <a href="https://www.esa.int/ESA_Multimedia/Videos/2022/03/Juice_s_flyby_of_Earth-Moon_system">the first time this has ever been done</a>. Other gravity assists will take it around Venus in 2025, and Earth (only) in 2026 and 2029, before being kicked out to Jupiter for an arrival in mid-2031. </p>
<hr>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/Fw17N3rdN7s?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<hr>
<h2>What happens when JUICE meets Jupiter?</h2>
<p>Fun fact: it will be the first spacecraft to orbit a moon other than our own! </p>
<p>Usually a spacecraft will orbit the main planet (in this case Jupiter) and merely flyby the moons as it loops past. JUICE will start like this, flying by Callisto, Europa and Ganymede a total of 35 times during its three-year tour of the moons. It will briefly meet <a href="https://europa.nasa.gov/">NASA’s Europa Clipper mission</a> around Europa, complimenting this mission nicely.</p>
<p>But in 2034, JUICE will actually switch its orbit from around Jupiter <a href="https://www.esa.int/ESA_Multimedia/Videos/2022/03/Juice_s_elliptical_orbit_around_Ganymede">to go around Ganymede</a>. This will give it an exceptional view, and nearly a year to study this fascinating moon, probing its internal, surface and atmospheric systems.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/520667/original/file-20230413-28-p06afj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A close-up photo of a conical white rocket with ESA logo on it and a cartoon of Jupiter and Earth" src="https://images.theconversation.com/files/520667/original/file-20230413-28-p06afj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/520667/original/file-20230413-28-p06afj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=900&fit=crop&dpr=1 600w, https://images.theconversation.com/files/520667/original/file-20230413-28-p06afj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=900&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/520667/original/file-20230413-28-p06afj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=900&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/520667/original/file-20230413-28-p06afj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1131&fit=crop&dpr=1 754w, https://images.theconversation.com/files/520667/original/file-20230413-28-p06afj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1131&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/520667/original/file-20230413-28-p06afj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1131&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Ariane 5 VA 260 with JUICE ready for launch on the ELA-3 launch pad at Europe’s Spaceport in Kourou, French Guiana on April 12 2023.</span>
<span class="attribution"><a class="source" href="https://www.esa.int/ESA_Multimedia/Images/2023/04/Ariane_5_VA_260_with_Juice_-_Ready_for_launch">ESA/S. Corvaja</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Of particular interest is the magnetic field. Ganymede is one of only three rocky bodies in our Solar System known to have one (Earth and Mercury being the other two). Questions JUICE will be able to investigate are not just the basic question of what is creating Ganymede’s magnetic field, but also what happens to it as Ganymede travels through the larger field produced by Jupiter itself, and how their complex interactions influence auroras on both Jupiter and Ganymede.</p>
<p>JUICE will also get a chance to study Jupiter itself, looking into characteristics of giant gas planets that might be universal. Could Jupiter be the key to understanding other solar systems, and the hundreds of <a href="https://theconversation.com/stars-with-planets-on-strange-orbits-whats-going-on-56511">exoplanets we have discovered orbiting other stars</a>?</p>
<p>So, we might be waiting a while for JUICE’s arrival at Jupiter, but it will be well worth the wait. Could any of these moons have once supported alien life, and what might we learn about our own Earth, its early oceans, and the conditions needed to spawn life?</p><img src="https://counter.theconversation.com/content/203669/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Eleanor K. Sansom receives funding from the International Centre for Radio Astronomy Research and is supported by the Space Science and Technology Centre at Curtin University and the Australian Research Council (DP230100301).</span></em></p>We’re going to get up close and personal with three of Jupiter’s largest moons like never before.Eleanor K. Sansom, Research Associate, Curtin UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2032042023-04-12T17:18:09Z2023-04-12T17:18:09ZScientists launch JUICE mission to explore Jupiter’s icy moons<figure><img src="https://images.theconversation.com/files/520598/original/file-20230412-24-axm9rh.jpeg?ixlib=rb-1.1.0&rect=0%2C20%2C1920%2C1057&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Jupiter has more than 80 moons, the largest of which were discovered by Galileo. Many will be studied in depth by the scientific instruments of ESA's JUICE mission.</span> <span class="attribution"><span class="source">ESA, NASA, JPL, ATG, DLR, University of Arizona, University of Leicester</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Could we discover conditions necessary for life outside the Earth in the solar system?</p>
<p>This is one of the mysteries that the space mission <a href="https://www.esa.int/Science_Exploration/Space_Science/Juice_factsheet">JUICE</a> (for <em>JUpiter ICy moons Explorer</em>) will look to elucidate. Initially set to be launched from Kourou, French Guiana, in the early afternoon of Thursday 13 April 2023, the spacecraft will now be <a href="https://www.rfi.fr/en/international/20230413-bad-weather-forces-postponement-of-jupiter-space-mission-juice">leaving on Friday</a> due to bad weather.</p>
<p>To propel this mission to a planet located more than 600 million kilometres away, the <a href="https://esamultimedia.esa.int/docs/corporate/This_is_ESA_FR_LR.pdf">European Space Agency</a> (ESA) has brought together no fewer than 13 European countries, as well as the United States, Japan, and Israel. Through this mission, the agency has also managed the feat of placing JUICE on the launch pad only 11 years after the project was greenlit. While the Covid pandemic slowed down the process, the delay was only nine months. France’s team, of which I am a part, also <a href="https://www.cnrs.fr/fr/cnrsinfo/cnrs-cnes-lespace-main-dans-la-main">helped develop</a> six of JUICE’s ten state-of-the-art scientific instruments. The probe is expected to arrive in the Jovian system in 2031.</p>
<h2>Stretching science’s boundaries</h2>
<p>Jupiter is both the largest planet in our solar system and the one with the most moons. To date, estimates of their number hover between <a href="https://fr.wikipedia.org/wiki/Satellites_naturels_de_Jupiter">82 and 95</a>, most of which have been discovered in the last two decades.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/519068/original/file-20230403-16-777o7x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Swirling clouds on the surface of Jupiter" src="https://images.theconversation.com/files/519068/original/file-20230403-16-777o7x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/519068/original/file-20230403-16-777o7x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=270&fit=crop&dpr=1 600w, https://images.theconversation.com/files/519068/original/file-20230403-16-777o7x.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=270&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/519068/original/file-20230403-16-777o7x.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=270&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/519068/original/file-20230403-16-777o7x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=340&fit=crop&dpr=1 754w, https://images.theconversation.com/files/519068/original/file-20230403-16-777o7x.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=340&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/519068/original/file-20230403-16-777o7x.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=340&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Jupiter’s turbulent atmosphere.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/jpl/churning-texture-in-jupiter-s-atmosphere">NASA/JPL-Caltech/SwRI/MSSS, Kevin M. Gill</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>JUICE is the first mission to receive more than 1 billion euros of funding as part of the ESA’s <a href="https://www.esa.int/Science_Exploration/Space_Science/ESA_s_Cosmic_Vision">Cosmic Vision</a> programme. It seeks to address four main questions:</p>
<ul>
<li><p>How do planets come to form and life to emerge?</p></li>
<li><p>How does the Solar System work?</p></li>
<li><p>What are the fundamental laws of physics in the universe?</p></li>
<li><p>How did the present universe come into being and what is it made of?</p></li>
</ul>
<p>JUICE was chosen ahead of other proposed missions because it was designed to address the first and last of these questions.</p>
<p>The <a href="https://theconversation.com/how-the-hubble-space-telescope-opened-our-eyes-to-the-first-galaxies-of-the-universe-133877">Hubble Space Telescope</a> and NASA’s space probes <a href="https://theconversation.com/what-the-voyager-space-probes-can-teach-humanity-about-immortality-and-legacy-as-they-sail-through-space-for-trillions-of-years-177513">Voyager</a>, <a href="https://www.jpl.nasa.gov/missions/galileo">Galileo</a>, <a href="https://www.nasa.gov/mission_pages/juno/main/index.html">Juno</a> have already picked up some clues either by direct observation or deduction.</p>
<h2>“Ocean moons” containing more water than the Earth</h2>
<p>NASA’s Galileo was the first to discover <a href="https://history.nasa.gov/sp4231.pdf">water on the moons</a> in 1995. Data captured by the space probe revealed gigantic liquid oceans not only under the crusts of its three icy moons, Callisto, Europa and Ganymede, but also on its <a href="https://theconversation.com/dou-viennent-les-aurores-boreales-et-pourquoi-sont-elles-si-differentes-sur-jupiter-186776">volcanic</a> moon, Io.</p>
<p>In 2014, the Hubble Space Telescope discovered geysers in Europa. Their bases appeared to be caked with <a href="https://planet-terre.ens-lyon.fr/planetterre/objets/Images/vie-Europe-Jupiter-2014/vie-Europe-satellite-Jupiter.pdf">salts</a>, including carbonates. It is therefore likely Europa could meet the <a href="https://planet-terre.ens-lyon.fr/ressource/habitabilite-vie-systeme-solaire.xml">four criteria</a> for habitability:</p>
<ul>
<li><p>The famous quartet of <a href="https://en.wikipedia.org/wiki/CHON">carbon, hydrogen, oxygen, nitrogen</a> (CHON), symbols of the main chemical elements that constitute living beings.</p></li>
<li><p>Liquid water that acts as a solvent.</p></li>
<li><p>Energy to enable the development of life.</p></li>
<li><p>A stable environment (orbits, rotation, average temperatures…)</p></li>
</ul>
<p>The Galilean moons further enjoy the gravitational energy of Jupiter, creating significant tidal effects and allowing the last two conditions above to be met.</p>
<h2>Why Ganymede is the main objective</h2>
<p>Ganymede is set to studied in much more depth by JUICE than Callisto and Europa. This is not only because it is the largest moon in the Solar System, but also an ocean moon with its own magnetic field. Similarly to the Earth’s magnetosphere, Ganymede’s has the potential to protect life by diverting the flow of cosmic rays and radiative particles from Jupiter’s <a href="https://fr.wikipedia.org/wiki/Ceinture_de_radiations">radiation belts</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/519078/original/file-20230403-22-1np1wb.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Ganymede’s northern lights" src="https://images.theconversation.com/files/519078/original/file-20230403-22-1np1wb.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/519078/original/file-20230403-22-1np1wb.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=503&fit=crop&dpr=1 600w, https://images.theconversation.com/files/519078/original/file-20230403-22-1np1wb.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=503&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/519078/original/file-20230403-22-1np1wb.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=503&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/519078/original/file-20230403-22-1np1wb.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=631&fit=crop&dpr=1 754w, https://images.theconversation.com/files/519078/original/file-20230403-22-1np1wb.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=631&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/519078/original/file-20230403-22-1np1wb.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=631&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Ganymede’s northern lights belts appear bathed in Jupiter’s magnetic field. When Jupiter’s magnetic field changes, the auroras sway – and this swaying motion indicates that a huge amount of salt water would be present under the crust of Ganymede, affecting its own magnetic field.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/sites/default/files/thumbnails/image/15-33i2.png">NASA/ESA</a></span>
</figcaption>
</figure>
<h2>JUICE, a probe of the extreme</h2>
<p>JUICE’s itinerary to the Jovian system will not be following a straight line. Instead, the spacecraft will fly by four different planets and moons that will alter and speed its trajectory, enabling it to save fuel as well – a trick also known as a <a href="https://en.wikipedia.org/wiki/Gravity_assist">gravity assist manoeuvre</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/519081/original/file-20230403-28-amat5c.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="timeline" src="https://images.theconversation.com/files/519081/original/file-20230403-28-amat5c.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/519081/original/file-20230403-28-amat5c.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=188&fit=crop&dpr=1 600w, https://images.theconversation.com/files/519081/original/file-20230403-28-amat5c.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=188&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/519081/original/file-20230403-28-amat5c.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=188&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/519081/original/file-20230403-28-amat5c.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=236&fit=crop&dpr=1 754w, https://images.theconversation.com/files/519081/original/file-20230403-28-amat5c.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=236&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/519081/original/file-20230403-28-amat5c.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=236&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">JUICE’s long journey to the Jovian system.</span>
<span class="attribution"><a class="source" href="https://esamultimedia.esa.int/docs/science/Juice-LaunchKit_FR.pdf">ESA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Along the way, JUICE will have to contend with the Solar System’s highest radiation levels. This means that its electronic modules have to be housed in lead-shielded cavities and <a href="https://fr.wikipedia.org/wiki/Durcissement_(%C3%A9electronics)">components have to be “hardened”</a> to help them resist the harsh environment.</p>
<p>JUICE will also have to cope with extreme temperatures, ranging from +250°C as it flies by Venus to -230°C in the Jovian system. To maintain a stable internal temperature, the spacecraft has been coated with a <a href="https://en.wikipedia.org/wiki/Multi-layer_insulation">multilayer thermal insulation</a> made out of grey silicon aluminium alloy, earning the probe the nickname “silver beauty”.</p>
<h2>An energy problem</h2>
<p>Around Jupiter, which is five times further from the Sun than Earth, the satellite will receive 25 times less solar energy than it would around Earth. The spacecraft does not carry a <a href="https://fr.wikipedia.org/wiki/G%C3%A9n%C3%A9rateur_thermo%C3%A9lectrique_%C3%A0_radioisotope">radioactive battery</a> because Europe is not yet able to produce them industrially, unlike the <a href="https://nuke.fas.org/space/bennett0706.pdf">United States</a>, Russia and China.</p>
<p>To enable the equipment and instruments to function with 1000W (the power of a small hairdryer), the craft will rely on huge solar panels – their surface area totals 85m2 – that have been tested to withstand the radiation and temperature variations.</p>
<figure class="align-center ">
<img alt="photo of the JUICE panels" src="https://images.theconversation.com/files/519083/original/file-20230403-20-ocb3ob.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/519083/original/file-20230403-20-ocb3ob.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/519083/original/file-20230403-20-ocb3ob.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/519083/original/file-20230403-20-ocb3ob.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/519083/original/file-20230403-20-ocb3ob.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/519083/original/file-20230403-20-ocb3ob.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/519083/original/file-20230403-20-ocb3ob.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Test deployment of JUICE’s solar panels, whose two wings comprise five panels of 2.5 times 3.5 metres each, arranged in a cross pattern.</span>
<span class="attribution"><a class="source" href="https://www.esa.int/ESA_Multimedia/Images/2022/06/Juice_solar_array_deployment_test2"> Airbus</a>, <span class="license">Fourni par l'auteur</span></span>
</figcaption>
</figure>
<p>Built by 80 European companies under the direction of EADS Toulouse, the JUICE probe has a wingspan of 28 metres (the length of a basketball court), a 2.5-metre long communications antenna (needed because of Jupiter’s distance from the Earth). It weighs nearly 6 tonnes at liftoff (most of which is propellant that will be consumed in manoeuvring the probe) and carries ten instruments (in all, less than 280 kg).</p>
<h2>Ten scientific instruments on board</h2>
<p>Of these instruments, France – with assistance from Italy – chiefly engineered the <a href="https://www.insu.cnrs.fr/fr/cnrsinfo/majis-un-spectro-imageur-pour-explorer-jupiter-et-ses-lunes">Moons and Jupiter Imaging Spectrometer</a> (MAJIS). It is the instrument that will allow the spacecraft to determine the physico-chemical compositions of the moons’ surfaces as it flies over them and thus detect the CHON associated with potential habitability.</p>
<p>MAJIS will also study their ice sheets and liquid water. This will enable us to identify landing sites for future <em>in situ</em> exploration, and evaluate the structure and dynamics of Jupiter’s atmosphere.</p>
<p>With an accuracy 10,000 times higher than the equivalent instrument on Galileo, the spatial resolution of MAJIS ranges between <a href="https://lejournal.cnrs.fr/articles/objectif-jupiter">100 metres and several kilometres</a> depending on the probe’s altitude at the time.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/zkKt9pyseQg?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">JUICE mission: Ask about the moons! (CNES).</span></figcaption>
</figure>
<p>Finally, it should be noted that JUICE’s plans may be revised based on the latest results from NASA’s Juno mission. Juno is still orbiting Jupiter and has been flying over its poles since 2016. Juno’s nominal mission has been extended to fly past each of Jupiter’s Galilean moons, starting with Ganymede in June 2021, and Europa in early 2023. These observations and subsequent data analysis will allow JUICE scientists to better target the observations they make – 12 years after Juno and 30 years after Galileo.</p>
<hr>
<p><em>The French laboratories involved in the development of JUICE are <a href="https://www.ias.u-psud.fr/fr">IAS</a>, <a href="https://astrophy.u-bordeaux.fr/">LAB</a>, <a href="https://www3.latmos.ipsl.fr/index.php/fr/">LATMOS</a>, <a href="https://ipag.osug.fr/">IPAG</a>, <a href="https://www.irap.omp.eu/">IRAP</a>, <a href="https://lerma.obspm.fr/">LERMA</a>, <a href="https://lesia.obspm.fr/">LESIA</a>, <a href="https://www.lpc2e.cnrs.fr/">LPC2E</a> and <a href="https://www.lpp.polytechnique.fr/">LPP</a>.</em></p><img src="https://counter.theconversation.com/content/203204/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Carole Larigauderie is a member of WIA (Women In Aerospace) and a sponsor within "Elles bougent".</span></em></p>One of Jupiter’s moons, Ganymede, could contain more water than the Earth.Carole Larigauderie, Sous-directrice adjointe des Projets en Sciences de l’Univers et Cheffe de Projet des contributions françaises à JUICE, Centre national d’études spatiales (CNES)Licensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2032072023-04-10T12:10:25Z2023-04-10T12:10:25ZJupiter’s moons hide giant subsurface oceans – two missions are sending spacecraft to see if these moons could support life<figure><img src="https://images.theconversation.com/files/519960/original/file-20230407-28-6r7tcb.jpg?ixlib=rb-1.1.0&rect=51%2C27%2C2224%2C1425&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The surface of Europa – one of Jupiter's moons – is a thick layer of solid ice.</span> <span class="attribution"><a class="source" href="https://solarsystem.nasa.gov/resources/204/europas-stunning-surface/?category=moons/jupiter-moons_europa">NASA/JPL-Caltech/SETI Institute</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>On April 13, 2023, the European Space Agency launched a rocket carrying a spacecraft destined for Jupiter. The <a href="https://www.esa.int/Science_Exploration/Space_Science/Juice">Jupiter Icy Moons Explorer</a> – or JUICE – will spend at least three years on Jupiter’s moons after it arrives in 2031. In October 2024, NASA is also planning to launch a robotic spacecraft named <a href="https://europa.nasa.gov/">Europa Clipper</a> to the Jovian moons, highlighting an increased interest in these distant, but fascinating, places in the solar system.</p>
<p><a href="https://www.michaelmsori.com/">I’m a planetary scientist</a> who studies the <a href="https://scholar.google.com/citations?user=NLWIrYoAAAAJ&hl=en&oi=ao">structure and evolution of solid planets and moons</a> in the solar system. </p>
<p>There are many reasons my colleagues and I are looking forward to getting the data that JUICE and Europa Clipper will hopefully be sending back to Earth in the 2030s. But perhaps the most exciting information will have to do with water. Three of Jupiter’s moons – Europa, Ganymede and Callisto – are home to large, underground oceans of liquid water that could support life.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/519961/original/file-20230407-16-27nqat.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Four moons next to a large red spot on the surface of Jupiter." src="https://images.theconversation.com/files/519961/original/file-20230407-16-27nqat.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/519961/original/file-20230407-16-27nqat.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=857&fit=crop&dpr=1 600w, https://images.theconversation.com/files/519961/original/file-20230407-16-27nqat.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=857&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/519961/original/file-20230407-16-27nqat.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=857&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/519961/original/file-20230407-16-27nqat.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1077&fit=crop&dpr=1 754w, https://images.theconversation.com/files/519961/original/file-20230407-16-27nqat.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1077&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/519961/original/file-20230407-16-27nqat.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1077&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">This composite image shows, from top to bottom, Io, Europa, Ganymede and Callisto next to Jupiter.</span>
<span class="attribution"><a class="source" href="https://solarsystem.nasa.gov/resources/2662/family-portrait-of-the-jovian-system/?category=moons/jupiter-moons_europa">NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Meet Io, Europa, Ganymede and Callisto</h2>
<p>Jupiter has dozens of moons. Four of them in particular are of interest to planetary scientists.</p>
<p>Io, Europa, Ganymede and Callisto are, like Earth’s Moon, relatively large, spherical complex worlds. Two previous NASA missions have sent spacecraft to orbit the Jupiter system and collected data on these moons. The <a href="https://solarsystem.nasa.gov/missions/galileo/overview/">Galileo mission</a> orbited Jupiter from 1995 to 2003 and led to geological discoveries on all four large moons. The <a href="https://www.nasa.gov/mission_pages/juno/main/index.html">Juno mission</a> is still orbiting Jupiter today and has provided scientists with an unprecedented view into Jupiter’s composition, structure and space environment. </p>
<p>These missions and other observations revealed that Io, the closest of the four to its host planet, is <a href="https://doi.org/10.1146/annurev.earth.31.100901.145428">abuzz with</a> <a href="https://doi.org/10.1038/nature22339">geological activity</a>, including lava lakes, volcanic eruptions and tectonically formed mountains. But it is not home to large amounts of water.</p>
<p>Europa, Ganymede and Callisto, in contrast, have icy landscapes. Europa’s surface is a frozen wonderland with a young but complex history, <a href="https://doi.org/10.1038/ngeo2245">possibly including icy analogs</a> of plate tectonics and volcanoes. Ganymede, the largest moon in the entire solar system, is bigger than Mercury and has its own magnetic field <a href="https://doi.org/10.1038/384544a0">generated internally from a liquid metal core</a>. Callisto appears somewhat inert compared to the others, but serves as a valuable time capsule of an ancient past that is no longer accessible on the youthful surfaces of Europa and Io.</p>
<p>Most exciting of all: Europa, Ganymede and Callisto all almost certainly possess <a href="https://www.nasa.gov/specials/ocean-worlds/">underground oceans of liquid water</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/519962/original/file-20230407-16-ddggzn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram showing a cutaway of Europa." src="https://images.theconversation.com/files/519962/original/file-20230407-16-ddggzn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/519962/original/file-20230407-16-ddggzn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=570&fit=crop&dpr=1 600w, https://images.theconversation.com/files/519962/original/file-20230407-16-ddggzn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=570&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/519962/original/file-20230407-16-ddggzn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=570&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/519962/original/file-20230407-16-ddggzn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=716&fit=crop&dpr=1 754w, https://images.theconversation.com/files/519962/original/file-20230407-16-ddggzn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=716&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/519962/original/file-20230407-16-ddggzn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=716&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Warmth from Europa’s interior and tidal energy from Jupiter likely maintain a massive liquid ocean beneath the moon’s icy surface.</span>
<span class="attribution"><a class="source" href="https://photojournal.jpl.nasa.gov/jpeg/PIA24477.jpg">NASA/JPL-Caltech/Michael Carroll</a></span>
</figcaption>
</figure>
