tag:theconversation.com,2011:/id/topics/enceladus-9753/articlesEnceladus – The Conversation2023-06-14T23:03:46Ztag:theconversation.com,2011:article/2077142023-06-14T23:03:46Z2023-06-14T23:03:46ZFor the first time, astronomers have found life-supporting molecules called phosphates on Enceladus<figure><img src="https://images.theconversation.com/files/531898/original/file-20230614-22-z3g0a3.png?ixlib=rb-1.1.0&rect=742%2C233%2C3155%2C1760&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">NASA/JPL/Space Science Institute</span></span></figcaption></figure><p>The search for habitable conditions beyond Earth has just become more interesting with the discovery of biologically available phosphorus from one of Saturn’s moons. Phosphorus is the most elusive of the six crucial elements needed for life.</p>
<p>In research <a href="https://doi.org/10.1038/s41586-023-05987-9">published today in Nature</a>, data from the Cassini spacecraft were used to find phosphorus compounds called phosphates in <a href="https://solarsystem.nasa.gov/news/13021/put-a-ring-on-it/">Saturn’s E ring</a> – one of the fainter outer rings of the planet.</p>
<p>These compounds likely came from the ice volcano (cryovolcano) plumes from the <a href="https://theconversation.com/waterworld-cassini-spots-the-motion-of-enceladuss-ocean-25069">sub-surface liquid water ocean</a> on Saturn’s moon Enceladus.</p>
<h2>A famous moon</h2>
<p>Enceladus seemed like a typical moon of Saturn until the Cassini spacecraft came to take a closer look. <a href="https://theconversation.com/a-look-back-at-cassinis-incredible-mission-to-saturn-before-its-final-plunge-into-the-planet-83226">Arriving at Saturn in 2005</a>, Cassini has been making <a href="https://solarsystem.nasa.gov/news/12892/cassini-10-years-at-saturn-top-10-discoveries/">discovery after discovery</a> that have catapulted Enceladus to one of the top places to look for life beyond Earth.</p>
<p>In particular, we learned Enceladus has a liquid water ocean beneath its icy surface, heated by gravitational tidal forcing – the kind of forcing that produces ocean tides on Earth.</p>
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<a href="https://images.theconversation.com/files/531895/original/file-20230614-19-jl252o.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/531895/original/file-20230614-19-jl252o.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/531895/original/file-20230614-19-jl252o.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=640&fit=crop&dpr=1 600w, https://images.theconversation.com/files/531895/original/file-20230614-19-jl252o.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=640&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/531895/original/file-20230614-19-jl252o.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=640&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/531895/original/file-20230614-19-jl252o.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=805&fit=crop&dpr=1 754w, https://images.theconversation.com/files/531895/original/file-20230614-19-jl252o.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=805&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/531895/original/file-20230614-19-jl252o.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=805&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">The process of organic compounds making their way onto ice grains emitted in plumes from Saturn’s moon Enceladus, where they were detected by NASA’s Cassini spacecraft.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech</span></span>
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<p>This environment is tantalisingly similar to the <a href="https://theconversation.com/origins-of-life-new-evidence-first-cells-could-have-formed-at-the-bottom-of-the-ocean-126228">hydrothermal vents thought by some</a> to be the place where life may have originated on Earth. Such vents certainly host life on Earth today.</p>
<p>Most life on Earth ultimately relies on photosynthesis – generating energy from sunlight. Meanwhile, the ultimate energy source for any life on Enceladus would be the gravity of Saturn producing tides far stronger than the Moon produces on Earth, allowing a liquid water ocean despite the very cold -200°C ice crust surface.</p>
<h2>Easy sampling</h2>
<p>The Enceladus plumes have been called a “gimme” for efforts to sample the oceans of alien worlds. One wouldn’t need to land to collect a sample, nor to then launch to return it for analysis.</p>
<p>An obvious approach to sampling an ice volcano is to simply fly through it. However, this is difficult because the speed at which a space probe would encounter the plume would likely kill most organics.</p>
<p>Instead, the easiest approach is to examine the accumulation of ejected material from Enceladus in Saturn’s E ring, which is what the team did in this latest study. </p>