<h2>Ocean worlds</h2>
<p>Europa, Ganymede and Callisto have chilly surfaces that are <a href="https://europa.nasa.gov/resources/114/daytime-temperatures-on-europa/">hundreds of degrees below zero</a>. At these temperatures, ice behaves like solid rock. </p>
<p>But <a href="https://theconversation.com/how-has-the-inside-of-the-earth-stayed-as-hot-as-the-suns-surface-for-billions-of-years-193277">just like Earth</a>, the deeper underground you go on these moons, the hotter it gets. Go down far enough and you eventually reach the temperature where ice melts into water. Exactly how far down this transition occurs on each of the moons is a <a href="https://doi.org/10.1016/j.icarus.2005.03.013">subject of debate</a> that scientists hope to resolve with JUICE and Europa Clipper. While the exact depths are still uncertain, scientists are confident that these oceans exist. </p>
<p>The best evidence of these oceans comes from Jupiter’s magnetic field. Saltwater is electrically conductive. So as these moons travel through Jupiter’s magnetic field, they <a href="https://doi.org/10.1126/science.289.5483.1340">generate a secondary, smaller magnetic field</a> that signals to researchers the presence of an underground ocean. Using this technique, planetary scientists have been able to show that the three <a href="https://doi.org/10.1038/27394">moons contain underground oceans</a>. And these oceans are not small – Europa’s ocean alone might have more than <a href="https://solarsystem.nasa.gov/moons/jupiter-moons/europa/overview/">double the water</a> of all of Earth’s oceans combined.</p>
<p>An obvious and tantalizing next question is whether these oceans can support extraterrestrial life. Liquid water is an important piece of what makes for a habitable world, but far from the only requirement for life. Life also needs <a href="https://astrobiology.nasa.gov/education/primer/">energy and certain chemical compounds</a> in addition to water to flourish. Because these oceans are hidden beneath miles of solid ice, <a href="https://doi.org/10.1126/science.1060081">sunlight and photosynthesis are out</a>. But it’s possible other sources could provide the needed ingredients.</p>
<p>On Europa, for example, the liquid water ocean <a href="https://solarsystem.nasa.gov/moons/jupiter-moons/europa/overview/">overlays a rocky interior</a>. That rocky seafloor could provide energy and chemicals through underwater volcanoes that could make Europa’s ocean habitable. But it is also possible that Europa’s ocean is a sterile, inhospitable place – scientists need more data to answer these questions. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/519967/original/file-20230407-16-sd1vga.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Artist's impression of the JUICE spacecraft approaching Jupiter and the jovian moons." src="https://images.theconversation.com/files/519967/original/file-20230407-16-sd1vga.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/519967/original/file-20230407-16-sd1vga.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=339&fit=crop&dpr=1 600w, https://images.theconversation.com/files/519967/original/file-20230407-16-sd1vga.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=339&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/519967/original/file-20230407-16-sd1vga.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=339&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/519967/original/file-20230407-16-sd1vga.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=426&fit=crop&dpr=1 754w, https://images.theconversation.com/files/519967/original/file-20230407-16-sd1vga.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=426&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/519967/original/file-20230407-16-sd1vga.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=426&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Jupiter Icy Moons Explorer spacecraft will travel for eight years before reaching Jupiter.</span>
<span class="attribution"><a class="source" href="https://sci.esa.int/web/juice/-/59334-exploring-jupiter">ESA/ATG medialab/NASA/JPL/University of Arizona/J. Nichols</a></span>
</figcaption>
</figure>
<h2>Upcoming missions from ESA and NASA</h2>
<p>JUICE and Europa Clipper are set up to give scientists game-changing information about the potential habitability of Jupiter’s moons. While both missions will gather data on multiple moons, JUICE will spend time orbiting and focusing on Ganymede, and Europa Clipper will make dozens of close flybys of Europa.</p>
<p>Both of the spacecraft will carry a suite of scientific instruments built specifically to investigate the oceans. Onboard radar will allow JUICE and Europa Clipper <a href="https://rslab.disi.unitn.it/rime/">to probe into the moons’</a> <a href="https://europa.nasa.gov/spacecraft/instruments/reason/">outer layers of solid ice</a>. Radar could reveal any small pockets of liquid water in the ice, or, in the case of Europa, which has a thinner outer ice layer than Ganymede and Callisto, <a href="https://doi.org/10.1016/j.icarus.2016.08.014">hopefully detect the larger ocean</a>. </p>
<p><a href="https://europa.nasa.gov/spacecraft/instruments/ecm/">Magnetometers will also be</a> <a href="https://www.esa.int/Science_Exploration/Space_Science/Juice_factsheet">on both missions</a>. These tools will give scientists the opportunity to study the secondary magnetic fields produced by the interaction of conductive oceans with Jupiter’s field in great detail and will hopefully give researchers clues to salinity and volumes of the oceans. </p>
<p>Scientists will also observe small variations in the moons’ gravitational pulls by tracking subtle movements in both spacecrafts’ orbits, which could help determine if Europa’s seafloor has volcanoes that <a href="https://doi.org/10.1016/j.icarus.2019.02.025">provide the needed energy and chemistry</a> for the ocean to support life.</p>
<p>Finally, both craft will carry a host of cameras and light sensors that will provide unprecedented images of the geology and composition of the moons’ icy surfaces. </p>
<p>Maybe one day, a spacecraft will be able to drill through the miles of solid ice on Europa, Ganymede or Callisto and explore oceans directly. Until then, observations from spacecraft like JUICE and Europa Clipper are scientists’ best bet for learning about these ocean worlds.</p>
<p>When Galileo discovered these moons in 1609, they were the first objects known to directly orbit another planet. Their discovery was the final nail in the coffin of the theory that Earth – and humanity – resides at the center of the universe. Maybe these worlds have another humbling surprise in store.</p><img src="https://counter.theconversation.com/content/203207/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Michael Sori receives funding from NASA. </span></em></p>The Jupiter Icy Moons Explorer and Europa Clipper missions will arrive at Jupiter in the 2030s and provide researchers with unprecedented access to the icy moons orbiting the gas giant.Mike Sori, Assistant Professor of Planetary Science, Purdue UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2023832023-03-23T17:33:31Z2023-03-23T17:33:31ZFour volcanic hotspots in the Solar System<figure><img src="https://images.theconversation.com/files/517213/original/file-20230323-14-dw9hgg.jpg?ixlib=rb-1.1.0&rect=0%2C2%2C1817%2C1811&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Io has volcanism.</span> <span class="attribution"><span class="source"> NASA/JPL/University of Arizona</span></span></figcaption></figure><p>Evidence for current volcanic activity on the planet Venus has recently <a href="https://theconversation.com/venus-proof-of-active-volcanoes-at-last-201931">made the news</a>. But Venus is far from the only world beyond Earth to exhibit signs of volcanic activity. </p>
<p>In fact, having volcanic activity (volcanism) on planets is quite common. Here are four worlds which have boasted active volcanism, from Mars to the far flung reaches of the outer Solar System.</p>
<h2>1. Mars</h2>
<p>Active volcanoes on Mars have not been directly observed, however, signs of volcanism are abundant on the Martian surface. Chief among them is the aptly named Olympus Mons, the tallest known volcano in the Solar System. </p>
<p>Olympus Mons stands at approximately 26km above the surrounding terrain, is roughly twice as high as Mount Everest, and is topped by a volcanic caldera some 70km across. The base of Olympus Mons is the size of Poland. The volcano is flanked by many solidified lava flows, the most recent of which are only a <a href="http://www.psrd.hawaii.edu/Jan05/MarsRecently.html">couple of million years old</a>, making them geologically recent. </p>
<p>Like many of Earth’s volcanoes, Olympus Mons is a basalt “shield volcano”, so termed because it has gentle slopes and a profile which resembles a shield laying on its side. A similar volcano on Earth is Mauna Kea in Hawaii. Elsewhere on Mars, a recent study has argued that another volcanic region, Elysium Planitia, is <a href="https://www.nature.com/articles/s41550-022-01836-3">active today</a> and powered by a sub-surface mantle plume similar to those which power volcanic hotspots on Earth. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/516951/original/file-20230322-16-ugxpio.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/516951/original/file-20230322-16-ugxpio.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=555&fit=crop&dpr=1 600w, https://images.theconversation.com/files/516951/original/file-20230322-16-ugxpio.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=555&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/516951/original/file-20230322-16-ugxpio.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=555&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/516951/original/file-20230322-16-ugxpio.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=698&fit=crop&dpr=1 754w, https://images.theconversation.com/files/516951/original/file-20230322-16-ugxpio.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=698&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/516951/original/file-20230322-16-ugxpio.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=698&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Olympus Mons.</span>
<span class="attribution"><span class="source">Nasa</span></span>
</figcaption>
</figure>
<p>The study used seismic data collected by <a href="https://mars.nasa.gov/insight/">Nasa’s Insight</a> lander spacecraft, which has detected numerous “Mars quakes” emanating from this region.</p>
<h2>2. Ceres</h2>
<p>The largest asteroid in the main asteroid belt is <a href="https://theconversation.com/dawn-breaks-over-distant-ceres-and-perhaps-reveals-signs-of-habitability-38967">Ceres</a>. It is a small world of just under 1,000km in diameter which orbits the Sun between Mars and Jupiter once every 4.6 years. It was visited for the first time by a robotic spacecraft in 2015 – Nasa’s <a href="https://solarsystem.nasa.gov/missions/dawn/overview/">Dawn mission</a>. One of the most interesting discoveries of this mission was the volcano <a href="https://www.science.org/doi/10.1126/science.aaf4286">Ahuna Mons</a>.</p>
<p>Most of Ceres’ surface is covered by impact craters, some of which show salt deposits inside them. However, Ahuna Mons stood out as an approximately 4km high mountain towering over the surrounding terrain. It is the only feature of its type on Ceres, and its flanks are covered with deposits of carbonate salts. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/516952/original/file-20230322-1533-l9gw62.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/516952/original/file-20230322-1533-l9gw62.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=339&fit=crop&dpr=1 600w, https://images.theconversation.com/files/516952/original/file-20230322-1533-l9gw62.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=339&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/516952/original/file-20230322-1533-l9gw62.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=339&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/516952/original/file-20230322-1533-l9gw62.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=426&fit=crop&dpr=1 754w, https://images.theconversation.com/files/516952/original/file-20230322-1533-l9gw62.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=426&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/516952/original/file-20230322-1533-l9gw62.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=426&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Ahuna Mons flanked by bright streaks of salt.</span>
<span class="attribution"><a class="source" href="https://photojournal.jpl.nasa.gov/jpegMod/PIA19631_modest.jpg">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA</a></span>
</figcaption>
</figure>
<p>The mechanism for the formation of this mountain is believed to be <a href="https://www.sciencedirect.com/science/article/pii/001910359190235L?via%3Dihub">cryovolcanism</a>, where liquid salty water rather than lava is ejected. On Ceres, these salt-water eruptions are produced due to the pressure from the asteroid’s icy surface. As the erupted water evaporated away under the vacuum of space, it left behind salt deposits. Ahuna Mons is the nearest known cryovolcano to the Sun. </p>
<h2>3. Io</h2>
<p>Io is the innermost large moon of Jupiter and orbits the host planet once every 43 hours. It was discovered by Galileo in 1610 and is about the same size as Earth’s Moon. But unlike our quiet celestial neighbour, Io is the most volcanically active body in the Solar System. Io has been surveyed several times by <a href="https://www.science.org/doi/10.1126/science.288.5469.1201">robotic spacecraft</a> and by telescopic observations from Earth.</p>
<p>Its surface is a distinctive yellow-red colour owing to vast deposits of sulphur, much of which has been erupted from the more than 400 active volcanoes on its surface. The source for much of the energy for all these volcanoes comes from tidal forces due to the proximity of the second nearest large Jovian moon, Europa, and the largest Jovian moon, Ganymede.</p>
<p>Io completes two orbits of Jupiter for every one orbit by the moon Europa, and four orbits of Jupiter for every one orbit of Ganymede. Hence, Io frequently lines up with one of these other moons and is stretched by the powerful gravity of Jupiter in one direction and the aligned gravitational fields of the outer moons in the other.</p>
<p>This continuous flexing releases large amounts of heat, which helps to keep the interior of Io molten and drive volcanism on its surface. Some of the most spectacular volcanoes of Io erupt plumes of material hundreds of miles above Io’s surface, as seen in the animation below.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/516954/original/file-20230322-28-8ah47s.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/516954/original/file-20230322-28-8ah47s.gif?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/516954/original/file-20230322-28-8ah47s.gif?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/516954/original/file-20230322-28-8ah47s.gif?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/516954/original/file-20230322-28-8ah47s.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/516954/original/file-20230322-28-8ah47s.gif?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/516954/original/file-20230322-28-8ah47s.gif?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Eruption plume from the Tvashtar volcano, Io.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<p>Io is quite the tortured world, with a surface pockmarked by violent volcanoes and showered with highly ionising radiation radiation from Jupiter’s radiation belts – even generating aurora in the moon’s thin atmosphere.</p>
<h2>4. Pluto</h2>
<p>Pluto lies in the <a href="https://solarsystem.nasa.gov/solar-system/kuiper-belt/overview/">Kuiper Belt</a>, an extended region of icy dwarf planets in the distant outer Solar System, of which Pluto is the largest known member. It orbits the Sun once every 247 years and the heat from the Sun at this distance is so feeble that Pluto’s average surface temperature is a frosty -230°C. </p>
<p>However, despite these frigid conditions, Pluto is an active world with many features of interest, including a thin atmosphere and glaciers made from frozen nitrogen. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/516955/original/file-20230322-1695-mzvn30.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/516955/original/file-20230322-1695-mzvn30.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=801&fit=crop&dpr=1 600w, https://images.theconversation.com/files/516955/original/file-20230322-1695-mzvn30.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=801&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/516955/original/file-20230322-1695-mzvn30.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=801&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/516955/original/file-20230322-1695-mzvn30.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1007&fit=crop&dpr=1 754w, https://images.theconversation.com/files/516955/original/file-20230322-1695-mzvn30.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1007&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/516955/original/file-20230322-1695-mzvn30.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1007&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Wright Mons, showing the distinctive summit depression.</span>
<span class="attribution"><span class="source">NASA/JHUAPL/SwRI</span></span>
</figcaption>
</figure>
<p>The first and, so far, only robotic visitor from Earth was Nasa’s New Horizons mission which famously <a href="https://theconversation.com/stunning-crystal-clear-images-of-pluto-but-what-do-they-mean-47517">flew past</a> the ice dwarf, and its moon Charon, in 2015. One remarkable feature discovered was <a href="https://www.science.org/content/article/ice-volcanoes-spotted-pluto-suggest-internal-heat-source">Wright Mons</a>.</p>
<p>This large mountain is about 150km across and over 4km high, with a distinctive large depression at the summit. The absence of recent meteorite impact craters on this feature strongly suggest that it is young, geologically speaking. </p>
<p>Wright Mons also resembles a shield volcano. It is believed to be a cryovolcano similar to those found on Saturn’s moon <a href="https://www.wired.com/2008/12/the-cryovolcano/">Titan</a> and Ceres, but it is the largest such feature known in the Solar System. The true nature of this intriguing mountain is still not fully agreed upon.</p><img src="https://counter.theconversation.com/content/202383/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gareth Dorrian does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Jupiter’s moon Io has more than 400 active volcanoes on its surface.Gareth Dorrian, Post Doctoral Research Fellow in Space Science, University of BirminghamLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2004542023-03-15T13:37:40Z2023-03-15T13:37:40ZCurious Kids: How are planets created?<figure><img src="https://images.theconversation.com/files/511620/original/file-20230222-16-c8nmnb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Eight planets, including Earth, revolve around our Sun.</span> <span class="attribution"><span class="source">Illustration by Tobias Roetsch/Future Publishing via Getty Images</span></span></figcaption></figure><p><em>Curious Kids is a <a href="https://theconversation.com/africa/topics/curious-kids-36782">series</a> for children in which we ask experts to answer questions from kids.</em></p>
<p><strong>How are planets created? - (Saba, 6, Kenya)</strong></p>
<p>Thanks for asking such an interesting question, Saba. When you talk about planets you’re probably thinking of the planets in our solar system – the ones orbiting (circling around) our sun. There are eight of these <a href="https://solarsystem.nasa.gov/planets/overview/#otp_planets_of_our_solar_system">planets</a>. One of them is where you and I live: Earth. The others are Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune.</p>
<p>There are many, many more planets way beyond our solar system and our galaxy, the Milky Way. Scientists like us, known as astronomers, have found <a href="https://www.planetary.org/worlds/exoplanets">over 5,000 planets</a> around other stars. We estimate that there may be trillions across the Universe.</p>
<p>How did they come into being? It all starts with a cloud of gas and dust.</p>
<h2>Gas and dust</h2>
<p>These clouds of gas and dust are called nebulae. They float around in space much like the clouds in our sky. There are some regions with more clouds and some with fewer and astronomers can see these using telescopes.</p>
<p>Nebulae contain gases like hydrogen, helium and carbon. When a nebula becomes dense enough its gravity pulls it together into a very dense core. This is a bit like the water in your bath swirling around the drain before getting sucked down. As the cloud gets dense it heats up. When it gets dense and hot enough the atoms – tiny building blocks for all the matter in the world – in the nebula start to fuse.</p>
<p>This process is called nuclear fusion and produces a lot of energy. And the cloud lights up like a firework. This is how a new star is born, just like our Sun was <a href="https://spaceplace.nasa.gov/sun-age/en/">4.5 billion years ago</a>.</p>
<p>A small amount of gas and dust remains around new stars in a spinning disc. Planets are formed from this disc of material.</p>
<h2>Protoplanets</h2>
<p>As the disc rotates, the material in it, small bits of rock and ice, lump together and get bigger and bigger. That forms what we call planetesimals, which collide with each other like bumper cars, creating even larger bodies known as protoplanets.</p>
<p>The protoplanets keep growing. While this is happening, they can attract gases from the surrounding disc, creating a thick atmosphere. This process is called accretion and it is how gas giant planets like Jupiter and Saturn are formed. If a protoplanet forms from heavier elements in the outer solar system it can create an ice giant. The planets Neptune and Uranus are ice giants.</p>
<p>Even after the planet is formed it can keep changing over time through processes such as volcanic activity, tectonic movement, and erosion. On Earth, mountains like Mount Kilimanjaro in Tanzania – the country next door to Kenya – formed from large volcanoes. And even larger mountains like the Himalayas have formed from tectonic plates colliding. Tectonic plates are big pieces of the Earth’s outer layer; sometimes they crash into each other and that creates things like mountains.</p>
<h2>Millions of years</h2>
<p>The way I’ve described this makes it sound as though planets are formed quickly. But the process which begins with those clouds of gas and dust takes millions of years to transform into the beautiful and diverse worlds we see in our Solar System and beyond.</p><img src="https://counter.theconversation.com/content/200454/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Daniel Cunnama receives funding from the National Research Foundation and the South African Astronomical Observatory. </span></em></p>It all starts with a cloud of gas and dust.Daniel Cunnama, Science Engagement Astronomer, South African Astronomical Observatory, South African Astronomical ObservatoryLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1862282022-07-04T13:13:01Z2022-07-04T13:13:01ZNasa considers sending swimming robots to habitable ‘ocean worlds’ of the Solar System<figure><img src="https://images.theconversation.com/files/472142/original/file-20220702-5543-5lvwti.jpg?ixlib=rb-1.1.0&rect=541%2C0%2C1355%2C576&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Realistic colour view of Jupiter's moon Europa.</span> <span class="attribution"><a class="source" href="https://photojournal.jpl.nasa.gov/jpeg/PIA19048.jpg">NASA/JPL-Caltech/SETI Institute</a></span></figcaption></figure><p>Nasa has recently <a href="https://www.jpl.nasa.gov/news/swarm-of-tiny-swimming-robots-could-look-for-life-on-distant-worlds">announced US$600,000 (£495,000) in funding for a study</a> into the feasibility of sending swarms of miniature swimming robots (known as independent micro-swimmers) to explore oceans beneath the icy shells of our Solar System’s many “ocean worlds”. But don’t imagine metal humanoids swimming frog-like underwater. They will probably be simple, triangular wedges.</p>
<p><a href="https://theconversation.com/life-inside-pluto-hot-birth-may-have-created-internal-ocean-on-dwarf-planet-140976">Pluto</a> is one example of a likely ocean world. But the worlds with oceans nearest to the surface, making them the most accessible, are <a href="https://theconversation.com/new-water-plumes-from-jupiters-moon-europa-raise-hopes-of-detecting-microbial-life-66019">Europa</a>, a moon of Jupiter, and <a href="https://theconversation.com/the-chemistry-that-could-feed-life-within-saturns-moon-enceladus-study-gives-clue-ahead-of-flyby-49683">Enceladus</a>, a moon of Saturn.</p>
<figure class="align-center ">
<img alt="Impression showing the cross-section of Europa." src="https://images.theconversation.com/files/139258/original/image-20160926-31840-4e1ocg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/139258/original/image-20160926-31840-4e1ocg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=405&fit=crop&dpr=1 600w, https://images.theconversation.com/files/139258/original/image-20160926-31840-4e1ocg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=405&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/139258/original/image-20160926-31840-4e1ocg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=405&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/139258/original/image-20160926-31840-4e1ocg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=508&fit=crop&dpr=1 754w, https://images.theconversation.com/files/139258/original/image-20160926-31840-4e1ocg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=508&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/139258/original/image-20160926-31840-4e1ocg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=508&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Cross-section through the outer zone of Europa’s south polar region showing plumes, the fractured ice shell, the liquid water ocean (cloudy at the base near hydrothermal plumes) and the rocky interior.</span>
<span class="attribution"><a class="source" href="http://www.nasa.gov/topics/solarsystem/features/pia16826.html">Nasa/JPL</a></span>
</figcaption>
</figure>
<h2>Life inside ocean worlds</h2>
<p>These oceans are of interest to scientists not just because they contain so much liquid water (Europa’s ocean probably has about <a href="https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/">twice as much water</a> as the whole of Earth’s oceans), but because chemical interactions between rock and the ocean water could support life. In fact, the environment in these oceans may be very similar to that on Earth <a href="https://theconversation.com/origins-of-life-new-evidence-first-cells-could-have-formed-at-the-bottom-of-the-ocean-126228">at the time life began</a>.</p>