<p>Using this approach, researchers have previously discovered <a href="https://academic.oup.com/mnras/article/489/4/5231/5573821">complex organic molecules</a> <a href="https://www.nature.com/articles/s41586-018-0246-4">coming from Enceladus</a>. These findings confirmed that the watery environment on Enceladus supports complex chemistry involving nitrogen and oxygen.</p>
<p>However, until now we didn’t know about the availability of phosphorus on Enceladus; in many environments this element is locked in rocks.</p>
<h2>A crucial element</h2>
<p>The discovery of phosphates in Saturn’s E ring suggests phosphates could be available within the oceans of Enceladus at a concentration 100 times higher than in Earth’s oceans.</p>
<p>Phosphorus is crucial for life as we know it, partly because it is a key building block of DNA and RNA, molecules essential to all life on Earth. Phosphate is also vital for a number of other metabolic processes in all life. </p>
<p>Many of the essential components necessary for the emergence of life as we know it have thus been discovered on Enceladus. This puts it at or near the top of lists of places to search for life beyond Earth in our Solar System. </p>
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<a href="https://theconversation.com/humans-are-still-hunting-for-aliens-heres-how-astronomers-are-looking-for-life-beyond-earth-197621">Humans are still hunting for aliens. Here's how astronomers are looking for life beyond Earth</a>
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<p>Nevertheless, this discovery is only the start of the story. For phosphate to form bonds with carbon – this type of bond is found in the backbone of DNA – we need specialised chemistry that’s very dependent on the environment.</p>
<p>We’ll need further study of the chemistry in and under the crust of Enceladus. But a future detection of organic phosphate compounds would be particularly interesting for the potential for life in the moon’s oceans.</p>
<h2>No ‘smoking gun’</h2>
<p>This research is reminiscent of the reported detection of <a href="https://theconversation.com/the-detection-of-phosphine-in-venus-clouds-is-a-big-deal-heres-how-we-can-find-out-if-its-a-sign-of-life-146185">phosphine on Venus</a> in September 2020, which was <a href="https://www.universetoday.com/158983/sofia-fails-to-find-phosphine-in-the-atmosphere-of-venus-but-the-debate-continues/">cast into doubt by later evidence</a>.</p>
<p>However, the detection method is quite different. On Venus the presence of phosphine was proposed by observing the atmosphere from Earth. The phosphates in this study were detected using an instrument orbiting Saturn called a mass spectrometer, which measured the mass of individual compounds found in the ice of the E ring.</p>
<p>To verify the analysis, the authors created a water solution on Earth very similar to the predicted Enceladus ocean.</p>
<p>That said, both detection methods carry a risk of misidentification, where a different molecule that’s not phosphine is actually responsible for the result. </p>
<p>It would be great to have a “smoking gun” for life beyond Earth, but realistically it will instead be a trickle of evidence that grows as we discover more about these environments. </p>
<p>The study published today is one more piece of evidence supporting the fact that Enceladus may be a great location in our search for extraterrestrial life. </p>
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<p><em>Acknowledgements: We thank Prof Steve Benner from The Foundation For Applied Molecular Evolution for his insight and contributions to this article.</em></p><img src="https://counter.theconversation.com/content/207714/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>Phosphorus is the most elusive element crucial for life as we know it – and we now have the first evidence there’s some available in the oceans of Enceladus.Laura McKemmish, Lecturer, UNSW SydneyAlbert Fahrenbach, Senior Lecturer, UNSW SydneyMartin Van Kranendonk, Professor and Director of the Australian Centre for Astrobiology, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2074562023-06-14T15:04:43Z2023-06-14T15:04:43ZAn element essential to life discovered on one of Saturn’s moons, raising hopes of finding alien microbes<figure><img src="https://images.theconversation.com/files/531159/original/file-20230609-25-9jhlr6.jpg?ixlib=rb-1.1.0&rect=17%2C19%2C974%2C358&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Ice particles, with just a trace of phosphates, venting from near Enceladus's south pole, as imaged by Cassini in 2010.</span> <span class="attribution"><a class="source" href="https://photojournal.jpl.nasa.gov/jpeg/PIA17184.jpg">NASA/JPL-Caltech/Space Science Institute</a></span></figcaption></figure><p>Enceladus is the tiny moon of Saturn that seems to have it all. Its icy surface is intricately carved by ongoing geological processes. Its icy shell overlies an internal, liquid ocean. There, chemically charged warm water seeps out of the rocky core onto the ocean floor – potentially providing nourishment for microbial life.</p>