<p>These are environments where water that has seeped into the rock of the ocean floor becomes hot and chemically enriched – water that is then expelled back into the ocean. Microbes can feed off this chemical energy, and can in turn be eaten by larger organisms. No sunlight or atmosphere is actually needed. Many warm, rocky structures of this sort, known as “hydrothermal vents”, have been documented on Earth’s ocean floors since they <a href="https://www.nhm.ac.uk/discover/survival-at-hydrothermal-vents.html">were discovered in 1977</a>. In these locations, the local food web is indeed supported by chemosynthesis (energy from chemical reactions) rather than photosynthesis (energy from sunlight).</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/472087/original/file-20220701-11-vo5zyy.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Image of a vent on the Earth’s ocean floor." src="https://images.theconversation.com/files/472087/original/file-20220701-11-vo5zyy.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/472087/original/file-20220701-11-vo5zyy.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/472087/original/file-20220701-11-vo5zyy.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/472087/original/file-20220701-11-vo5zyy.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/472087/original/file-20220701-11-vo5zyy.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/472087/original/file-20220701-11-vo5zyy.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/472087/original/file-20220701-11-vo5zyy.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A vent on the floor of the northeast Pacific. A bed of tube worms feeding on chemosynthetic microbes covers the base.</span>
<span class="attribution"><span class="source">NOAA/PMEL</span></span>
</figcaption>
</figure>
<p>In most of our Solar System’s ocean worlds, the energy that heats their rocky interiors and prevents the oceans from freezing all the way to the base comes principally from tides. This is in contrast to the largely radioactive heating of the Earth’s interior. But the chemistry of the water-rock interactions is similar.</p>
<p>Enceladus’s ocean has already been sampled <a href="https://theconversation.com/nasa-saturn-moon-enceladus-is-able-to-host-life-its-time-for-a-new-mission-76102">by flying the Cassini spacecraft through plumes</a> of ice crystals that erupt through cracks in the ice. And there are hopes that Nasa’s <a href="https://europa.nasa.gov/">Europa Clipper mission</a> may find similar plumes to sample when it begins a series of close Europa flybys in 2030. However, getting inside the ocean to go exploring would potentially be much more informative than merely sniffing at a freeze-dried sample.</p>
<figure class="align-center ">
<img alt="Artist’s impression of swimming robotic devices." src="https://images.theconversation.com/files/472141/original/file-20220702-24-5c4qf4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/472141/original/file-20220702-24-5c4qf4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=911&fit=crop&dpr=1 600w, https://images.theconversation.com/files/472141/original/file-20220702-24-5c4qf4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=911&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/472141/original/file-20220702-24-5c4qf4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=911&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/472141/original/file-20220702-24-5c4qf4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1145&fit=crop&dpr=1 754w, https://images.theconversation.com/files/472141/original/file-20220702-24-5c4qf4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1145&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/472141/original/file-20220702-24-5c4qf4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1145&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A lander of Europa uses a probe to melt a hole through the ice, which then releases a swarm of swimming robots. Conceptual impression, not to scale.</span>
<span class="attribution"><span class="source">Nasa/JPL-Caltech</span></span>
</figcaption>
</figure>
<h2>In the Swim</h2>
<p>This is where the <a href="https://www.jpl.nasa.gov/news/swarm-of-tiny-swimming-robots-could-look-for-life-on-distant-worlds">sensing with independent micro-swimmers (Swim)</a> concept comes. The idea is to land on Europa or Enceladus (which would be neither cheap nor easy) at a place where the ice is relatively thin (not yet located) and use a radioactively heated probe to melt a 25cm-wide hole through to the ocean – located hundreds or thousands of metres below.</p>
<p>Once there, it would release up to about four dozen 12cm long, wedge-shaped micro-swimmers to go exploring. Their endurance would be much less than that of the 3.6m long autonomous underwater vehicle famously named <a href="https://noc.ac.uk/education/educational-resources/boaty-mcboatface">Boaty McBoatface</a>, with a range of 2,000km that has already achieved a cruise of more than 100km below the Antarctic ice.</p>
<figure class="align-center ">
<img alt="Artist’s impression of swimming robotic devices, deployed from a probe that has penetrated the ice crust of a moon." src="https://images.theconversation.com/files/472081/original/file-20220701-20-fpy5b5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/472081/original/file-20220701-20-fpy5b5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=345&fit=crop&dpr=1 600w, https://images.theconversation.com/files/472081/original/file-20220701-20-fpy5b5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=345&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/472081/original/file-20220701-20-fpy5b5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=345&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/472081/original/file-20220701-20-fpy5b5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=434&fit=crop&dpr=1 754w, https://images.theconversation.com/files/472081/original/file-20220701-20-fpy5b5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=434&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/472081/original/file-20220701-20-fpy5b5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=434&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Independent micro-swimmers, deployed from a probe that has penetrated the ice crust of a moon. Not to scale.</span>
<span class="attribution"><span class="source">Nasa/JPL</span></span>
</figcaption>
</figure>
<p>At this stage, Swim is merely one of five “phase 2 studies” into a range of “advanced concepts” funded in the 2022 round of Nasa’s <a href="https://www.nasa.gov/directorates/spacetech/niac/index.html">Innovative Advanced Concepts (NIAC) programme</a>. So there are still long odds against Swim becoming a reality, and no complete mission has been scoped out or funded.</p>
<p>The micro-swimmers would communicate with the probe acoustically (through sound waves), and the probe would send its data via cable to the lander on the surface. The study will trial prototypes in a test tank with all subsystems integrated. </p>
<p>Each micro-swimmer could explore maybe only tens of metres away from the probe, limited by their battery power and the range of their acoustic data link, but by acting as a flock they could map changes (in time or location) in temperature and salinity. They may even be able to measure changes in the <a href="https://www.geo-ocean.fr/en/Science-pour-tous/Nos-salles-d-etudes/Systemes-hydrothermaux/Hydrothermalism/Plumes">cloudiness of the water</a>, which could indicate the direction towards the nearest hydrothermal vent.</p>
<p>Power limitations of the micro-swimmers may mean that none could carry cameras (these would need their own light source) or sensors that could specifically sniff out organic molecules, though. But at this stage, nothing is ruled out.</p>
<p>I think finding signs of hydrothermal vents is a long shot, however. The ocean floor would, after all, be many kilometres below the micro-swimmer’s release point. But, to be fair, pinpointing vents is not explicitly suggested in the Swim proposal. To locate and examine the vents themselves, we probably do need Boaty McBoatface in space. That said, Swim would be a good start.</p><img src="https://counter.theconversation.com/content/186228/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Rothery is Professor of Planetary Geosciences at the Open University. He is co-leader of the European Space Agency's Mercury Surface and Composition Working Group, and a Co-Investigator on MIXS (Mercury Imaging X-ray Spectrometer) that is now on its way to Mercury on board the European Space Agency's Mercury orbiter BepiColombo. He has received funding from the UK Space Agency and the Science & Technology Facilities Council for work related to Mercury and BepiColombo, and from the European Commission under its Horizon 2020 programme for work on planetary geological mapping (776276 Planmap). He is author of Planet Mercury - from Pale Pink Dot to Dynamic World (Springer, 2015), Moons: A Very Short Introduction (Oxford University Press, 2015) and Planets: A Very Short Introduction (Oxford University Press, 2010). He is Educator on the Open University's free learning Badged Open Course (BOC) on Moons and its equivalent FutureLearn Moons MOOC, and chair of the Open University's level 2 course on Planetary Science and the Search for Life.</span></em></p>There may be life on Jupiter’s moon Europa or Saturn moon’s Enceladus.David Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1805732022-04-28T02:24:34Z2022-04-28T02:24:34ZCurious Kids: will the big storm on Jupiter ever go away?<figure><img src="https://images.theconversation.com/files/457543/original/file-20220411-36999-z8jc37.jpg?ixlib=rb-1.1.0&rect=0%2C15%2C5330%2C2226&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The red and stormy planet. </span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/planet-jupiter-some-69-known-moons-1104597008">Shutterstock</a></span></figcaption></figure><blockquote>
<p><strong>Will the big storm on Jupiter ever go away? — Edgar Nuttall, age 5, Brisbane</strong></p>
</blockquote>
<p><a href="https://theconversation.com/au/topics/curious-kids-36782"><img src="https://images.theconversation.com/files/291898/original/file-20190911-190031-enlxbk.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=90&fit=crop&dpr=1" width="100%"></a></p>
<p>Hi Edgar! Thank you for such a unique question.</p>
<p>Jupiter is the largest planet in our solar neighbourhood, and its weather is very wild. We have beautiful images of Jupiter which show striped, stormy clouds covering the whole planet. </p>
<p>In fact, Jupiter is covered with storms. Some are only small, but some are so big they could cover all of Earth. </p>
<p>The largest of these storms is the famous Great Red Spot — which I see you already know about. This spot is actually a cyclone, similar to hurricanes and cyclones here on Earth. </p>
<p>It is made of powerful winds blowing in circles, a bit like tea swirling in a cup when you stir it. These winds are more than five times faster than any hurricane winds on Earth. </p>
<p>The Great Red Spot is like the grandfather of Jupiter’s storms. It has been roaming for many, many years – but recently we’ve seen it get smaller. </p>
<p>Does that mean it will one day go away? Well, not necessarily. </p>
<h2>Stormy stripes</h2>
<p>Jupiter looks like a giant, stripy ball that spins very fast. The light-coloured stripes are clouds with rising air, while the dark-coloured stripes are clouds that are sinking. </p>
<p>When you see dark and light stripes next to each other on Jupiter, you’re actually seeing winds blowing in opposite directions. When this happens, they can spin up big cyclones, kind of like how pushing a beach ball with one hand and pulling it with other will make it spin.</p>
<p>Humans have been watching the Great Red Spot for at least 200 years and it has been blowing strong winds almost this whole time. </p>
<p>Like all storms, it can change from day to day. Sometimes it looks round, sometimes like an egg. Its colour can also change from brownish-red to pale red. Sometimes it looks almost white. </p>
<p>But recently, scientists have noticed the enormous cyclone shrinking. About 100 years ago, the Great Red Spot was almost three times larger than it is today. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/curious-kids-why-are-some-planets-surrounded-by-rings-130318">Curious Kids: why are some planets surrounded by rings?</a>
</strong>
</em>
</p>
<hr>
<h2>Why is it shrinking?</h2>
<p>To understand why it’s shrinking, it helps to first understand why cyclones shrink (and eventually stop) on Earth. </p>
<p>On Earth, cyclones often form above deep, warm oceans before moving onto the hard land or cooler water. When a cyclone’s winds rub against the hard land, the winds slow down (and therefore the cyclone slows down).</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/457840/original/file-20220413-22-qzpstx.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Cartoon depiction of cyclone over the land" src="https://images.theconversation.com/files/457840/original/file-20220413-22-qzpstx.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/457840/original/file-20220413-22-qzpstx.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=440&fit=crop&dpr=1 600w, https://images.theconversation.com/files/457840/original/file-20220413-22-qzpstx.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=440&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/457840/original/file-20220413-22-qzpstx.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=440&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/457840/original/file-20220413-22-qzpstx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=553&fit=crop&dpr=1 754w, https://images.theconversation.com/files/457840/original/file-20220413-22-qzpstx.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=553&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/457840/original/file-20220413-22-qzpstx.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=553&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">On Earth, cyclones usually begin over large warm oceans, but slow down as they move into cooler areas or break up against the land.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
</figcaption>
</figure>
<p>Cyclones on Earth are also hit by other weather and winds around them, which can makes the cyclone “flake” away within a few days. </p>
<p>But Jupiter doesn’t have a hard, rocky surface like Earth. And even though the air in Jupiter’s clouds is freezing, the air towards the inside is very hot. This hot air gives storms plenty of energy to rage on for months, or even years. </p>
<p>So even while the Great Red Storm is shrinking, it can actually still get a bit taller as it does. And it has plenty of energy to keep spinning. </p>
<p>We can also see it “flaking” away at the edges as it slams into other storms and winds around it. But astronomers still don’t know if this will make it go away entirely. Some think it might one day break up into many smaller storms.</p>
<p>Recently, the Juno space probe (which has been flying around Jupiter <a href="https://www.space.com/topics/juno-mission">since 2016</a>) took many beautiful pictures of Jupiter’s storms while flying by the planet. We may learn something new from these images. </p>
<p>Until then, we may as well admire the Great Red Spot as it rages on.</p>
<p><em>The animation below shows the Juno space probe. You can spin it around to see it more clearly. On the top is the antenna. The big ‘wings’ are covered with solar panels which provide electricity for its different parts.</em></p>
<p><iframe src="https://solarsystem.nasa.gov/gltf_embed/2376" width="100%" height="450px" frameborder="0"></p></iframe></p><img src="https://counter.theconversation.com/content/180573/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Lucyna Kedziora-Chudczer does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The biggest storm on Jupiter is so big it could swallow all of Earth. But it’s now shrinking, and we’re not sure what that means.Lucyna Kedziora-Chudczer, Program Manager / Adjunct Research Fellow, Swinburne University of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1759182022-02-21T05:00:45Z2022-02-21T05:00:45ZJupiter, Saturn, Uranus, Neptune: why our next visit to the giant planets will be so important (and just as difficult)<figure><img src="https://images.theconversation.com/files/446075/original/file-20220213-17-1s3hlo0.jpg?ixlib=rb-1.1.0&rect=15%2C3%2C2580%2C1191&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.youtube.com/watch?v=sOpMrVnjYeY&t=2225s&ab_channel=SpaceX">SpaceX</a></span></figcaption></figure><p>The giant planets – Jupiter, Saturn, Uranus and Neptune - are some of the most awe-inspiring in our Solar System, and have great importance for space research and our comprehension of the greater universe.</p>
<p>Yet they remain the least explored – especially the “ice giants” Uranus and Neptune – due to their distance from Earth, and the extreme conditions spacecraft must survive to enter their atmospheres. As such, they’re also the least understood planets in the Solar System.</p>
<p>Our <a href="https://arc.aiaa.org/doi/10.2514/1.A34282">ongoing</a> <a href="https://doi.org/10.2514/1.J060560">research</a> looks at how to overcome the harsh entry conditions experienced during giant planet missions. As we look forward to potential future missions, here’s what we might expect.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=227&fit=crop&dpr=1 600w, https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=227&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=227&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=286&fit=crop&dpr=1 754w, https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=286&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=286&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Jupiter is about ten times as large as Earth – with a 69,911km radius (compared to Earth’s 6,371km radius).</span>
<span class="attribution"><span class="source">Beinahegut</span></span>
</figcaption>
</figure>
<h2>But first, what are giant planets?</h2>
<p>Unlike rocky planets, giant planets don’t have a surface to land on. Even in their lower atmospheres they remain gaseous, reaching extremely high pressures that would crush any spacecraft well before it could land on anything solid.</p>
<p>There are two types of giant planets: gas giants and ice giants. </p>
<p>The larger Jupiter and Saturn are gas giants. These are mainly made of hydrogen and helium, with an outer gaseous layer and a partially liquid “metallic” layer below that. They’re also believed to have a small rocky core. </p>
<p>Uranus and Neptune have similar outer atmospheres and rocky cores, but their inner layer is made up of about 65% water and other so-called “ices” (although these technically remain liquid) such as <a href="https://www.lpi.usra.edu/icegiants/mission_study/Exec-Summary.pdf">methane and ammonia</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Relative size and composition of the giant planets in our solar system (with Earth also shown for comparison).</span>
<span class="attribution"><span class="source">JPL/Caltech (based on material from the Lunar and Planetary Institute)</span></span>
</figcaption>
</figure>
<h2>Slingshots to the edge of the Solar System</h2>
<p>Any giant planet mission is extremely difficult. Still, there have been some past missions sent to the gas giants.</p>
<p>NASA’s 1989 Galileo mission had to slingshot around Venus and Earth to give it enough momentum to <a href="https://www.nasa.gov/feature/30-years-ago-galileo-off-to-orbit-jupiter">get to Jupiter</a>, which it orbited for eight years. The 2011 <a href="https://spaceflight101.com/juno/juno-mission-trajectory-design/">Juno mission</a> spent five years in transit, using a flyby around Earth to reach Jupiter (which it still orbits).</p>
<p>Similarly, the Cassini-Huygens mission run by NASA and the European Space Agency (ESA) <a href="https://sci.esa.int/web/cassini-huygens/-/31240-getting-to-saturn">took seven years</a> to reach Saturn. The spacecraft spent 13 years exploring the planet and its surrounds, and launched a probe to explore Saturn’s moon, <a href="https://solarsystem.nasa.gov/missions/cassini/science/titan/">Titan</a>.</p>
<p>Flight times get even longer for the two ice giants, which are much further from the Sun. Neither has had a dedicated mission so far. </p>
<h2>A complex journey</h2>
<p>The last and only spacecraft to visit the ice giants was <a href="https://solarsystem.nasa.gov/missions/voyager-2/in-depth/">Voyager 2</a>, which flew by Uranus in 1986 and Neptune in 1989. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Voyager 2, the only spacecraft ever to have visited Neptune, took a photo of the planet in 1989.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<p>While momentum is building for a return, it won’t be simple. If we launch during the next convenient <a href="https://www.lpi.usra.edu/icegiants/mission_study/Exec-Summary.pdf">launch windows</a> of 2030–34 for Uranus and 2029–30 for Neptune, flight times would vary from 11 to 15 years.</p>
<p>A major issue is power. The Juno spacecraft is the most distant object from the Sun to have <a href="https://www.jpl.nasa.gov/news/nasas-juno-spacecraft-breaks-solar-power-distance-record">used solar panels</a>. It orbits Jupiter, which is <a href="https://solarsystem.nasa.gov/planets/jupiter/in-depth/">five times further away</a> from the Sun than Earth is. Yet, where Juno’s solar cells would generate 14 kilowatts of continuous power on Earth, they only <a href="https://www.jpl.nasa.gov/news/nasas-juno-spacecraft-breaks-solar-power-distance-record">generate 0.5kW at Jupiter</a>. </p>
<p>Meanwhile, Uranus and Neptune are <a href="https://solarsystem.nasa.gov/planets/uranus/in-depth/">20</a> and <a href="https://solarsystem.nasa.gov/planets/neptune/in-depth/">30</a> times further away, respectively, from the Sun than Earth is. Power for these missions would have to be generated from the radioactive <a href="https://solarsystem.nasa.gov/missions/galileo/in-depth/#otp_spacecraft_and_instruments">decay of plutonium</a> (the power source for both the Galileo and Cassini missions). </p>
<p>This radioactive decay can damage and interfere with instruments. It is therefore reserved for spacecraft which really need it, such as missions operating far away from the Sun. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/so-a-helicopter-flew-on-mars-for-the-first-time-a-space-physicist-explains-why-thats-such-a-big-deal-159334">So a helicopter flew on Mars for the first time. A space physicist explains why that's such a big deal</a>
</strong>
</em>
</p>
<hr>
<h2>Fighting the heat</h2>
<p>The massive scale of giant planets means orbit speeds for incoming spacecraft are incredibly fast. And these speeds greatly heat up the spacecraft. </p>
<p>The Galileo probe entered Jupiter’s atmosphere at <a href="https://solarsystem.nasa.gov/missions/galileo-probe/in-depth/">47.5 kilometres per second</a>, surviving the harshest entry conditions ever experienced by an entry probe. The shock layer which formed at the front of the spacecraft during entry reached a temperature of 16,000°C – around three times the temperature of the Sun’s surface.</p>
<p>Even so, the distribution of the <a href="https://arc.aiaa.org/doi/10.2514/2.3293">heat shield’s</a> mass was found to be inefficient – showing we still have a lot to learn about entering giant planets.</p>
<p>Proposed future probe missions to Uranus and Neptune would occur at slower entry speeds of <a href="https://link.springer.com/article/10.1007/s11214-020-0638-2">22km/s and 26km/s</a>, respectively. </p>
<p>For this, NASA have developed a tough but relatively lightweight material woven from carbon fibre, called <a href="https://www.nasa.gov/ames/heeet">HEEET</a> (Heatshield for Extreme Entry Environment Technology), designed specifically for surviving giant planet and Venusian entry. </p>
<p>While the material has been tested with a <a href="https://www.nasa.gov/centers/ames/entry-systems-vehicle-development/tps-materials.html">full-scale prototype</a>, it has yet to fly on a mission.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">It’s planned NASA’s HEEET material will be used for future ice giant entry missions.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<h2>The next steps</h2>
<p>In 2024, NASA’s Europa Clipper mission <a href="https://europa.nasa.gov/">will launch</a> to investigate Jupiter’s moon Europa, which is believed to house an <a href="https://europa.nasa.gov/why-europa/overview/">ocean of liquid water</a> below its icy surface, where signs of life may be found. The <a href="https://www.nasa.gov/press-release/nasas-dragonfly-will-fly-around-titan-looking-for-origins-signs-of-life/">Dragonfly</a> mission, planned to launch in 2026, will similarly aim to search for signs of life on Saturn’s moon Titan.</p>
<p>There are plans for a joint <a href="https://www.sciencedirect.com/science/article/pii/S0032063318303507">NASA-ESA mission</a> to visit one of the ice giants within the upcoming launch window. But while there has been <a href="https://www.lpi.usra.edu/icegiants/documents_presentations/">extensive</a> <a href="https://sci.esa.int/web/future-missions-department/-/61307-cdf-study-report-ice-giants">preparation</a>, it’s undecided which ice giant will be visited. </p>
<p>A single mission to both planets is being considered. An entry probe is planned, too. But if the mission visits both planets, it’s undecided which planet’s <a href="https://www.sciencedirect.com/science/article/pii/S003206331830350">atmosphere the probe would explore</a>.</p>
<p>If we want to meet the upcoming launch window, it’s expected mission concepts will need to be finalised <a href="https://www.sciencedirect.com/science/article/pii/S0032063320300040">by 2025</a>, at the latest. In other words, crunch time is coming. </p>
<p>Should a mission go forward, the two most important <a href="https://www.lpi.usra.edu/icegiants/mission_study/Full-Report.pdf">goals</a> for NASA’s scientists will be to determine the interior makeup of ice giants (exactly what they are made of) and their composition (how they are formed).</p>
<p>Other objectives will include studying their magnetic fields, which are <a href="https://www.lpi.usra.edu/icegiants/mission_study/Full-Report.pdf">very different</a> to gas giants and all other types of planets. </p>
<p>They’ll also want to study the heat released by both Uranus and Neptune, which both have average temperatures of around -200°C. All giant planets are meant to be very slowly cooling down, as they release energy gained during their formation. </p>
<p>This heat release can be detected for Jupiter, Saturn and Neptune. Uranus, however, doesn’t seem to release heat – and scientists don’t know why.</p><img src="https://counter.theconversation.com/content/175918/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chris James receives funding from the University of Queensland, the Australian Research Council, and the U.S. Office of Naval Research. </span></em></p><p class="fine-print"><em><span>Yu Liu does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>There has never been a dedicated mission sent to the “ice giants”, Uranus and Neptune. But there may be one on the horizon.Chris James, ARC DECRA Fellow, Centre for Hypersonics, School of Mechanical and Mining Engineering, The University of QueenslandYu Liu, Honorary Fellow, The University of QueenslandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1698772021-12-23T11:47:30Z2021-12-23T11:47:30ZFive of the most exciting telescope pictures of the universe<figure><img src="https://images.theconversation.com/files/438943/original/file-20211223-36920-y1fny7.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C3000%2C1410&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/multimedia/imagegallery/image_feature_1578.html">Hubble: NASA, ESA, and Q.D. Wang (University of Massachusetts, Amherst); Spitzer: NASA, Jet Propulsion Laboratory, and S. Stolovy (Spitzer Science Center/Caltech)</a></span></figcaption></figure><p>The forthcoming launch of the <a href="https://jwst.nasa.gov/content/webbLaunch/index.html">James Webb Space Telescope</a> offers unprecedented new opportunities for astronomers. It’s also a timely opportunity to reflect on what previous generations of telescopes have shown us.</p>
<p>Astronomers rarely use their telescopes to simply take pictures. The pictures in astrophysics are usually generated by a process of scientific inference and imagination, sometimes visualised in artist’s impressions of what the data suggests.</p>
<p>Choosing just a handful of images was not easy. I limited my selection to images produced by publicly-funded telescopes and which reveal some interesting science. I tried to avoid very popular images which have already been viewed widely.</p>
<p>The selection below is a personal one and I’m sure many readers could advocate for different choices. Feel free to share them in the comments.</p>
<h2>1. Jupiter’s poles</h2>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/438322/original/file-20211219-15-hopw43.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438322/original/file-20211219-15-hopw43.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=509&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438322/original/file-20211219-15-hopw43.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=509&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438322/original/file-20211219-15-hopw43.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=509&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438322/original/file-20211219-15-hopw43.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=640&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438322/original/file-20211219-15-hopw43.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=640&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438322/original/file-20211219-15-hopw43.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=640&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">This image is sometimes called ‘Jupiter Blues’.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/jpl/pia21972/jupiter-blues">Enhanced Image by Gerald Eichstädt and Sean Doran (CC BY-NC-SA) based on images provided Courtesy of NASA/JPL-Caltech/SwRI/MSSS</a></span>
</figcaption>
</figure>
<p>The first image I’ve chosen was produced by Nasa’s <a href="https://www.nasa.gov/mission_pages/juno/main/index.html">Juno mission</a>, which is currently orbiting Jupiter. The image was taken <a href="https://www.nasa.gov/image-feature/jpl/pia21972/jupiter-blues">in October 2017</a> when the spacecraft was 18,906 kilometres away from the tops of Jupiter’s clouds. It captures a cloud system in the planet’s northern hemisphere, and represents our first view of Jupiter’s poles (the north pole).</p>
<p>The images this picture is based on reveal complex flow patterns, akin to cyclones in Earth’s atmosphere, and striking effects caused by the variety of clouds at different altitudes, sometimes casting shadows on layers of clouds below.</p>
<p>I chose this image for its beauty as well as the surprise it produced: the parts of the planet near its north pole look very different to the parts we had previously seen closer to the equator. By looking down on the poles of Jupiter, Juno showed us a different view of a familiar planet.</p>
<h2>2. The Eagle Nebula</h2>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/438325/original/file-20211219-18663-az74ch.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438325/original/file-20211219-18663-az74ch.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=692&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438325/original/file-20211219-18663-az74ch.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=692&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438325/original/file-20211219-18663-az74ch.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=692&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438325/original/file-20211219-18663-az74ch.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=870&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438325/original/file-20211219-18663-az74ch.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=870&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438325/original/file-20211219-18663-az74ch.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=870&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">This image allows us to see into the dense, dusty regions of space where star formation takes place.</span>
<span class="attribution"><a class="source" href="https://www.esa.int/ESA_Multimedia/Images/2016/04/Herschel_s_view_of_the_Eagle_Nebula">G. Li Causi, IAPS/INAF, Italy</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Astronomers can obtain unique information by building telescopes which are sensitive to light of “colours” beyond those our eyes can see. The familiar rainbow of colours is only a tiny fraction of what physicists call the electromagnetic spectrum. </p>
<p>Beyond red is the infrared, which carries less energy than optical light. An infrared camera can see objects too cool to be detectable by the human eye. In space, it can also see through dust, which otherwise completely obscures our view.</p>
<p>The James Webb Space Telescope will be the largest infrared observatory ever launched. Until now, the European Space Agency’s <a href="https://www.esa.int/Science_Exploration/Space_Science/Herschel_overview">Herschel Space Observatory</a> has been the largest. The next image I’ve chosen is Herschel view of star formation in the Eagle Nebula, also known as M16.</p>
<p>A nebula is a cloud of gas in space. The Eagle Nebula is 6,500 light years away from Earth, which is quite close by astronomical standards. This nebula is a site of vigorous star formation.</p>
<p>A close-up view of a feature near the centre of this image has been called the “<a href="https://hubblesite.org/contents/media/images/3862-Image?keyword=eagle">Pillars of Creation</a>”. Appearing a bit like a thumb and forefinger pointing upwards and slightly to the left, these pillars protrude into a cavity in a giant cloud of molecular gas and dust. The cavity is being swept out by winds emanating from energetic new stars which have recently formed deeper within the cloud. </p>
<h2>3. The Galactic Centre</h2>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/438943/original/file-20211223-36920-y1fny7.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C3000%2C1410&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438943/original/file-20211223-36920-y1fny7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=283&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438943/original/file-20211223-36920-y1fny7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=283&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438943/original/file-20211223-36920-y1fny7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=283&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438943/original/file-20211223-36920-y1fny7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=356&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438943/original/file-20211223-36920-y1fny7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=356&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438943/original/file-20211223-36920-y1fny7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=356&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Can you spot the Quintuplet cluster and the Arches?</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/multimedia/imagegallery/image_feature_1578.html">Hubble: NASA, ESA, and Q.D. Wang (University of Massachusetts, Amherst); Spitzer: NASA, Jet Propulsion Laboratory, and S. Stolovy (Spitzer Science Center/Caltech)</a></span>
</figcaption>
</figure>
<p>This image looks <a href="https://www.eso.org/sci/publications/messenger/archive/no.173-sep18/messenger-no173.pdf">deeper into space</a> to the centre of our Milky Way Galaxy. It also uses infrared light, this time combining data from two Nasa telescopes, <a href="https://www.nasa.gov/mission_pages/hubble/main/index.html">Hubble</a> and <a href="https://www.nasa.gov/mission_pages/spitzer/main/index.html">Spitzer</a>.</p>
<p>The bright white region in the lower right of the image is the very centre of our Galaxy. It contains a massive black hole called <a href="https://www.britannica.com/topic/Sagittarius-A-black-hole">Sagittarius A*</a>, a cluster of stars and the remains of a massive star which exploded as a supernova about 10,000 years ago. </p>
<p>Other <a href="https://hubblesite.org/contents/media/images/1999/30/863-Image.html?news=true">star clusters</a> are visible too. There’s the Quintuplet cluster in the lower left of the image within a bubble where the stars’ winds have cleared the local gas and dust. In the upper left there’s a cluster called the Arches, which was named for the illuminated arcs of gas which extend above it and out of the image. These two clusters include some of the most massive stars known.</p>
<h2>4. Abell 370</h2>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/438855/original/file-20211222-18663-11hzdzc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438855/original/file-20211222-18663-11hzdzc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=539&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438855/original/file-20211222-18663-11hzdzc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=539&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438855/original/file-20211222-18663-11hzdzc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=539&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438855/original/file-20211222-18663-11hzdzc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=678&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438855/original/file-20211222-18663-11hzdzc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=678&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438855/original/file-20211222-18663-11hzdzc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=678&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Abell 370 is a cluster of hundreds of galaxies about five billion light years away from Earth.</span>
<span class="attribution"><a class="source" href="https://hubblesite.org/contents/articles/gravitational-lensing">NASA, ESA, and J. Lotz and the HFF Team (STScI)</a></span>
</figcaption>
</figure>
<p>On much larger scales than individual galaxies, the universe is structured as a web of filaments (long connected strands) of dark matter. Some of the most dramatic visible objects are clusters of galaxies which form at the intersection of filaments.</p>
<p>If we look at galaxy clusters nearby (relatively speaking, of course), we can see dramatic proof that Einstein was right when he asserted that mass curves space. One of the prettiest examples which reveals this warping of space can be seen in Hubble’s image of <a href="https://hubblesite.org/contents/articles/gravitational-lensing">Abell 370</a>, released in 2017.</p>
<p>Abell 370 is a cluster of hundreds of galaxies about five billion light years away from us. In the picture you can see elongated arcs of light. These are the magnified and distorted images of far more distant galaxies. The mass of the cluster distorts spacetime and bends the light from the more distant objects, magnifying them and in some cases creating multiple images of the same distant galaxy. This phenomenon is called gravitational lensing, because the warped spacetime acts like an optical lens. </p>
<p>The most prominent of these magnified images is the thickest bright arc above and to the left of the centre of the picture. Called “the Dragon”, this arc consists of two images of the same distant galaxy at its head and tail. Overlapping images of several other distant galaxies comprise the arc of the dragon’s body.</p>
<p>These gravitationally magnified images are useful to astronomers, because the magnification reveals more detail of the distant lensed object than would otherwise be seen. In this case the lensed galaxy’s population of stars can be examined in detail. </p>
<h2>5. The Hubble Ultra Deep Field</h2>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/438347/original/file-20211219-48250-pwyos1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438347/original/file-20211219-48250-pwyos1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438347/original/file-20211219-48250-pwyos1.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438347/original/file-20211219-48250-pwyos1.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438347/original/file-20211219-48250-pwyos1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438347/original/file-20211219-48250-pwyos1.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438347/original/file-20211219-48250-pwyos1.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Sometimes, less is more.</span>
<span class="attribution"><a class="source" href="https://esahubble.org/images/heic0611b/">NASA, ESA, and S. Beckwith (STScI) and the HUDF Team</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>In an inspired idea, astronomers decided to point Hubble at a blank patch of sky for several days to discover what extremely distant objects might be seen at the edge of the observable universe. </p>
<p>The <a href="https://esahubble.org/images/heic0611b/">Hubble Ultra Deep Field</a> contains nearly 10,000 objects, almost all of which are very distant galaxies. The light from some of these galaxies has been travelling for over 13 billion years, since the universe was only about half a billion years old. </p>
<p>Some of these objects are among the oldest and most distant known. Here we’re seeing light from ancient stars whose local contemporaries have long since been extinguished. </p>
<p>The oldest galaxies formed during the epoch of reionisation, when the tenuous gas in the universe first became bathed in starlight which was capable of separating electrons from hydrogen. This was the last major change in properties of the universe as a whole. </p>
<hr>
<p>
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<strong>
Read more:
<a href="https://theconversation.com/james-webb-space-telescope-what-astronomers-hope-it-will-reveal-about-the-beginning-of-the-universe-podcast-173436">James Webb Space Telescope: what astronomers hope it will reveal about the beginning of the universe – podcast</a>
</strong>
</em>
</p>
<hr>
<p>The fact that light carries so much information, allowing us to piece together the history of the universe, is remarkable. The launch of the James Webb Space Telescope will give us some vastly improved infrared images, and will inevitably raise new questions to challenge future generations of scientists.</p><img src="https://counter.theconversation.com/content/169877/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Carole Haswell receives funding from STFC. </span></em></p>As we await the launch of the James Webb Space telescope, it’s timely to look back on what previous generations of telescopes have shown us.Carole Haswell, Professor of Astrophysics, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1701882021-11-09T21:24:05Z2021-11-09T21:24:05ZWeird weather: Metal rain and super-high temperatures on an ultra-hot exoplanet<figure><img src="https://images.theconversation.com/files/430843/original/file-20211108-27-1tbrtjs.jpeg?ixlib=rb-1.1.0&rect=0%2C0%2C9985%2C5000&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">An artist's impression of the exoplanet WASP-76b, which is hot enough to vaporize metals. </span> <span class="attribution"><a class="source" href="https://www.eso.org/public/images/eso2005a/">(European Southern Observatory/M. Kornmesser)</a></span></figcaption></figure><iframe style="width: 100%; height: 175px; border: none; position: relative; z-index: 1;" allowtransparency="" src="https://narrations.ad-auris.com/widget/the-conversation-canada/weird-weather--metal-rain-and-super-high-temperatures-on-an-ultra-hot-exoplanet" width="100%" height="400"></iframe>
<p>Ultra-hot Jupiters — named as such because of their physical similarities to the planet Jupiter — are exoplanets that orbit stars other than the sun with temperatures so high that the molecules in their atmospheres are <a href="https://www.nasa.gov/feature/jpl/water-is-destroyed-then-reborn-in-ultrahot-jupiters">completely torn apart</a>. They are among the most extreme environments in our galaxy. </p>
<p>They also whip around their parent stars in orbits that only last a few days, and astronomers <a href="https://doi.org/10.3847/2041-8213/abd84f">still aren’t sure</a> how it’s possible for them to form.</p>
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<p>
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<strong>
Read more:
<a href="https://theconversation.com/the-seven-most-extreme-planets-ever-discovered-78959">The seven most extreme planets ever discovered</a>
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<p>While these harsh conditions might sound like they’re as extreme as it gets, astronomers are starting to realize they may just be the tip of the (very hot) iceberg. In a recent study published in <a href="https://doi.org/10.3847/2041-8213/ac2513"><em>The Astrophysical Journal Letters</em></a>, my colleagues and I discovered that one of these exotic worlds in particular is even more extreme than we’d ever thought.</p>
<h2>Ultra-hot worlds</h2>
<p><a href="http://dx.doi.org/10.1051/0004-6361/201527276">Discovered in 2016</a>, WASP-76b is perhaps the most well-known of these ultra-hot worlds. At double the size of our own planet Jupiter, WASP-76b has day-side temperatures reaching a whopping 2,400 C, and takes less than two days to orbit its parent star. Its claim to fame, however, is a 2020 study suggesting that <a href="https://doi.org/10.1038/s41586-020-2107-1">liquid iron might literally be raining down from its skies</a>.</p>
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<em>
<strong>
Read more:
<a href="https://theconversation.com/from-iron-rain-on-exoplanets-to-lightning-on-jupiter-four-examples-of-alien-weather-160403">From iron rain on exoplanets to lightning on Jupiter: four examples of alien weather</a>
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<p>More recent research, yet to be peer-reviewed, has <a href="https://arxiv.org/abs/2109.00163">called this result into question</a>. But there’s no doubt that the conditions on WASP-76b are totally unlike anything here on Earth. WASP-76b can therefore offer us a window into the most extreme physical and chemical processes in our galaxy, and studying its harsh alien conditions can help us place our own solar system into context.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/430847/original/file-20211108-19-13xs3rq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An illustration of a planet orbiting a large star" src="https://images.theconversation.com/files/430847/original/file-20211108-19-13xs3rq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/430847/original/file-20211108-19-13xs3rq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/430847/original/file-20211108-19-13xs3rq.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/430847/original/file-20211108-19-13xs3rq.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/430847/original/file-20211108-19-13xs3rq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/430847/original/file-20211108-19-13xs3rq.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/430847/original/file-20211108-19-13xs3rq.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=471&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An artist’s impression of a hot Jupiter planet, similar to WASP-76b, orbiting close to one of the stars in the rich old star cluster Messier 67.</span>
<span class="attribution"><a class="source" href="https://www.eso.org/public/images/eso1621a/">(European Space Observatory/L. Calçada)</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Atmospheric knowledge</h2>
<p>Unfortunately, studying exoplanets — even massive ones like WASP-76b — is often easier said than done. The <a href="https://exoplanetarchive.ipac.caltech.edu/">4,500 exoplanets already discovered</a> are incredibly far away from us, and their parent stars are so bright that light from the exoplanets themselves gets completely washed out. </p>
<p>Rather than looking at the exoplanets directly, we often have to find ways to infer their presence instead. These indirect methods have actually been responsible for most of the exoplanets we’ve discovered. As a bonus, we can use these methods to peer into the exoplanets’ atmospheres as well.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-how-to-find-an-exoplanet-part-1-56682">Explainer: How to find an exoplanet (part 1)</a>
</strong>
</em>
</p>
<hr>
<p>This is the idea behind <a href="https://exoplanets.nasa.gov/discovery/how-we-find-and-characterize/">transit spectroscopy</a>. When an exoplanet passes in front of, or transits, its parent star, the light from the star gets filtered through the exoplanet’s atmosphere. Different atmospheric gases leave unique chemical imprints — like fingerprints — on the starlight, and by studying these fingerprints, we’re able to learn which gases are present. This can help us learn more about what conditions on the exoplanet are actually like.</p>
<p>In theory, you can do this for any exoplanet with an atmosphere, but it’s easiest with atmospheres that are hot and puffed-up. Large, extended atmospheres leave stronger chemical imprints on their starlight, which makes them much easier for us to observe.</p>
<p>This is precisely why our team chose WASP-76b as one of the first exoplanets to be observed by our new ExoGemS (Exoplanets with Gemini Spectroscopy) survey. Led by <a href="https://sites.google.com/site/astrojaketurner/home">Jake Turner</a>, <a href="https://astro.cornell.edu/ray-jayawardhana">Ray Jayawardhana</a> and <a href="https://research.astro.cornell.edu/andrew-ridden-harper">Andrew Ridden-Harper</a> at Cornell University, the goal of the survey is to glimpse into the atmospheres of more than 40 exoplanets using the <a href="https://www.gemini.edu/">Gemini North telescope in Hawaii</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/430829/original/file-20211108-25-k0zl0c.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Researchers stand next to an observatory at sunset" src="https://images.theconversation.com/files/430829/original/file-20211108-25-k0zl0c.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/430829/original/file-20211108-25-k0zl0c.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/430829/original/file-20211108-25-k0zl0c.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/430829/original/file-20211108-25-k0zl0c.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/430829/original/file-20211108-25-k0zl0c.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/430829/original/file-20211108-25-k0zl0c.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/430829/original/file-20211108-25-k0zl0c.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The ExoGemS survey uses the Gemini North telescope in Hawaii to study the atmospheres of more than 40 planets.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Gemini_Observatory_at_sunset.jpg">(Mailseth/Wikimedia Commons)</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>Extreme atmospheres</h2>
<p>In this particular study, we observed WASP-76b for a period of four hours as it transited in front of its parent star. We were searching for the chemical fingerprints of metals in its atmosphere, because at these extreme temperatures, metals will actually vaporize into gas.</p>
<p>WASP-76b had already been observed many times in the past, but our observations from the Gemini North telescope reached redder wavelengths of light than previously published results. This meant that we could search for chemical fingerprints that previous studies didn’t have access to, shedding a much broader light on the exotic composition of this extreme world.</p>
<p>What immediately stood out to us in our data was a series of three very strong absorption features at infrared wavelengths of light. We recognized these as the chemical fingerprint of ionized calcium — calcium atoms that have lost an electron — and the signal was so strong that we could actually see it moving around as the exoplanet orbited its parent star.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/BAN-l2rkOHE?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">A video by the European Southern Observatory showing the ultra-hot giant exoplanet WASP-76b.</span></figcaption>
</figure>
<p>Finding calcium in WASP-76b’s atmosphere wasn’t particularly surprising — a different set of calcium signals had <a href="https://doi.org/10.1051/0004-6361/202039511">already been detected earlier this year</a>. What did surprise us was just how much ionized calcium we were seeing — much more than any of our theoretical models predicted we would.</p>
<p>So what’s going on? One possibility is that WASP-76b’s atmosphere is even hotter than the 2,400 C we’d previously thought. These extreme temperatures would strip electrons off of regular calcium atoms and the hotter the temperature, the more frequently this is going to occur.</p>
<p>Another possibility is that powerful winds are unearthing ionized calcium atoms from the exoplanet’s depths. A recent study actually suggested that <a href="https://doi.org/10.1051/0004-6361/202140569">WASP-76b may have winds as fast as 22 kilometres per second</a>. For reference, <a href="https://public.wmo.int/en/media/news/new-world-record-wind-gust">the fastest winds ever measured on the Earth had a speed of less than one kilometre per second</a>.</p>
<p>In a fortunate coincidence, another team of astronomers used observations from the <a href="https://www.caha.es/">Calar Alto Observatory in Spain</a> to detect this same ionized calcium signal in infrared light. Like us, their data showed <a href="https://doi.org/10.1051/0004-6361/202141669">more ionized calcium than expected</a>. There’s clearly much more going on in WASP-76b’s atmosphere than we’d thought.</p>
<h2>Weird, wild atmosphere</h2>
<p>WASP-76b has been observed by just about every major telescope out there, from the Gemini North telescope in Hawaii to the <a href="https://www.eso.org/public/teles-instr/paranal-observatory/vlt/">Very Large Telescope in Chile</a> all the way up to the <a href="https://www.nasa.gov/mission_pages/hubble/main/index.html">Hubble Space Telescope</a> in outer space. To fully piece together the puzzle of what’s going on its atmosphere, we’ll need to wait for observations from the powerful new <a href="https://theconversation.com/james-webb-space-telescope-an-astronomer-on-the-team-explains-how-to-send-a-giant-telescope-to-space-and-why-167516">James Webb Space Telescope</a> set to launch in December 2021.</p>
<p>In the meantime, our ExoGemS survey will allow us to continue investigating the atmospheres of dozens of exoplanets — many of which have never been characterised — from right here on Earth. There’s no doubt that WASP-76b’s weird, wild atmosphere is just the beginning of what we’re going to uncover.</p><img src="https://counter.theconversation.com/content/170188/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Emily Deibert receives funding from NSERC (Natural Science and Engineering Research Council of Canada). </span></em></p>On the ultra-hot exoplanet WASP-76b, metal is vaporized in the heat. Studying the atmosphere of extreme planets will reveal more wild and weird weather.Emily Deibert, PhD Candidate in Astronomy & Astrophysics, University of TorontoLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1706002021-10-28T18:55:59Z2021-10-28T18:55:59ZJupiter: mission unveils the depth and structure of planet’s shrinking red spot and colourful bands<figure><img src="https://images.theconversation.com/files/428567/original/file-20211026-23-up80yc.png?ixlib=rb-1.1.0&rect=29%2C15%2C1735%2C976&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Enhanced artist impression based on JunoCam image of Jupiter acquired on July 21, 2021.
</span> <span class="attribution"><a class="source" href="https://www.missionjuno.swri.edu/junocam/processing?id=11191">NASA / SwRI / MSSS / Tanya Oleksuik</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-sa/4.0/">CC BY-NC-SA</a></span></figcaption></figure><p>Nasa’s <a href="https://www.missionjuno.swri.edu/">Juno mission</a>, the solar-powered robotic explorer of Jupiter, has completed its <a href="https://theconversation.com/the-first-results-from-the-juno-mission-are-in-and-they-already-challenge-our-understanding-of-jupiter-78203">five-year prime mission</a> to reveal the inner workings of the solar system’s biggest planet. Since 2016, the spacecraft has flown within a few thousand kilometres of Jupiter’s colourful cloud tops every 53 days, using a carefully selected array of instruments to peer deeper into the planet than ever before.</p>
<p>The most recent findings from these measurements have now been published in a series of papers, revealing the three-dimensional structure of Jupiter’s weather systems – including of its famous Great Red Spot, a centuries-old storm big enough to swallow the Earth whole.</p>
<p>Before Juno, decades of observations had revealed the famous striped appearance of Jupiter’s atmosphere, with white bands known as zones, and red-brown bands known as belts. The bands are separated by powerful winds zipping east and west, known as the jet streams, and are punctuated by gigantic vortices, such as the <a href="https://theconversation.com/six-mysteries-of-jupiters-great-red-spot-80829">red spot</a>.</p>
<p>But scientists had long suspected that these weather patterns were the mere tip of the iceberg, and that hidden and unforeseen phenomena might be shaping the atmosphere deep below the veil of clouds. Unlike the Earth, Jupiter’s atmosphere lacks a surface, so could be considered as a bottomless abyss. </p>
<p>Juno has three ways to <a href="https://theconversation.com/nasas-juno-arrives-at-jupiter-to-lift-cloudy-veil-60879">peer down beneath</a> the maelstrom of these cloudy upper layers. It can measure tiny changes to Jupiter’s gravity to sense the distribution of mass all the way down to the fuzzy core. It can measure Jupiter’s magnetic field to determine the flows within deep, magnetised fluid layers. And it can use microwave light to look straight through the clouds. </p>
<h2>The Great Red Spot</h2>
<p>Jupiter’s <a href="https://theconversation.com/six-mysteries-of-jupiters-great-red-spot-80829">Great Red Spot</a> has had a hard time in recent years. It has been steadily shrinking in the east-west direction for decades, and recent encounters with smaller vortices has led to enormous flakes of reddish material being drawn out of the spot itself. These flaking events, though troublesome for fans of the best-known storm in the solar system, do appear to be superficial, only affecting the reddish hazes that sit atop the vortex. </p>
<figure class="align-center ">
<img alt="Image of the red spot." src="https://images.theconversation.com/files/428570/original/file-20211026-25-t81asy.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/428570/original/file-20211026-25-t81asy.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=412&fit=crop&dpr=1 600w, https://images.theconversation.com/files/428570/original/file-20211026-25-t81asy.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=412&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/428570/original/file-20211026-25-t81asy.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=412&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/428570/original/file-20211026-25-t81asy.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=518&fit=crop&dpr=1 754w, https://images.theconversation.com/files/428570/original/file-20211026-25-t81asy.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=518&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/428570/original/file-20211026-25-t81asy.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=518&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Jupiter’s Great Red Spot at PJ18 (2019), showing large flakes of red material to the west (left) of the vortex.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill</span></span>
</figcaption>
</figure>
<p>But fans of the storm can take comfort from Juno’s latest findings. In 2017, Juno was able to observe the red spot in <a href="https://www.science.org/doi/10.1126/science.abf1015">microwave light</a>. Then, in 2019, as Juno flew at more than 200,000 kilometres per hour above the vortex, <a href="https://www.nasa.gov/directorates/heo/scan/services/networks/deep_space_network/about">Nasa’s Deep Space Network</a> was <a href="https://www.science.org/doi/10.1126/science.abf1396">monitoring the spacecraft’s velocity</a> from millions of kilometres away. Tiny changes as small as 0.01 millimetres per second were detected, caused by the gravitational force from the massive spot.</p>
<p>By modelling these microwave and gravity data, my colleagues and I were able to determine that the famous storm is at least 300km deep, maybe as deep as 500km. That’s deeper than the expected cloud-forming “weather layer” that reaches down to around 65km below the surface, but higher than the jet streams which might extend down to 3,000km. The deeper the roots, the more likely the Red Spot is to persist in the years to come, despite the superficial battering it has been receiving from passing storms.</p>
<p>To place the depth in perspective, the International Space Station orbits ~420km above Earth’s surface. Yet despite these new findings, the spot could still be a “pancake-like” structure floating in the bottomless atmosphere, with the spot’s 12,000km width being 40 times larger than its depth.</p>
<h2>The mystery of belts and zones</h2>
<p>In the cloud-forming weather layer, Juno’s microwave antennae saw the expected structure of belts and zones. The cool zones appeared dark, indicating the presence of ammonia gas, which absorbs microwave light. Conversely, the belts were bright in microwave light, consistent with a lack of ammonia. These bright and dark bands in the weather layer were perfectly aligned with the winds higher up, measured at the top of the clouds. But what happens when we probe deeper?</p>
<figure class="align-center ">
<img alt="Graphs of the belts and zones observed in microwave light." src="https://images.theconversation.com/files/428571/original/file-20211026-23-juxhj3.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/428571/original/file-20211026-23-juxhj3.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=523&fit=crop&dpr=1 600w, https://images.theconversation.com/files/428571/original/file-20211026-23-juxhj3.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=523&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/428571/original/file-20211026-23-juxhj3.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=523&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/428571/original/file-20211026-23-juxhj3.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=657&fit=crop&dpr=1 754w, https://images.theconversation.com/files/428571/original/file-20211026-23-juxhj3.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=657&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/428571/original/file-20211026-23-juxhj3.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=657&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Jupiter’s belts and zones observed in microwave light, compared to the colours of the cloud-tops (left), and the winds at the cloud tops (right).</span>
<span class="attribution"><span class="source">Credit: NASA/JPL/SwRI/Univ. Leicester</span></span>
</figcaption>
</figure>
<p>The temperature of Jupiter’s atmosphere is just right for the formation of a water cloud around 65km down below the cloud tops. When Juno peered through this layer, it found something unexpected. The belts became microwave-dark and the zones became microwave-bright. This is the complete reverse of what we saw in the shallower cloudy regions, and we are calling this <a href="https://doi.org/10.1029/2021JE006858">transition layer</a> the “jovicline” – some 45-80km below the visible clouds.</p>
<p>A “cline” is a layer within a fluid where properties change dramatically. Earth’s oceans have a <a href="https://en.wikipedia.org/wiki/Thermocline">thermocline</a>, dividing mixed surface waters from cold and deep water below. This isn’t a new idea - the legendary science fiction author Arthur C. Clarke envisaged the voyage of the <em>Kon Tiki</em> balloon down into Jupiter’s atmosphere in his 1971 short story, <a href="https://en.wikipedia.org/wiki/A_Meeting_with_Medusa">A Meeting with Medusa</a>. He describes the balloon travelling down towards a Jovian thermocline and its associated bank of clouds.</p>
<p>The jovicline may separate the shallow cloud-forming weather layer from the deep abyss below. This unexpected result implies something is moving all that ammonia around. </p>
<h2>A conveyor belt?</h2>
<p>One possibility is that each jetstream is associated with a <a href="https://arxiv.org/abs/1907.01822">“circulation cell”</a>, a climate phenomenon that moves gases around via currents of rising and falling air. The rising could cause ammonia enrichment, and the sinking ammonia depletion. If true, there would be about <a href="https://dx.doi.org/10.1029/2021GL095651">eight of these circulation cells</a> in each hemisphere. Earth displays <a href="https://en.wikipedia.org/wiki/Atmospheric_circulation">similar phenomena</a> – the Hadley cell, named after the English physicist and meteorologist George Hadley, in the tropics, and the Ferrel cells, named after the American meteorologist William Ferrel, at mid-latitudes both influence the Earth’s weather and climate.</p>
<p>Other meteorological phenomena might be responsible for moving the ammonia around within this deep atmosphere. For example, vigorous storms in Jupiter’s belts might create mushy ammonia-water hailstones (known as “<a href="https://www.nasa.gov/feature/jpl/shallow-lightning-and-mushballs-reveal-ammonia-to-nasas-juno-scientists">mushballs</a>”), which deplete ammonia within the shallow belts before falling deep, eventually evaporating to enrich the belts at great depths.</p>
<p>What’s clear is that Juno has opened a new window onto the dark, deep atmosphere, and that the results are challenging our understanding of this giant planet. As Juno embarks on its <a href="https://www.jpl.nasa.gov/news/nasas-juno-mission-expands-into-the-future">extended mission</a>, scientists will be working to make sense of these new findings.</p><img src="https://counter.theconversation.com/content/170600/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Leigh Fletcher is a Juno Participating Scientist and receives funding from a European Research Council (ERC) Consolidator Grant.</span></em></p>Jupiter’s Great Red Spot is deep, meaning it may persist even though it is shrinking.Leigh Fletcher, Associate Professor in Planetary Sciences, University of LeicesterLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1699742021-10-15T11:48:28Z2021-10-15T11:48:28ZNasa: imminent asteroid missions could reveal our origins – and help save Earth from deadly strike<figure><img src="https://images.theconversation.com/files/426665/original/file-20211015-30-1rt7iub.jpg?ixlib=rb-1.1.0&rect=40%2C17%2C2955%2C1661&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">DART will change the orbit of a moon around an asteroid.</span> <span class="attribution"><span class="source">Nasa</span></span></figcaption></figure><p>Asteroids are remnants of the early Solar System, with the potential to reveal secrets of our planet’s origins. But they could also bring an end to life on Earth. Now two missions, <a href="https://www.nasa.gov/mission_pages/lucy/overview/index">Lucy</a> and <a href="https://dart.jhuapl.edu/">DART</a> (Double Asteroid Redirection Test) will provide further insights into both of these features – with DART even attempting to redirect the orbit of a moon around an asteroid. </p>
<p>Space rocks are generally considered to be asteroids if they are larger than approximately 1km in size, and made principally of “non-volatile” materials – chemicals which can be easily vaporised. Carbon monoxide, for example, is volatile as it vaporises at a temperature of -191°C. But iron, with a vaporisation point of 2,862°C is non-volatile. </p>
<p>This is somewhat different to <a href="https://theconversation.com/neowise-an-increasingly-rare-opportunity-to-spot-a-comet-with-the-naked-eye-142764">comets</a>. Asteroids are found more commonly in the inner Solar System, whereas comets with their volatile-rich composition tend to lurk in the outer part, far from the heat of the Sun. Some 500,000 asteroids have been catalogued to date, and many have small moons of their own.</p>
<figure class="align-center ">
<img alt="Images of asteroids." src="https://images.theconversation.com/files/426526/original/file-20211014-21-13f62fp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/426526/original/file-20211014-21-13f62fp.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=475&fit=crop&dpr=1 600w, https://images.theconversation.com/files/426526/original/file-20211014-21-13f62fp.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=475&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/426526/original/file-20211014-21-13f62fp.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=475&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/426526/original/file-20211014-21-13f62fp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=596&fit=crop&dpr=1 754w, https://images.theconversation.com/files/426526/original/file-20211014-21-13f62fp.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=596&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/426526/original/file-20211014-21-13f62fp.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=596&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Asteroid montage (Not to scale)</span>
<span class="attribution"><span class="source">NASA/ESA</span></span>
</figcaption>
</figure>
<p>Asteroids are thought to be the remnants of <a href="https://www.britannica.com/science/planetesimal">planetesimals</a> – precursors of the planets in the early Solar System, which coalesced under gravity to form the familiar worlds we know today. Asteroids somehow escaped this process, preserving something of the conditions of our early Solar System, from a time before even the planets had formed. This epoch is quite mysterious. How tiny dust particles, which constituted the bulk of solid material at the time, were able to clump together and form larger objects like asteroids, given that they lack significant gravitational fields of their own, is still being <a href="https://link.springer.com/article/10.1007/s11214-018-0486-5">investigated</a>. </p>
<p>The most well known of the asteroids are those which reside in the main belt, a million-strong swarm orbiting the Sun between Mars and Jupiter. This sounds like a lot, but space is vast and the distances between one asteroid and a neighbour are typically millions of kilometres. Thus the odds of successfully navigating an asteroid field, at least in our Solar System, are significantly better than <a href="https://giphy.com/gifs/bcbPzkSCytDH2">3,720 to 1</a>. </p>
<p>The US$980 million (£714 million) Lucy spacecraft – set to launch on October 16 – will <a href="https://www.nasa.gov/mission_pages/lucy/overview/index">fly through</a> three asteroid fields over the course of its 12-year mission. It is named Lucy after the famous hominin <a href="https://iho.asu.edu/about/lucys-story">fossil</a>, because it is hoped it could be similarly revolutionary for our knowledge of the Solar System’s origins. Lucy will fly first through the main belt, then travel outwards to visit two other less familiar asteroid fields – <a href="https://astronomy.com/magazine/news/2018/09/exploring-jupiters-trojan-asteroids">the Jupiter Trojans</a>. </p>
<p>Trojan asteroids orbit the Sun at the “Lagrange points”. These are positions in space where the gravitational pull of the Sun and a planet balance out such that an object located there will naturally remain in place, potentially for billions of years. There are five such points for all planets in the solar system and they are numbered L1-L5 (see image below). The Jupiter Trojans, clustered at L4 and L5, are two enormous and unexplored asteroid fields, which between them harbour at least as many asteroids as the main belt. </p>
<figure class="align-center ">
<img alt="Drawing of Lagrange points." src="https://images.theconversation.com/files/426522/original/file-20211014-27-15pz8ve.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/426522/original/file-20211014-27-15pz8ve.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=691&fit=crop&dpr=1 600w, https://images.theconversation.com/files/426522/original/file-20211014-27-15pz8ve.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=691&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/426522/original/file-20211014-27-15pz8ve.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=691&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/426522/original/file-20211014-27-15pz8ve.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=869&fit=crop&dpr=1 754w, https://images.theconversation.com/files/426522/original/file-20211014-27-15pz8ve.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=869&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/426522/original/file-20211014-27-15pz8ve.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=869&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The Lagrange points.</span>
<span class="attribution"><span class="source">Author provided</span></span>
</figcaption>
</figure>
<p>Lucy will first venture to the L4 Jupiter Trojans, which it will reach in 2027. It will then fly back towards Earth, using our planet’s gravity to slingshot it back out towards the L5 Jupiter Trojans, which it will reach in 2033. This remarkable flight path will be accomplished with <a href="https://www.nasa.gov/mission_pages/tdm/sep/index.html">solar-electric propulsion</a>.</p>
<p>The spacecraft carries a suite of instruments including sophisticated cameras and spectrometers to map the asteroids and discern their composition. It is expected that the chemical composition of the Jupiter Trojans will be somewhat different from the main belt asteroids, containing a higher concentration of volatile material, blurring the distinction between asteroids and comets. Indeed, one Jupiter Trojan was recently found to have a <a href="http://www.sci-news.com/astronomy/first-trojan-asteroid-comet-like-tail-2019-ld2-08457.html">comet-like tail</a>.</p>
<h2>Asteroid strikes</h2>
<p>Not all asteroids are confined to a belt. Some wander throughout the Solar System on orbits which can bring them into close proximity with planets like Earth. The impact hazard of asteroids is relatively well publicised, particularly after the <a href="https://theconversation.com/chelyabinsk-meteor-explosion-a-wake-up-call-scientists-warn-19874">Chelyabinsk meteor</a> which exploded over a Russian town in 2013, injuring over 1,000 people and causing extensive damage. </p>
<p>At some point in late November, Nasa will attempt to launch DART. This spacecraft will attempt to intercept <a href="https://www.sciencedirect.com/science/article/abs/pii/S0019103520301640">65803 Didymos</a>, a near-Earth asteroid with a small moon of its own, called Dimorphos. The approximately 170 metre sized moon will be struck by the 500kg DART spacecraft with an impact velocity of 6.6 kilometres per second. The objective is to observe a change in orbital motion of Dimorphos about Didymos as a result of the collision. </p>
<p>This will be accomplished by a follow up mission launched by ESA, called <a href="https://www.esa.int/Safety_Security/Hera/Hera">Hera</a>, which will reach Didymos in 2026 and perform a detailed survey of Dimorphos’ orbit. By measuring the change in orbit of the little moon, scientists and engineers will be able to better calculate how much energy is required to alter the orbit of a hypothetical future threatening asteroid. It must be stressed that, currently, there are no known future asteroid-Earth collisions, but clearly it is best to prepare for such an eventuality.</p>
<p>There are even more asteroid missions in the near future. In August 2022, Nasa will launch <a href="https://www.nasa.gov/psyche">Psyche</a> to visit its namesake asteroid, <a href="https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/16-psyche/in-depth/">16 Psyche</a>, which orbits in the main belt. This peculiar world is over 200km across and contains a lot of metal. So much in fact that it is believed to be the exposed core of a once growing planet in the early Solar System, which suffered a catastrophic impact at some point in the distant past. This collision sheared off the outer layers of the fledgling planet, leaving the exposed metal-rich core behind. If this theory turns out to be correct, then it will be the first time that scientists have had a chance to directly observe a planetary core.</p>
<p>This slew of upcoming missions, and many recent <a href="https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/exploration/?page=0&per_page=10&order=launch_date+desc%2Ctitle+asc&search=&tags=Asteroids&category=33">previous ones</a>, represent something of a golden era in asteroid research. Asteroids still have many stories to tell, hold vast <a href="https://theconversation.com/mining-asteroids-could-unlock-untold-wealth-heres-how-to-get-started-95675">economic potential</a> as mining resources, and pose an obvious danger to civilisation on Earth.</p><img src="https://counter.theconversation.com/content/169974/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gareth Dorrian does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The Lucy mission could revolutionise our knowledge of the Solar System’s history, while the DART mission could help redirect hazardous asteroids in the future.Gareth Dorrian, Post Doctoral Research Fellow in Space Science, University of BirminghamLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1621922021-06-07T09:59:19Z2021-06-07T09:59:19ZNasa has just rejected missions to moons of Jupiter and Neptune – here’s what we would have found out<figure><img src="https://images.theconversation.com/files/404489/original/file-20210604-27-1ybuyah.png?ixlib=rb-1.1.0&rect=114%2C45%2C2378%2C1363&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A volcanic eruption on Jupiter's moon Io.</span> <span class="attribution"><a class="source" href="https://solarsystem.nasa.gov/resources/1039/galileo-sees-io-erupt/?category=moons/jupiter-moons_io">NASA/JPL/DLR</a></span></figcaption></figure><p>It’s been 30 years since Nasa last visited Venus, with <a href="https://theconversation.com/astronomers-spot-strange-bow-like-structure-in-venus-atmosphere-71347">the Magellan orbiter</a> in 1990. Now, <a href="https://www.nasa.gov/press-release/nasa-selects-2-missions-to-study-lost-habitable-world-of-venus/">two new missions</a> have been selected to explore the deadly atmosphere, crushing pressures and volcanic landscape. </p>
<p>The process dates back to February 2020, when Nasa announced that four missions were to undergo a nine-month peer-review process for feasibility. They were all part of the <a href="https://www.nasa.gov/planetarymissions/discovery.html">Discovery program</a>, started by Nasa in 1992 to bring together scientists and engineers to create exciting, groundbreaking missions. Set aside from the flagship missions – such as <a href="https://theconversation.com/curiosity-catches-a-whiff-of-methane-on-mars-and-a-possibility-of-past-life-35595">Curiosity</a> and <a href="https://theconversation.com/uk/search?q=perseverance">Perseverance</a> – the missions operating under Discovery have taken unique and innovative approaches to exploring the solar system.</p>
<p>The two winning Venus missions, <a href="https://theconversation.com/nasa-has-announced-two-missions-to-venus-by-2030-heres-why-thats-exciting-162133">Davinci and Veritas</a>, have been awarded US$500 million (£354 million) and will be launched some time between 2028 and 2030. But the competition was tough from the two losing missions, which would have gone to <a href="https://www.nasa.gov/planetarymissions/io-volcano-observer">Io</a> and <a href="https://www.nasa.gov/feature/jpl/proposed-nasa-mission-would-visit-neptunes-curious-moon-triton">Triton</a>, respectively moons of Jupiter and Neptune. So what are we missing out on as a result?</p>
<h2>Exploring Jupiter’s bizarre moon</h2>
<p>Io is a strange moon – even among moons, which are strange to begin with. As Jupiter’s innermost moon, orbiting a mere 350,000 km above the cloud tops, it gives Io an extreme <a href="https://www.youtube.com/watch?v=9qHrzs6Mbp4">heating mechanism</a> that makes it the <a href="https://ui.adsabs.harvard.edu/abs/2004Icar..169..140L/abstract">most volcanically active</a> object in the solar system, sporting over four hundred volcanoes. </p>
<p>You might think, given we live on a planet with a fair share of volcanoes, that we’d have a good idea of where all this heat is coming from. In fact, according to Alfred McEwen, principal investigator on the proposed Io Volcanic Explorer or IVO mission, we’re still profoundly ignorant of how it actually works.</p>
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Read more:
<a href="https://theconversation.com/nasa-has-announced-two-missions-to-venus-by-2030-heres-why-thats-exciting-162133">Nasa has announced two missions to Venus by 2030 – here's why that's exciting</a>
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<p>IVO was designed to perform multiple fly-bys of the moon and use a suite of instruments to map the activity on and below the surface. By collecting information on Io’s magnetic and gravitational fields, taking videos of the enormous lava eruptions and analysing the gas and dust escaping from the moon, IVO would help scientists learn how Io’s heat is generated and lost.</p>
<p>All of this information is crucial – not just for awesome videos of space volcanoes – because this kind of extreme activity is believed to be an <a href="https://www.nasa.gov/planetarymissions/io-volcano-observer">important aspect</a> of planetary formation and evolution. By understanding the processes that drive change on Io, we can ultimately learn more about how planets and moons came to be.</p>
<h2>The ice giants</h2>
<p>The least explored and understood planets are Uranus and Neptune, and they are home to some of the most bizarre things in the solar system. Uranus has an axial tilt – the angle of its axis of rotation compared to the plane it orbits the Sun – so extreme that it spins on its side. This is thought to be the result of a <a href="http://icc.dur.ac.uk/giant_impacts/">giant collision</a> in the solar system’s past. </p>
<p>Meanwhile, Neptune is home to the <a href="https://www.nasa.gov/feature/jpl/proposed-nasa-mission-would-visit-neptunes-curious-moon-triton">only large moon</a> that orbits backwards around its parent planet, the curious Triton. The peculiar orbital arrangement isn’t where the oddities end. The plane in which Triton orbits is offset by an extreme 23 degrees compared to Neptune’s, and it is believed to have moved to <a href="https://ui.adsabs.harvard.edu/abs/2006Natur.441..192A/abstract">Neptune from the Kuiper Belt</a>, the region beyond Neptune’s orbit filled with icy leftovers from the solar system’s formation. </p>
<figure class="align-center ">
<img alt="A diagram showing the surface of Triton and what the Trident mission was aiming to do." src="https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=628&fit=crop&dpr=1 600w, https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=628&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=628&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=789&fit=crop&dpr=1 754w, https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=789&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=789&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">What the Trident mission would’ve done.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/sites/default/files/thumbnails/image/pia23874-1041.jpg">NASA/JPL-Caltech</a></span>
</figcaption>
</figure>
<p>Triton also has an <a href="https://ui.adsabs.harvard.edu/abs/1992AdSpR..12k.113L/abstract">active ionosphere</a> – a layer of charged particles in its atmosphere ten times more active than any other moon, which isn’t powered by the Sun – as well as a constantly changing and dynamic surface, coated in what might be nitrogen snow. When <a href="https://solarsystem.nasa.gov/moons/neptune-moons/triton/in-depth/">Voyager 2 photographed the moon</a>, it discovered cryovolcanoes – geysers erupting ice and gas up to 8km high, which might indicate a subsurface ocean. </p>
<p>The proposed Trident mission would have explored these many strange things about the moon. It proposed a three-pronged approach using instruments to measure the magnetic field of Triton. It would have identified the presence and structure of a subsurface ocean. High resolution infrared cameras would have allowed the spacecraft to image the entire surface, using the sunlight reflected from Neptune, showing scientists what had changed since the last visit in 1989. Finally, the spacecraft would have tried to discover how Triton’s surface remains so dynamic and young.</p>
<p>Ultimately, Trident and IVO lost out to the Venus missions. It would have been fascinating to once again explore the outer reaches of the solar system, or see the colossal volcanoes of Io. But Venus is a fascinating planet, with mysteries and potential all of its own.</p><img src="https://counter.theconversation.com/content/162192/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ashley Spindler does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>When two missions to Venus were announced, two others missed out.Ashley Spindler, STFC Innovation Fellow, University of HertfordshireLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1523702020-12-21T17:13:00Z2020-12-21T17:13:00ZThe ‘Christmas Star’ appears again: Jupiter and Saturn align in the ‘great conjunction’ on Dec. 21, 2020<figure><img src="https://images.theconversation.com/files/376184/original/file-20201221-15-1sejjhd.jpg?ixlib=rb-1.1.0&rect=40%2C62%2C2690%2C1931&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">On Dec. 21, Jupiter and Saturn will be so close together they will almost appear to be touching.</span> <span class="attribution"><span class="source">(Unsplash)</span></span></figcaption></figure><p>Jupiter and Saturn lined up on Dec. 21, so close together that they appeared as one bright shining star. Many referred to it as the “Christmas Star.” It’s the <a href="https://www.scientificamerican.com/article/jupiter-and-saturns-great-conjunction-is-the-best-in-800-years-heres-how-to-see-it/">closest the two planets have appeared together in about 800 years</a>, and won’t occur again until 2080.</p>
<p>This conjunction of Jupiter and Saturn may have an even closer tie to the Biblical story of the birth of Jesus Christ than its occurrence so close to Christmas this year. As noted by Johannes Kepler in the 17th century, a similar conjunction occurred in 7 BCE and could be the astronomical origin of the Star of Bethlehem that guided the wise men.</p>
<p>But there are notable differences between the two events, and the full story has several interesting ties to the history of astronomy, starting with the origins of the word “planet,” which <a href="https://www.etymonline.com/word/planet">comes from the Greek word meaning “wanderer.”</a></p>
<p>The planets have always been recognizable to astronomers, not only because they are relatively bright points of light among the stars, but because of their unique wandering nature. This posed a problem to ancient astronomers, which lasted more than 2,000 years and was only resolved during the Scientific Revolution.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/376231/original/file-20201221-15-dgnu5g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Sunset view of planets" src="https://images.theconversation.com/files/376231/original/file-20201221-15-dgnu5g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/376231/original/file-20201221-15-dgnu5g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=406&fit=crop&dpr=1 600w, https://images.theconversation.com/files/376231/original/file-20201221-15-dgnu5g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=406&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/376231/original/file-20201221-15-dgnu5g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=406&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/376231/original/file-20201221-15-dgnu5g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=511&fit=crop&dpr=1 754w, https://images.theconversation.com/files/376231/original/file-20201221-15-dgnu5g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=511&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/376231/original/file-20201221-15-dgnu5g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=511&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A graphic made from a simulation program, showing a view of the 2020 great conjunction through the naked eye just after sunset at approximately 5:15 p.m. (EST) on Dec. 21.</span>
<span class="attribution"><span class="source">(NASA)</span></span>
</figcaption>
</figure>
<h2>The motion of the planets</h2>
<p>As the Earth spins on its axis every 24 hours, the sun, moon, stars and planets all appear to move across our sky, rising in the East and setting in the West. But because the planets orbit the sun, all travelling in counterclockwise directions when viewed from above the North Pole, to us on Earth the sun and planets all appear to move with respect to the background stars. </p>
<p>As Earth moves around the sun, the sun in turn appears to move slowly to the East, by about a degree each day as it travels through the <a href="https://earthsky.org/space/what-is-the-ecliptic">Zodiac constellations</a>. Mercury and Venus move from one side of the sun to the other as they circle it. And the outer planets of the solar system — Mars, Jupiter and Saturn are visible to the naked eye — appear to move East through the stars as they orbit the sun.</p>
<p>But a peculiar thing happens to the positions of the outer planets when the Earth passes between them and the sun: They appear to briefly reverse direction, and travel West, against the background stars. This <a href="https://earthsky.org/space/what-is-retrograde-motion">apparent retrograde motion</a> is caused by a parallax shift that occurs for the same reason your thumb hops back and forth if you hold it fixed in front of your face and wink first with one eye, then the other; it’s an optical illusion caused by a shift in our perspective.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/376170/original/file-20201221-17-ww488o.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/376170/original/file-20201221-17-ww488o.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/376170/original/file-20201221-17-ww488o.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=623&fit=crop&dpr=1 600w, https://images.theconversation.com/files/376170/original/file-20201221-17-ww488o.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=623&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/376170/original/file-20201221-17-ww488o.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=623&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/376170/original/file-20201221-17-ww488o.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=783&fit=crop&dpr=1 754w, https://images.theconversation.com/files/376170/original/file-20201221-17-ww488o.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=783&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/376170/original/file-20201221-17-ww488o.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=783&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Earth-centred model of planetary motions could explain why other planets sometimes appeared to be moving backwards. Nicolaus Copernicus proposed a sun-centred theory in 1543.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Ptolemaic_system.png">(Muhammad/Wikimedia)</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>While the Ancient Greeks had considered this explanation of the retrograde motions of planets, they mainly preferred an alternate, <a href="http://galileo.rice.edu/sci/theories/ptolemaic_system.html">Earth-centred model</a> in which the planets move around a fixed Earth on small circular orbits, the centres of which went around larger, Earth-centred circles. Thus, as the planets orbited an empty point in space while that point went around the Earth, the planets would occasionally stop and move backwards in their motion against the background stars. </p>
<p>Astronomers mainly described the solar system in this Earth-centred way until Nicolaus Copernicus proposed a <a href="https://earthobservatory.nasa.gov/features/OrbitsHistory">sun-centred theory in 1543</a>. Copernicus’s theory didn’t do any better job of describing planetary motion than the Earth-centred models did, but the idea gained traction.</p>
<p>The German 17th-century astronomer Johannes Kepler eventually found the key to describing planetary motion in a sun-centred system. Rather than orbiting the sun in circles, Kepler found the <a href="https://solarsystem.nasa.gov/resources/310/orbits-and-keplers-laws/">planets moved in ellipses</a>, a distinction that allowed him to precisely predict their observed positions. </p>
<h2>Christmas Star</h2>
<p>A conjunction is said to happen when two astronomical objects pass each other due to their movement along the direction of the stars’ daily rotation. Since solar system objects do not all move within precisely the same plane, conjunctions can sometimes happen with a wide separation. Since Jupiter orbits the sun every 11.9 years while Saturn’s orbit takes 29.5 years, it happens that a conjunction of Jupiter and Saturn — called a “great conjunction” due to its rarity — occurs roughly every 20 years.</p>
<figure class="align-center ">
<img alt="Night sky with two bright planets." src="https://images.theconversation.com/files/376190/original/file-20201221-23-1y3eslv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/376190/original/file-20201221-23-1y3eslv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/376190/original/file-20201221-23-1y3eslv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/376190/original/file-20201221-23-1y3eslv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/376190/original/file-20201221-23-1y3eslv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/376190/original/file-20201221-23-1y3eslv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/376190/original/file-20201221-23-1y3eslv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Conjunction of Jupiter (top) and Venus (bottom) in 2015.</span>
<span class="attribution"><span class="source">(Mike Pennington)</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Most great conjunctions are not particularly notable. But occasionally, like this year, Jupiter and Saturn cross paths so close to each other that they can be barely distinguishable to the naked eye. Or sometimes the two planets cross paths when they are opposite the sun, so their apparent retrograde motion results in a triple conjunction, as was the case in 7 BCE.</p>
<p>In 1604, while he was working in Prague, <a href="https://www.nature.com/articles/462987a">Kepler observed the tight arrangement of three planets — Mars, Saturn and Jupiter — and a bright new star, a supernova, that would slowly fade over the course of a year</a>. This occurrence inspired him to consider a similar set of events that might have led the wise men to Bethlehem in time for Jesus Christ’s birth. </p>
<p>Knowing that Herod the Great had died in 4 BCE, he placed the birth of Christ before that date. And using his knowledge of planetary motion, he found that Jupiter and Saturn underwent a triple conjunction in 7 BCE, that conjunctions of Mars with each planet in 6 BCE were shortly followed by conjunctions of the planets with the sun. Kepler <a href="http://articles.adsabs.harvard.edu//full/1937JRASC..31..417B/0000421.000.html">suggested that these solar conjunctions aligned with the conception of Christ and that the wise men arrived the following year to witness Christ’s birth beneath the Star of Bethlehem</a>.</p>
<h2>Significance of the great conjunction</h2>
<p>On Dec. 21 of this year, Jupiter and Saturn were only one-tenth of a degree apart, well within the field of any telescope’s view. With this year’s event, it is worth keeping in mind the historical significance previous conjunctions have had. </p>
<p>Kepler’s fascination with planetary motion led, only a handful of years later, to his discovery that planets follow elliptical paths around the sun. And Kepler’s discovery would, before the end of that century, <a href="https://www.pbs.org/wgbh/nova/newton/principia.html">inspire Newton’s work on his most important contribution</a>, the great <em>Philosophiae Naturalis Principia Mathematica</em>, where he laid down his ideas on the law of gravity, and which forever changed the world of science. </p>
<p>Without fear of exaggeration, it’s possible to link the wandering motion of the planets — never more clearly on display than when we can simultaneously see Saturn’s rings and the Galilean moons of Jupiter through a telescope — with the discovery that Earth is a planet within a solar system in which motions are dominated by a universal gravitation that acts between all massive bodies.</p>
<p><em>This is a corrected version of a story originally published on Dec. 21. The earlier story said a conjunction aligned with Immaculate Conception instead of the conception of Christ.</em></p><img src="https://counter.theconversation.com/content/152370/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Daryl Janzen does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>From the birth of Jesus Christ to Newton’s discovery of gravity, great conjunctions of Jupiter and Saturn have many notable connections in human history.Daryl Janzen, Observatory Manager and Instructor, Astronomy, University of SaskatchewanLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1522242020-12-18T13:26:06Z2020-12-18T13:26:06ZWhat you need to know about this year’s winter solstice and the great conjunction<p><em>Editor’s note: Dr. William Teets is the director of Vanderbilt University’s Dyer Observatory. In this interview, he explains what does and doesn’t happen during the winter solstice on Dec. 21. Another cosmic phenomenon is also going to occur on the same day called “<a href="https://www.nasa.gov/feature/the-great-conjunction-of-jupiter-and-saturn/">the great conjunction</a>,” where Saturn and Jupiter, both of which can be seen with the naked eye, will appear extremely close to one another.</em></p>
<h2>What happens on the winter solstice?</h2>
<p>The winter solstice this year happens on Dec. 21. This is when the Sun appears the lowest in the Northern Hemisphere sky and is at its farthest southern point over Earth – directly over the Tropic of Capricorn. For folks living at 23.5 degrees south latitude, not only does this day mark their summer solstice, but they also see the Sun directly over them at local noon. After that, the Sun will start to creep back north again. </p>
<p><img src="https://cdn.theconversation.com/static_files/files/1401/sun_over_here_2.gif?1608652248"></p>
<p>The sequence of images below shows the path of the Sun through the sky at different times of the year. You can see how the Sun is highest in the Northern Hemisphere sky in June, lowest in December, and halfway in between these positions in March and September during the equinoxes.</p>
<p><img src="https://cdn.theconversation.com/static_files/files/1389/Comp_1.gif?1608247458"></p>
<h2>The winter solstice is the shortest day in the Northern Hemisphere but not the day with the latest sunrise and earliest sunset. How is that possible?</h2>
<p>The winter solstice doesn’t coincide with the latest sunrise or the earliest sunset. Those actually occur about two weeks before and two weeks after the winter solstice. This is because we are changing our distance from the sun due to our elliptical, not circular, orbit, which changes the speed at which we orbit. </p>
<p>If you were to look at where the Sun is at exactly the same time of day over different days of the year, you would see that it’s not always in the same spot. Yes, the Sun is higher in the summer and lower in the winter, but it also moves from side to side of the average noontime position, which also plays a role in when the Sun rises and sets. </p>
<p>One should also keep in mind that the seasons are due to the Earth’s axial tilt, not our distance from the Sun. Believe it or not, we are closest to the Sun in January.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/375778/original/file-20201217-23-sp6uaa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Analemma of the sun over Callanish, Scotland." src="https://images.theconversation.com/files/375778/original/file-20201217-23-sp6uaa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/375778/original/file-20201217-23-sp6uaa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=693&fit=crop&dpr=1 600w, https://images.theconversation.com/files/375778/original/file-20201217-23-sp6uaa.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=693&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/375778/original/file-20201217-23-sp6uaa.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=693&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/375778/original/file-20201217-23-sp6uaa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=870&fit=crop&dpr=1 754w, https://images.theconversation.com/files/375778/original/file-20201217-23-sp6uaa.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=870&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/375778/original/file-20201217-23-sp6uaa.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=870&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A photograph of the position of the Sun, taken at the same time on different days throughout the year, shows a figure-eight pattern known as an analemma. This photo was taken in Callanish, Scotland.</span>
<span class="attribution"><a class="source" href="https://apod.nasa.gov/apod/ap180923.html">Giuseppe Petricca, NASA</a></span>
</figcaption>
</figure>
<h2>What is ‘the great conjunction’?</h2>
<p><img src="https://cdn.theconversation.com/static_files/files/1387/travelling.gif?1608244561"></p>
<p>Saturn and Jupiter have appeared fairly close together in our sky throughout the year. But on Dec. 21, Saturn and Jupiter will appear so close together that some folks may have a difficult time seeing them as two objects. </p>
<p>If you have a pair of binoculars, you’ll easily be able to spot both planets. In even a small telescope, you’d see both planets at the same time in the same field of view, which is really unheard of. That’s what makes this conjunction so rare. Jupiter and Saturn appear to meet up about every 20 years. Most of the time, however, they’re not nearly as close together as we’re going to see them on Monday, Dec. 21. </p>
<p>For a comparison, there was a great conjunction back in 2000, but the two planets were separated by about two full-Moon widths. This year, the orbits will bring them to where they appear to be about one-fifth of a full-Moon diameter. </p>
<p><img src="https://cdn.theconversation.com/static_files/files/1388/december.gif?1608244618"></p>
<p>We have been encouraging folks to go out and look at these planets using just their eyes between now and Dec. 21. You’ll actually be able to see how much they appear to move over the course of a single day. </p>
<p>The next time they will get this close together in our sky won’t be for another 60 years, so this is going to be a once-in-a-lifetime event for many people. In fact, the last time they got this close together was in the year <a href="https://whenthecurveslineup.com/2020/02/20/1623-the-great-conjunction-of-jupiter-and-saturn/">1623</a>, but it was really difficult, if not impossible, to see them then because they appeared much closer to the Sun and set soon after it. Go back another 400 years to <a href="https://thehill.com/changing-america/opinion/529835-the-great-conjunction-of-jupiter-and-saturn-the-dawn-of-a-new-era">1226</a> and this would have been the last time that we would have had a good view of this type of conjunction.</p>
<h2>What advice would you give to people who want to see the great conjunction?</h2>
<p>If weather permits at <a href="https://dyer.vanderbilt.edu/">Dyer Observatory</a>, we’ll be streaming a live view of the conjunction from one of the observatory’s telescopes, and I’ll be available to answer questions. Even if you don’t have a telescope or a pair of binoculars, definitely go out and check out this very rare alignment with your own eyes. Remember that they set soon after sunset, so be ready to view right at dusk!</p>
<p>[<em>Understand new developments in science, health and technology, each week.</em> <a href="https://theconversation.com/us/newsletters/science-editors-picks-71/?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=science-understand">Subscribe to The Conversation’s science newsletter</a>.]</p><img src="https://counter.theconversation.com/content/152224/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>William Teets does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The 2020 winter solstice is also when Saturn and Jupiter appear closest to each other for 60 years, Here’s what you need to know about both the events.William Teets, Acting Director and Astronomer, Dyer Observatory, Vanderbilt UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1463582020-09-18T11:50:53Z2020-09-18T11:50:53ZThe four most promising worlds for alien life in the solar system<figure><img src="https://images.theconversation.com/files/358659/original/file-20200917-14-1qdn4n4.jpg?ixlib=rb-1.1.0&rect=36%2C36%2C1238%2C782&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">NASA's Curiosity Rover takes a selfie on Mars in June, 2018.</span> <span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA22486">NASA/JPL-Caltech/MSSS</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>The Earth’s biosphere contains all the known ingredients necessary for life as we know it. Broadly speaking these are: liquid water, at least one source of energy, and an inventory of biologically useful elements and molecules.</p>
<p>But the recent discovery of possibly biogenic phosphine <a href="https://theconversation.com/venus-could-it-really-harbour-life-new-study-springs-a-surprise-145981">in the clouds of Venus</a> reminds us that at least some of these ingredients exist elsewhere in the solar system too. So where are the other most promising locations for extra-terrestrial life?</p>
<h2>Mars</h2>
<p>Mars is one of the most Earth-like worlds in the solar system. It has a 24.5-hour day, polar ice caps that expand and contract with the seasons, and a large array of surface features that were sculpted by water during the planet’s history.</p>
<figure class="align-center ">
<img alt="Red planet Mars in space with polar ice caps visible" src="https://images.theconversation.com/files/358594/original/file-20200917-24-fwl0h9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/358594/original/file-20200917-24-fwl0h9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/358594/original/file-20200917-24-fwl0h9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/358594/original/file-20200917-24-fwl0h9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/358594/original/file-20200917-24-fwl0h9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/358594/original/file-20200917-24-fwl0h9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/358594/original/file-20200917-24-fwl0h9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Mars has polar ice caps.</span>
<span class="attribution"><a class="source" href="https://upload.wikimedia.org/wikipedia/commons/0/02/OSIRIS_Mars_true_color.jpg">ESA & MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA)</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>The detection of <a href="https://www.sciencemag.org/news/2018/07/liquid-water-spied-deep-below-polar-ice-cap-mars">a lake beneath</a> the southern polar ice cap and methane in the Martian atmosphere (which varies with the seasons and even the <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL083800">time of day</a>) make Mars a very interesting candidate for life. Methane is significant as it can be produced by biological processes. But the actual source for the methane on Mars is not yet known.</p>
<p>It is possible that life may have gained a foothold, given the <a href="https://advances.sciencemag.org/content/4/6/eaar3330">evidence</a> that the planet once had a much more benign environment. Today, Mars has a very thin, dry atmosphere comprised almost entirely of carbon dioxide. This offers scant protection from solar and cosmic radiation. If Mars has managed to retain some reserves of water beneath its surface, it is not impossible that life may still exist. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/life-on-mars-europe-commits-to-groundbreaking-mission-to-bring-back-rocks-to-earth-128328">Life on Mars? Europe commits to groundbreaking mission to bring back rocks to Earth</a>
</strong>
</em>
</p>
<hr>
<h2>Europa</h2>
<p>Europa was discovered by Galileo Galilei in 1610, along with Jupiter’s three other larger moons. It is slightly smaller than Earth’s moon and orbits the gas giant at a distance of some 670,000km once every 3.5 days. Europa is constantly squeezed and stretched by the competing gravitational fields of Jupiter and the other <a href="https://www.universetoday.com/44796/galilean-moons/">Galilean moons</a>, a process known as tidal flexing. </p>
<p>The moon is believed to be a geologically active world, like the Earth, because the strong tidal flexing heats its rocky, metallic interior and keeps it partially molten.</p>
<figure class="align-center ">
<img alt="Jupiter's white with brown streaks moon Europa in space," src="https://images.theconversation.com/files/358637/original/file-20200917-14-1hfzvc8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/358637/original/file-20200917-14-1hfzvc8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=444&fit=crop&dpr=1 600w, https://images.theconversation.com/files/358637/original/file-20200917-14-1hfzvc8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=444&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/358637/original/file-20200917-14-1hfzvc8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=444&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/358637/original/file-20200917-14-1hfzvc8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=557&fit=crop&dpr=1 754w, https://images.theconversation.com/files/358637/original/file-20200917-14-1hfzvc8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=557&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/358637/original/file-20200917-14-1hfzvc8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=557&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Europa’s icy surface is a good sign for alien hunters.</span>
<span class="attribution"><a class="source" href="https://photojournal.jpl.nasa.gov/catalog/PIA19048">NASA/JPL-Caltech/SETI Institute</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>The surface of Europa is a vast expanse of water ice. Many scientists think that beneath the frozen surface is a layer of liquid water – a global ocean – which is prevented from freezing by the heat from flexing and which maybe over 100km deep. </p>
<p>Evidence for this ocean includes geysers erupting through <a href="https://www.nature.com/articles/s41550-019-0933-6">cracks in the surface ice</a>, a <a href="http://ffden-2.phys.uaf.edu/webproj/212_spring_2015/Justin_Long/Justin_Long/magnetic.html">weak magnetic field</a> and <a href="https://www.sciencedirect.com/science/article/abs/pii/S0019103599961870?via%3Dihub">chaotic terrain</a> on the surface, which could have been deformed by ocean currents swirling beneath. This icy shield insulates the subsurface ocean from the extreme cold and vacuum of space, as well as Jupiter’s ferocious radiation belts.</p>
<p>At the bottom of this ocean world it is conceivable that we might find <a href="https://oceanservice.noaa.gov/facts/vents.html">hydrothermal vents</a> and ocean floor volcanoes. On Earth, such features often support very rich and diverse ecosystems.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/europa-there-may-be-life-on-jupiters-moon-and-two-new-missions-will-pave-the-way-for-finding-it-122551">Europa: there may be life on Jupiter's moon and two new missions will pave the way for finding it</a>
</strong>
</em>
</p>
<hr>
<h2>Enceladus</h2>
<p>Like Europa, <a href="https://solarsystem.nasa.gov/moons/saturn-moons/enceladus/in-depth/">Enceladus</a> is an ice-covered moon with a subsurface ocean of liquid water. Enceladus orbits Saturn and first came to the attention of scientists as a potentially habitable world following the <a href="https://solarsystem.nasa.gov/resources/806/bursting-at-the-seams-the-geyser-basin-of-enceladus/">surprise discovery</a> of enormous geysers near the moon’s south pole.</p>
<p>These jets of water escape from large cracks on the surface and, given Enceladus’ weak gravitational field, spray out into space. They are clear evidence of an underground store of liquid water.</p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"906891543780323328"}"></div></p>
<p>Not only was water detected in these geysers but also an array of organic molecules and, crucially, tiny grains of rocky silicate particles that can only be present if the sub-surface ocean water was in physical contact with the rocky ocean floor at a <a href="https://solarsystem.nasa.gov/missions/cassini/science/enceladus/">temperature of at least 90˚C</a>. This is very strong evidence for the existence of hydrothermal vents on the ocean floor, providing the chemistry needed for life and localised sources of energy. </p>
<h2>Titan</h2>
<p>Titan is the largest moon of Saturn and the only moon in the solar system with a substantial atmosphere. It contains a thick orange haze of complex organic molecules and a methane weather system in place of water – complete with seasonal rains, dry periods and surface sand dunes created by wind.</p>
<figure class="align-center ">
<img alt="Yellow/orange moon Titan in space" src="https://images.theconversation.com/files/358645/original/file-20200917-16-17xgwya.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/358645/original/file-20200917-16-17xgwya.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/358645/original/file-20200917-16-17xgwya.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/358645/original/file-20200917-16-17xgwya.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/358645/original/file-20200917-16-17xgwya.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/358645/original/file-20200917-16-17xgwya.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/358645/original/file-20200917-16-17xgwya.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Titan’s atmosphere makes it look like a fuzzy orange ball.</span>
<span class="attribution"><a class="source" href="https://photojournal.jpl.nasa.gov/catalog/PIA14602">NASA/JPL-Caltech/Space Science Institute</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>The atmosphere consists mostly of nitrogen, an important chemical element used in the construction of proteins in all known forms of life. Radar observations have detected the presence of <a href="https://theconversation.com/titan-first-global-map-uncovers-secrets-of-a-potentially-habitable-moon-of-saturn-126985">rivers and lakes</a> of liquid methane and ethane and possibly the presence of cryovolcanoes – volcano-like features that erupt liquid water rather than lava. This suggests that Titan, like Europa and Enceladus, has a sub-surface reserve of liquid water.</p>
<p>At such an enormous distance from the Sun, the surface temperatures on Titan are a frigid -180˚C – way too cold for liquid water. However, the bountiful chemicals available on Titan has raised speculation that lifeforms – potentially with fundamentally different chemistry to terrestrial organisms – <a href="https://www.space.com/8547-strange-discovery-titan-leads-speculation-alien-life.html">could exist</a> there.</p><img src="https://counter.theconversation.com/content/146358/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gareth Dorrian does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The clouds of Venus may harbour alien life. But where else?Gareth Dorrian, Post Doctoral Research Fellow in Space Science, University of BirminghamLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1303182020-01-29T13:00:18Z2020-01-29T13:00:18ZCurious Kids: why are some planets surrounded by rings?<figure><img src="https://images.theconversation.com/files/311372/original/file-20200122-117933-hxjvkf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Saturn is one of a few planets in our solar system surrounded by rings.</span> <span class="attribution"><span class="source">Vadim Sadovski/Shutterstock/Elements of this image furnished by NASA</span></span></figcaption></figure><p><em>Curious Kids is a series for children in which we ask experts to answer questions from kids.</em></p>
<p><strong>Why do Uranus and other planets have rings around them? (Lesedi, 6, Soweto)</strong></p>
<p>For a very long time, Saturn was thought to be the only planet in our solar system with rings. The rings around Saturn were discovered by an astronomer called <a href="https://www.britannica.com/biography/Galileo-Galilei">Galileo Galilei</a> nearly 400 years ago. He used a very simple telescope that he constructed himself from lenses and pointed it at the planets in the night sky. One of the first objects he looked at was Saturn. At first he thought that Saturn had two large moons on either side of the planet because his telescope wasn’t very good and only produced very blurry images.</p>
<p>Since then, astronomers – who study the universe and everything in it, like planets – have used bigger and better telescopes to find rings around all of the outer gas giant planets: Jupiter, Saturn, Neptune and Uranus. These planets, unlike others in our system, consist largely of gas. </p>
<p>We’re not sure how the rings work or how they formed, but there are a few theories.</p>
<h2>Different theories</h2>
<p>The first <a href="https://www.universetoday.com/127197/127197/">theory</a> states that the rings formed at the same time as the planet. Some particles of gas and dust that the planets are made of were too far away from the core of the planet and could not be squashed together by gravity. They remained behind to form the ring system. </p>
<p>The second <a href="https://www.planetsforkids.org/why-does-saturn-have-rings.html">theory</a>, and my personal favourite, is that the rings were formed when two of the moons of the planet, which had formed at the same time as the planet, somehow got disturbed in their orbits and eventually crashed into each other (an orbit is the circular path that the moon travels on around the planet). The stuff that was left behind in this huge smash could not come together again to form a new moon. Instead, it spread out into the ring systems we see today.</p>
<p>Since we don’t have the answers yet, we keep exploring and testing different theories.</p>
<p>What we do know is that the rings around the various planets are all slightly different from one another, but they all share some characteristics too.</p>
<p>First, they are all much wider than they are thick. The rings of Saturn, for example, are about <a href="https://solarsystem.nasa.gov/planets/saturn/in-depth/">280,000km wide</a> (stretching away from the planet) but only 200 metres thick. That’s like having a normal pancake on your plate for breakfast that is 14km wide.</p>
<p>The other thing that all rings systems share is that they are all made of small particles of ice and rock. The smallest of these particles are no bigger than dust grains, while the largest of the particles are about 20 metres in diameter – about the size of a school hall. All the rings around the planets also contain gaps that are sometimes many kilometres wide and at first nobody could figure out why. We later learned that the gaps were caused by small <a href="https://www.universetoday.com/849/gaps-in-saturns-rings/">moons</a> that had gobbled up all the material in that particular part of the ring system.</p>
<p>The biggest difference between the rings of Saturn and the other gas giant planets is that the particles that make up the rings of Saturn are very good at reflecting the light from the sun back towards the Earth. That means they appear to be very bright, which is why we can see the rings from Earth using a normal telescope. The extremely large number of particles trapped in the rings of Saturn also make the rings much bigger and wider; that’s another reason they’re easier to see than the rings of the other gas giant planets.</p>
<p>The particles that make up the rings of Uranus and Neptune contain elements that were darkened by the sun. These dark particles look very similar to pieces of coal or charcoal like you’d use to make a fire. This makes them much more difficult to see because they don’t reflect as much of the sun’s light back to us.</p>
<h2>New discoveries</h2>
<p>This is an exciting time for astronomy. More and more satellites and space probes are being launched from all over the <a href="https://www.planetary.org/explore/space-topics/space-missions/missions-beyond-mars.html">world</a>, which allows us to investigate the outer planets of our solar system. That means astronomers will have the chance to study these rings – and one day, hopefully, we’ll be able to answer all of your questions and more.</p>
<p><em>Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question to africa-curiouskids@theconversation.com. Please tell us your name, age, and which city you live in. We won’t be able to answer every question but we will do our best.</em></p><img src="https://counter.theconversation.com/content/130318/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dr Rudi Kuhn does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>We’re not sure how the rings work or how they formed, but there are a few theories.Dr Rudi Kuhn, SALT Astronomer, South African Astronomical ObservatoryLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1241582019-12-16T13:43:04Z2019-12-16T13:43:04ZPlanetary confusion – why astronomers keep changing what it means to be a planet<figure><img src="https://images.theconversation.com/files/300167/original/file-20191104-88409-13vd5ta.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">In 2015, NASA’s New Horizons spacecraft looked back toward the sun and captured this near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto’s horizon. </span> <span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/pluto-s-majestic-mountains-frozen-plains-and-foggy-hazes">NASA/JHUAPL/SwRI</a></span></figcaption></figure><p>As an astronomer, the question I hear the most is why isn’t Pluto a planet anymore? More than 10 years ago, astronomers famously voted to change <a href="https://www.nytimes.com/2006/08/24/science/space/25pluto.html">Pluto’s classification</a>. But the question still comes up. </p>
<p>When I am asked directly if I think Pluto is a planet, I tell everyone my answer is no. It all goes back to the origin of the word “planet.” It comes from the Greek phrase for “wandering stars.” Back in ancient times before the telescope was invented, the mathematician and astronomer Claudius Ptolemy called stars “fixed stars” to distinguish them from the seven wanderers that move across the sky in a very specific way. <a href="https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/modeling-the-cosmos/ancient-greek-astronomy-and-cosmology">These seven objects are the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn</a>.</p>
<p>When people started using the word “planet,” they were referring to those seven objects. Even Earth was not originally called a planet – but the Sun and Moon were. </p>
<p>Since people use the word “planet” today to refer to many objects beyond the original seven, it’s no surprise we argue about some of them.</p>
<p>Although I am trained as an astronomer and I studied more distant objects like stars and galaxies, I have an interest in the objects in our Solar System because I teach several classes on planetary science.</p>
<h2>Asteroids, the first demoted planets</h2>
<p>The word “planet” is used to describe <a href="https://www.skyandtelescope.com/observing/ice-giants-neptune-and-uranus/">Uranus and Neptune</a>, which were discovered in 1781 and 1846 respectively, because they move in the same way that the other “wandering stars” move. Like Saturn and Jupiter, if you look at them through a telescope, they appear bigger than stars, so they were recognized to be more like planets than stars. </p>
<p>Not long after the discovery of Uranus, astronomers discovered additional wandering objects – these were named <a href="https://www.esa.int/About_Us/Welcome_to_ESA/ESA_history/Asteroids_The_discovery_of_asteroids">Ceres, Pallas, Juno and Vesta</a>. At the time they were considered planets, too. Through a telescope they look like pinpoints of light and not disks. With a small telescope, even <a href="http://www.astronomy.com/observing/sky-this-month/2018/08/neptune-at-its-best">distant Neptune appears fuzzier than a star</a>. Even though these other, new objects were called planets at first, astronomers thought they needed a different name since they appear more star-like than planet-like. </p>
<p>William Herschel (who discovered Uranus) is often said to have named them “asteroids” which means “star-like,” but <a href="https://www.skyandtelescope.com/astronomy-news/why-do-we-call-them-asteroids/">recently, Clifford Cunningham</a> claimed that the person who coined that name was Charles Burney Jr., a preeminent Greek scholar. </p>
<p>Today, just like the word “planet,” we use the word “asteroid” differently. Now it refers to objects that are rocky in composition, mostly found between Mars and Jupiter, mostly irregularly shaped, smaller than planets, but bigger than meteoroids. Most people assume there is a strict definition for what makes an object an asteroid. But there isn’t, just like there never was for the word “planet.”</p>
<p>In the 1800s the large asteroids were called planets. Students at the time likely learned that the planets were Mercury, Venus, Earth, Mars, Ceres, Vesta, Pallas, Juno, Jupiter, Saturn, Uranus and, eventually, Neptune. Most books today write that asteroids are different than planets, but there is a <a href="https://arxiv.org/abs/1805.04115">debate among astronomers</a> about whether the term “asteroid” was originally used to mean a small type of planet, rather than a different type of object altogether. </p>
<h2>How are moons different than planets?</h2>
<p>These days, scientists consider properties of these celestial objects to figure out whether an object is a planet or not. For example, you might say that shape is important; planets should be mostly spherical, while asteroids can be lumpy. As astronomers try to fix these definitions to make them more precise, we then create new problems. If we use roundness as an important distinction for objects, what should we call moons? Should moons be considered planets if they are round and asteroids if they are not round? Or are they somehow different from planets and asteroids altogether? </p>
<p>I would argue we should again look to how the word “moon” came to refer to objects that orbit planets. </p>
<p>When astronomers talk about the Moon of Earth, we capitalize the word “Moon” to indicate that it’s a proper name. That is, <a href="http://curious.astro.cornell.edu/observational-astronomy/seti-and-extraterrestrial-life/159-our-solar-system/the-sun/the-solar-system/4-what-are-the-names-of-the-earth-moon-sun-and-solar-system-beginner">the Earth’s moon has the name, Moon</a>. For much of human history, it was the only Moon known, so there was no need to have a word that referred to one celestial body orbiting another. This changed when <a href="http://galileo.rice.edu/sci/observations/jupiter_satellites.html">Galileo discovered four large objects orbiting Jupiter</a>. These are now called Io, Europa, Ganymede and Callisto, the moons of Jupiter. </p>
<p>This makes people think the technical definition of moon is a satellite of another object, and so we call lots of objects that orbit Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Eris, Makemake, Ida and a <a href="https://stardate.org/radio/program/2018-08-28">large number of other asteroids</a> moons. When you start to look at the variety of moons, some, like Ganymede and Titan, are larger than Mercury. Some are similar in size to the object they orbit. Some are small and irregularly shaped, and some have odd orbits. </p>
<p>So they are not all just like Earth’s Moon. If we try to fix the definition for what is a moon and how that differs from a planet and asteroid, we are likely going to have to reconsider the classification of some of these objects, too. You can argue that Titan has more properties in common with the planets than Pluto does, for example. You can also argue that every single particle in Saturn’s rings is an individual moon, which would mean that Saturn has billions upon billions of moons.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Family portrait of Pluto’s moons: This composite image shows a sliver of Pluto’s large moon, Charon, and all four of Pluto’s small moons.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/charon-and-the-small-moons-of-pluto">NASA/JHUAPL/SwRI</a></span>
</figcaption>
</figure>
<h2>Planets around other stars</h2>
<p>The most recent naming challenge astronomers face arose when they discovering planets far from our Solar System orbiting around distant stars. These objects have been called extrasolar planets, exosolar planets or <a href="http://exoplanets.org/">exoplanets</a>. </p>
<p>Astronomers are currently searching for <a href="https://arxiv.org/abs/1904.10618">exomoons</a> orbiting exoplanets. Exoplanets are being discovered that have properties unlike the planets in our Solar System, so astronomers have started putting them in categories like “hot Jupiter,” “warm Jupiter,” “super-Earth” and “mini-Neptune.” </p>
<p>Ideas for how planets form also suggest that there are planetary objects that have been flung out of orbit from their parent star. This means there are <a href="https://exoplanets.nasa.gov/resources/28/free-floating-planets-may-be-more-common-than-stars/">free-floating planets</a> not orbiting any star. Should planetary objects that are flung out of a solar system also get ejected from the elite club of planets? </p>
<p>When I teach, I end this discussion with a recommendation. Rather than arguing over planet, moon, asteroid and exoplanet, I think we need to do what Herschel and Burney did and coin a new word. For now, I use “world” in my class, but I do not offer a rigorous definition of what makes something a world and what does not. Instead, I tell my students that all of these objects are of interest to study. </p>
<h2>The Sun was once a planet</h2>
<p>A lot of people seem to feel that scientists wronged Pluto by changing its classification. I look at it that Pluto was only originally called a planet because of an accident; scientists were looking for planets beyond Neptune, and when they found Pluto they called it a planet, even though its observable properties should have led them to call it an asteroid. </p>
<p>As our understanding of this object has grown, I feel like the evidence now leads me to call Pluto something besides planet. There are other scientists who disagree, feeling Pluto still should be classified as a planet. </p>
<p>But remember: The Greeks started out calling the Sun a planet given how it moved on the sky. We now know that the properties of the Sun show it to belong in a very different category from the planets; it’s a star, not a planet. If we can stop calling the Sun a planet, why can’t we do the same to Pluto?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Kepler-90 planets have a similar configuration to our solar system with small planets found orbiting close to their star, and the larger planets found farther away.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/ames/kepler-90-system-planet-sizes">NASA/Ames Research Center/Wendy Stenzel</a></span>
</figcaption>
</figure>
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<p class="fine-print"><em><span>Christopher Palma does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Many people are still upset that Pluto was demoted from being a planet. But definitions of various celestial objects are fairly fluid. So whether it is an asteroid or moon or planet is up for debate.Christopher Palma, Associate Dean for Undergraduate Students and Teaching Professor of Astronomy & Astrophysics, Penn StateLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1276732019-11-25T21:49:08Z2019-11-25T21:49:08ZContrary to recent reports, Jupiter’s Great Red Spot is not in danger of disappearing<figure><img src="https://images.theconversation.com/files/303380/original/file-20191125-74576-1fzpz6q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Measuring in at 10,159 miles (16,350 kilometers) in width (as of April 3, 2017) Jupiter’s Great Red Spot is 1.3 times as wide as Earth. </span> <span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/jpl/pia21774/jupiter-s-great-red-spot-swallows-earth">NASA/JPL-Caltech/SwRI/MSSS/Christopher Go</a></span></figcaption></figure><p>In the last 10 years, but in the last five months in particular, <a href="https://www.sciencetimes.com/articles/22314/20190603/amateur-astronomer-discovers-jupiters-great-red-spot-dying.htm">the press has reported dire warnings</a> that the <a href="https://www.cbc.ca/news/technology/jupiter-great-red-spot-1.5154387">Great Red Spot of Jupiter is dying</a>. However, some astronomers believe, to paraphrase Mark Twain, that the reports of its death are greatly exaggerated, or at least premature.</p>
<p>Robert Hooke, an early British physicist who discovered cells, <a href="https://www.jstor.org/stable/101402">first described the Great Red Spot in 1665</a>. In 1979, when two <a href="https://voyager.jpl.nasa.gov/">Voyager</a> spacecraft flew close by Jupiter, images showed that the spot was a red cloud that rotated as part of a huge vortex several times larger than the Earth.</p>
<p>Concerns for the Great Red Spot’s “health” arose when astronomers realized that the cloud’s area in 1979 was only half of its size in the 1800s, as determined from old photographic plates. Recent images showed <a href="https://doi.org/10.1006/icar.2002.6867">more cloud shrinkage</a>, leading to headlines that the spot could die within 20 years. In spring 2019, astronomers reported that it was “unraveling,” and shedding large “blades” and “flakes” of red clouds. </p>
<p><a href="https://cfd.me.berkeley.edu/people/philip-marcus/">I</a> have been intrigued by the Great Red Spot since 1979, when I viewed the Voyager images only days after NASA processed them. The beautiful structure of this extraordinary atmospheric intrigued me since my <a href="https://cfd.me.berkeley.edu/list-of-publications/">career was evolving from astrophysics to fluid dynamics</a> – the study of how liquids and gases move. What better way to begin exploring the fundamental physics and math of fluid dynamics than to study the Great Red Spot?</p>
<h2>Jupiter’s clouds and vortices</h2>
<p>I believe that the <a href="http://meetings.aps.org/Meeting/DFD19/Session/L13.1">Great Red Spot is in no danger of disappearing</a>. By analyzing the cloud images with computer models that incorporate the physics of how fluids move, my research group at Berkeley was able to <a href="http://doi.org/doi:%2010.1016/j.icarus.2010.06.026">determine the area</a> of the spot. We discovered that the area of the spot cloud is larger than its underlying vortex, the swirling gas that defines it. The question then becomes: Does a decrease in the area of the cloud mean that the vortex itself is shrinking? </p>
<p>It is difficult to determine the relationship between the cloud’s size and the vortex’s size or even how Jovian clouds form and dissipate. Therefore, to understand the health of the spot, planetary scientists need to study the health of its vortex and not its cloud; the cloud’s shrinkage is not a harbinger of death. Based on the spot’s interactions with other vortices my Berkeley group found there is no evidence that that vortex itself has changed its size or intensity.</p>
<p>Jupiter’s atmosphere contains vortices besides the Great Red Spot, some of which are useful for monitoring its health. Some, like this spot, are anticyclones that rotate in the opposite direction of the planet’s spin; others are cyclones that rotate in the same direction as the planet’s spin. Anticyclones appear as bright clouds and so are easily detectable, but cyclones (except at the poles) often have filamentary clouds or no clouds at all. </p>
<p>How do we know that Jovian cyclones exist when clouds are not visible? For more than a century astronomers documented the motions of cloud-covered anticyclones as they slowly drifted across Jupiter. Changes in their speeds were often abrupt and seemed to occur for no reason. However, by assuming that these observable vortices interact with cloud-free (and unobservable) cyclones, we can explain the abrupt changes.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=377&fit=crop&dpr=1 600w, https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=377&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=377&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=474&fit=crop&dpr=1 754w, https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=474&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=474&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A (false color) series of images capturing the repeated flaking of red clouds from the GRS in the Spring of 2019. In the earliest image, the flaking is predominant on the east side of the giant red vortex. The flake then breaks off from the GRS, but a new flake starts to detach in the fifth image.</span>
<span class="attribution"><span class="source">Chris Go</span></span>
</figcaption>
</figure>
<h2>Two simultaneous events that led to flaking</h2>
<p>Anticyclones merge with each other. However, anticyclones repel cyclones. In spring 2019, when the “flaking” was observed, the Great Red Spot was also observed to merge with a series of small clouds (likely small anticyclones) on its northwest side. Such mergers are common; Voyager 1 first observed these and they have subsequently been observed every few months. Typically, small anticyclones are not “digested” immediately, but produce lumps on the spot’s boundary that orbit around it, slowly migrating into the center. </p>
<p>I believe that the shedding of clouds from the spot as “flakes” and “blades” observed in 2019 was due to two simultaneous events: undigested lumps of merged anticyclones traveling along the spot’s boundary and a close encounter with one or more “unobservable” cyclones. </p>
<p>When a large anticyclone and smaller cyclone approach each other before repelling, they create a “stagnation” point near the boundary of the anticyclone where the local winds abruptly change direction, going off approximately perpendicular to their original directions. Think of two fire hoses aimed at each other so that their streams of water collide – the streams momentarily halt at the point of impact (the stagnation point) and then scatter outward. Any cloud or undigested lump on the spot that encounters a stagnation point will similarly shatter and flake away in opposite directions.</p>
<p>The numerical calculations of my Berkeley research group show that the recent observations of cloud shedding can be explained by the collision of undigested red clouds at the edge of the Great Red Spot with stagnation points produced during a close encounter with a cyclone. </p>
<p>Pieces of the red cloud scatter outward from the stagnation point, appearing as flakes and blades. Neither the mergers that created the lumps nor the close encounters with cyclones are unusual by themselves, but it is not that common for them to occur at the same time. However, neither event is a sign of ill health for the Great Red Spot. My colleagues and I believe it will survive for many more years.</p>
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<p class="fine-print"><em><span>Philip Marcus received funding from NASA and NSF. </span></em></p>Little bits of Jupiter’s Great Red Spot seem to be flaking off. Is it a sign of the demise of this enigmatic red cloud, or just a consequence of atmospheric chaos we can’t see from above?Philip Marcus, Professor of Mechanical Engineering, University of California, BerkeleyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1241632019-10-02T20:13:42Z2019-10-02T20:13:42ZWhat moons in other solar systems reveal about planets like Neptune and Jupiter<figure><img src="https://images.theconversation.com/files/295058/original/file-20191001-173407-17sejqu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Exomoons orbiting an exoplanet outside our solar system.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/beautiful-exoplanet-exomoons-orbiting-alien-binary-744849175?src=2ZJlWkTDSoIH0Tay3OeqBw-1-1">Dotted Yeti/Shutterstock.com</a></span></figcaption></figure><p>What is the difference between a planet-satellite system as we have with the Earth and Moon, versus a binary planet – two planets orbiting each other in a cosmic do-si-do?</p>
<p><a href="http://www.astro.ucla.edu/%7Ehansen">I am an astronomer</a> interested in planets orbiting nearby stars, and gas giants – Jupiter, Saturn, Uranus and Neptune in our solar system – are the largest and easiest planets to detect. The crushing pressure within their gassy atmosphere means they are unlikely to be hospitable to life. But the rocky moons orbiting such planets could have conditions that are more welcoming. Last year, astronomers discovered a planet-sized exomoon orbiting another gas giant planet outside our solar system.</p>
<p><a href="https://advances.sciencemag.org/content/5/10/eaaw8665">In a new paper</a>, I argue that this exomoon is really what is called a captured planet.</p>
<h2>Is the first detected ‘exomoon’ really a moon?</h2>
<p>True Earth analogues, that orbit Sun-like stars, are very hard to detect, even with the large <a href="http://keckobservatory.org">Keck telescopes</a>. The task is easier if the host star is less massive. But then the planet has to be closer to the star to be warm enough, and the star’s gravitational tides may trap the planet in a state with a permanent hot side and a permanent cold side. This makes such planets less attractive as a potential location that could harbor life. When gas giants orbiting Sun-like stars have rocky moons, these may be more likely places to find life.</p>
<p>In 2018, two astronomers from Columbia University reported the first tentative <a href="http://doi.org/10.1126/sciadv.aav1784">observation of an exomoon</a> – a satellite orbiting a planet that itself orbits another star. One curious feature was that this exomoon <a href="http://exoplanet.eu/catalog/kepler-1625_b_i/">Kepler-1625b-i</a> was much more massive than any moon found in our solar system. It has a mass similar to Neptune and orbits a planet similar in size to Jupiter. </p>
<p>Astronomers expect moons of planets like Jupiter and Saturn to have masses only a few percent of Earth. But this new exomoon was almost a thousand times larger than the corresponding bodies of our solar system – moons like Ganymede and Titan which orbit Jupiter and Saturn, respectively. It is very difficult to explain the formation of such a large satellite using current models of moon formation.</p>
<p>In a new model I developed, I discuss how <a href="https://advances.sciencemag.org/content/5/10/eaaw8665">such a massive exomoon</a> forms through a different process, wherein it is really a captured planet.</p>
<p>All planets, large and small, start by gathering together asteroid-sized bodies to make a rocky core. At this early stage in the evolution of a planetary system, the rocky cores are still surrounded by a gaseous disk left over from the formation of the parent star. If a core can grow fast enough to reach a mass equivalent to 10 Earths, then it will have the gravitational strength to pull gas in from the surrounding space and grow to the massive size of Jupiter and Saturn. However, this gaseous accumulation is short-lived, as the star is draining away most of the gas in the disk, the dust and gas surrounding a newly formed star.</p>
<p>If there are two cores growing in close proximity, then they compete to capture rock and gas. If one core gets slightly larger, it gains an advantage and can capture the bulk of the gas in the neighborhood for itself. This leaves the second body without any further gas to capture. The increased gravitational pull of its neighbor drags the smaller body into the role of a satellite, albeit a very large one. The former planet is left as a super-sized moon, orbiting the planet that beat it out in the race to capture gas.</p>
<h2>A remnant core as a look back into history</h2>
<p>Viewed in this context, the captured planet is unlikely to be habitable. Growing planetary cores have gaseous envelopes, which make them more like Uranus and Neptune – a mix of rocks, ice and gas that would have become a Jupiter if it had not been so rudely cut off by its larger neighbor. </p>
<p>However, there are other implications that are almost as interesting. Studying the cores of giant planets is very difficult, because they are buried under several hundred Earth masses of hydrogen and helium. Currently, the <a href="https://www.nasa.gov/mission_pages/juno/main/index.html">JUNO mission</a> is attempting to do this for Jupiter. However, studying the properties of this exomoon may enable astronomers to see the naked core of a giant gaseous planet when it is stripped of its gaseous envelope. This can provide a snapshot of what Jupiter may have looked like before it grew to its current enormous size.</p>
<p>This exomoon system Kepler-1625b-i is right at the edge of what is detectable with current technology. There may be many more objects like this that could be uncovered with future improvements in telescope capabilities. As astronomers’ census of exoplanets continues to grow, systems like the exomoon and its host highlight an issue that will become more important as we go forward. This exomoon reveals that the properties of a planet are not solely a consequence of its mass and position, but can depend on its history and the environment in which it formed. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=474&fit=crop&dpr=1 600w, https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=474&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=474&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=596&fit=crop&dpr=1 754w, https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=596&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=596&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Exomoons may reveal secrets about how gas giants like Jupiter formed and what is in their core.</span>
<span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/news/press_kits/juno/science/">JPL/NASA</a></span>
</figcaption>
</figure><img src="https://counter.theconversation.com/content/124163/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Bradley Hansen does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>A giant exomoon hundreds of times the size of Earth is revealing secrets about how giant planets like Jupiter and Saturn formed. They might also help astronomers find planets where life may thrive.Bradley Hansen, Professor of Physics and Astronomy, University of California, Los AngelesLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1225512019-08-28T10:21:49Z2019-08-28T10:21:49ZEuropa: there may be life on Jupiter’s moon and two new missions will pave the way for finding it<figure><img src="https://images.theconversation.com/files/289801/original/file-20190828-184217-tucepa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Enigmatic Europa.</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p>It’s brilliant news. In just over a decade, there will be two spacecraft exploring <a href="https://theconversation.com/the-moon-was-a-first-step-mars-will-test-our-capabilities-but-europa-is-the-prize-37253">one of the most habitable worlds</a> in the solar system – Jupiter’s moon Europa. That’s thanks to a recent announcement by NASA that the orbiter <a href="https://www.jpl.nasa.gov/news/news.php?feature=7480">Europa Clipper</a> has been given the go ahead, scheduled to reach the moon at the beginning of the 2030s.</p>
<p>In April this year, the European Space Agency also <a href="http://sci.esa.int/juice/61286-review-board-gives-juice-the-all-clear/">approved development</a> of the Jupiter Icy Moons Explorer (<a href="http://sci.esa.int/juice/">JUICE</a>), which is currently slated to reach the Jupiter system in 2029.</p>
<p>At the dawn of the space age, it was imagined that all life was ultimately dependent on energy from the sun. The frozen ice ball moons of the outer planets seemed unlikely abodes for any kind of life. Discoveries of thriving ecosystems at the bottom the oceans of Earth, relying on hydrothermal vents for both energy and molecular fuel, changed all that. Now we know that life can thrive in environments that are completely isolated from the sun.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/XotF9fzo4Vo?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
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<p>Europa is thought to be able to harbour simple, microbial life in its liquid, <a href="https://europa.nasa.gov/about-europa/ocean/">internal ocean</a> beneath its icy surface. That is because it has each of three essential prerequisites for life in abundance: a source of biochemically useful molecules, a source of energy and a liquid solvent (water) in which dissolved substances can chemically react with each other. </p>
<p>Europa’s energy comes from a combination of its slightly elliptical orbit about Jupiter and its gravitational interaction with two other moons. This combination of forces subject Europa to a tidal variation in gravity with each orbit, causing it to <a href="https://www.nature.com/articles/325133a0">flex and release heat</a>, which prevents the water from freezing. </p>
<p>Europa’s biochemically useful molecules may come from <a href="https://theconversation.com/building-blocks-of-life-found-among-organic-compounds-on-comet-67p-what-philae-discoveries-mean-45379">impacts by comets</a> or from deep within the moon’s rocky core.</p>
<h2>Ice penetrating radar</h2>
<p>Both Europa Clipper and JUICE will carry special radar instruments to probe beneath Europa’s surface ice. This is not a new technique, radar has been used since the 1970s to find sub-glacial lakes in <a href="http://www.antarcticglaciers.org/glacier-processes/glacial-lakes/subglacial-lakes/">Antarctica</a> and, more recently, on <a href="https://science.sciencemag.org/content/361/6401/490">Mars</a>. </p>
<p>As it happens, Europa may offer an even more suitable environment in which to try this out because the colder ice gets, the more transparent it becomes to radar. Being so far from the sun, typical daytime surface temperatures at Europa are -170°C. The goal at Europa is to establish the depth at which the ice sheet gives way to a global ocean of liquid water. Current models predicts that is at a depth of <a href="https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/">15-25km</a>. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/289823/original/file-20190828-184234-ylqt4i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/289823/original/file-20190828-184234-ylqt4i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=528&fit=crop&dpr=1 600w, https://images.theconversation.com/files/289823/original/file-20190828-184234-ylqt4i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=528&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/289823/original/file-20190828-184234-ylqt4i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=528&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/289823/original/file-20190828-184234-ylqt4i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=664&fit=crop&dpr=1 754w, https://images.theconversation.com/files/289823/original/file-20190828-184234-ylqt4i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=664&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/289823/original/file-20190828-184234-ylqt4i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=664&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Europa Clipper with Jupiter in the background.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<p>However, liquid water may also be found much closer to the surface, which would be easier to get to. Evidence from Hubble Space Telescope images <a href="https://www.nasa.gov/press-release/nasa-s-hubble-spots-possible-water-plumes-erupting-on-jupiters-moon-europa">appear to show</a> plumes of liquid water erupting from the southern hemisphere. Production of these plumes might function something like a volcano, with liquid water welling up from the ocean below. </p>
<p>Water, under sufficient pressure, will force its way through fractures and voids inside the ice, eventually reaching the surface to erupt as geysers. During this process, any liquid water that doesn’t quite make it to the surface may nonetheless fill <a href="https://science.nasa.gov/science-news/science-at-nasa/2011/16nov_europa">voids and cracks</a> within the ice, forming something very similar to the sub-glacial lakes of Mars and Antarctica. </p>
<p>The missions should be able to find these features if they exist. All of this contributes to one of the ultimate goals of these missions, which is to scout out the best location for a future lander which could one day drill through the ice and reach the enigmatic ocean realm beneath.</p>
<h2>Gravity maps</h2>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/289815/original/file-20190828-184196-xifp4j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/289815/original/file-20190828-184196-xifp4j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=776&fit=crop&dpr=1 600w, https://images.theconversation.com/files/289815/original/file-20190828-184196-xifp4j.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=776&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/289815/original/file-20190828-184196-xifp4j.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=776&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/289815/original/file-20190828-184196-xifp4j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=976&fit=crop&dpr=1 754w, https://images.theconversation.com/files/289815/original/file-20190828-184196-xifp4j.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=976&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/289815/original/file-20190828-184196-xifp4j.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=976&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Europa’s interior.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>Spacecraft travelling near the surface of a planet or moon can use slight changes in rocket speed to detect subtle variations in the gravitational field of that object. Such “gravitational anomalies” are caused by changes in the density of material under the planetary surface as the spacecraft flies overhead. </p>
<p>For example, denser rock that one might find in a mountain range can cause the spacecraft to experience a measureable extra gravitational tug. Detection of gravitational anomalies on Earth has been used for many years to identify subterranean structures such as oil fields, metal deposits and the famous dinosaur-destroying impact crater at <a href="https://www.lpi.usra.edu/science/kring/Chicxulub/discovery/">Chixculub</a> in Mexico.</p>
<p>JUICE and Europa Clipper will also be able to detect gravitational anomalies and potentially allow scientists to find interesting features at the bottom of the ocean. A smooth ocean floor with small gravitational anomalies would actually be a <a href="https://www.sciencedirect.com/science/article/abs/pii/S0019103517308527?via%3Dihub">boon to the prospects of life</a>, as it would imply more heat flow from the moon’s interior. </p>
<h2>Getting through the ice</h2>
<p>But to ultimately find life on Europa, we have to get beneath the ice by one day putting a lander on the surface, potentially carrying a submarine. Even if Europa Clipper and JUICE do identify where the ice is thinnest, this will be challenging. </p>
<p>Europa is close to Jupiter, meaning that spacecraft need lots of fuel to change their velocity enough that they can get out of the planet’s massive gravity field and enter into orbit about the moon. JUICE, in fact, will become the first spacecraft to perform this manoeuvre at Ganymede, one of Jupiter’s other moons, and it will use 3,000 kg of fuel to do it on the same journey.</p>
<p>There are also huge amounts of harmful radiation at Jupiter, which can <a href="https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1134214">damage spacecraft</a> in the long run. Europa Clipper will therefore stay in long looping orbits about Jupiter, repeatedly taking it out of the radiation field. It will study Europa by instead performing flybys of the moon.</p>
<p>The lack of substantial atmosphere at Europa poses another problem. It means we can’t slow a lander with heat shields and parachutes. Everything must be done with rockets, requiring yet more fuel. The lack of atmosphere also offers little protection from radiation while the lander is on the surface.</p>
<p>Even if a spacecraft survives a landing, there is the matter of the ice itself. Using a mechanical drill to bore through many miles of super-cold ice, which is as hard as granite, is unlikely. Instead more exotic means of getting through are being considered, such as using <a href="https://www.universetoday.com/140499/archimedes-digging-into-the-ice-on-europa-with-lasers/">lasers</a> or heat from a <a href="https://interestingengineering.com/nuclear-powered-drilling-bot-to-look-for-life-on-europa">nuclear reactor</a> to melt through the ice. </p>
<p>Another consideration is that Europa, currently, is a pristine environment. That means these complex tasks must be done without inadvertently contaminating the ocean with pollutants from the spacecraft, or any terrestrial microbes which may have hitched a ride.</p>
<p>But one way or another, we will get there. The final challenge might then be ensuring that the spacecraft or submarine, having finally reached the ocean, doesn’t get eaten by something swimming around in the deep.</p><img src="https://counter.theconversation.com/content/122551/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gareth Dorrian does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>NASA’s Europa Clipper mission just got the green light - here’s what it could achieve.Gareth Dorrian, Post Doctoral Research Fellow in Space Science, University of BirminghamLicensed as Creative Commons – attribution, no derivatives.