<p>Now, a new study, <a href="https://www.nature.com/articles/s41586-023-05987-9">published in Nature</a>, has uncovered more evidence. It presents the first proof that Enceladus’s ocean contains phosphorus, an element that is essential to life.</p>
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<a href="https://images.theconversation.com/files/531290/original/file-20230612-29-6bcdcp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Cross-cutting ridges and grooves on the surface of Enceladus" src="https://images.theconversation.com/files/531290/original/file-20230612-29-6bcdcp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/531290/original/file-20230612-29-6bcdcp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=431&fit=crop&dpr=1 600w, https://images.theconversation.com/files/531290/original/file-20230612-29-6bcdcp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=431&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/531290/original/file-20230612-29-6bcdcp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=431&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/531290/original/file-20230612-29-6bcdcp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=542&fit=crop&dpr=1 754w, https://images.theconversation.com/files/531290/original/file-20230612-29-6bcdcp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=542&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/531290/original/file-20230612-29-6bcdcp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=542&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">A complicated history of fracturing of the icy crust is apparent in this 80 km wide view of the Samarkand Sulci region of Enceladus.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/Space Science Institute</span></span>
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<p>The <a href="https://theconversation.com/bittersweet-feeling-as-cassini-mission-embarks-on-its-grand-finale-ahead-of-death-plunge-76670">Cassini spacecraft</a>, operated in orbit about Saturn 2004-17 by Nasa and the European Space Agency (Esa), found plumes of ice particles venting from cracks. These penetrate right through the icy shell so that the ocean water at the bottom of each crack is exposed to the vacuum of space, where the lack of confining pressure causes it to bubble and vaporise in the form of plumes. </p>
<p>These plumes provided samples of spray from Enceladus’s internal ocean that were scooped up for analysis by Cassini during several close fly-bys – a bonus that wasn’t anticipated when the mission was initially planned.</p>
<p>Particles analysed during these brief passages through the plumes demonstrated that the ice is contaminated by traces of <a href="https://theconversation.com/nasa-saturn-moon-enceladus-is-able-to-host-life-its-time-for-a-new-mission-76102">simple organic molecules as well as molecular hydrogen and tiny particles of silica</a>. Taken together, these indicate that chemical reactions between water and warm rock take place on the ocean floor, most probably at “<a href="https://oceanexplorer.noaa.gov/facts/mid-ocean-ridge.html">hydrothermal vents</a>” (a fissure releasing heated water) similar to those on Earth.</p>
<p>This is significant. It means Enceladus has all the ingredients for microbial life to sustain itself (in the absence of sunlight). It is in fact the setting considered most likely to have helped life on Earth begin. If it happened on Earth it could have happened inside Enceladus too.</p>
<h2>Missing link</h2>
<p>All life on Earth requires six essential elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur – known collectively by the scarcely pronounceable acronym CHNOPS. Five of these six essential elements were detected in Enceladus plume samples several years ago, but phosphorus had never been found. </p>
<p>Phosphorus is a vital ingredient, because it is needed for the phosphate groups (phosphorus plus oxygen) that link the long chains of <a href="https://www.genome.gov/genetics-glossary/Nucleic-Acids">nucleic acids</a> such as DNA and RNA that store genetic information. It also allows cells to store energy by means of molecules such as <a href="https://www.ncbi.nlm.nih.gov/books/NBK553175/">adenoside triphosphate</a> (ATP for short).</p>
<p>Of course, we don’t know for sure that life inside Enceladus (if it exists) is obliged to use nucleic acids or ATP. However, because the presence of phosphorus is essential for life as we know it, it makes Enceladus a more likely prospect now that we are certain that there is enough phosphorus available there.</p>
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<img alt="Against black space, a diffuse arc which is invisibly small icy particles scattering sunlight. A bright dot within the arc in Enceladus." src="https://images.theconversation.com/files/531170/original/file-20230609-27-zq3ly3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/531170/original/file-20230609-27-zq3ly3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=397&fit=crop&dpr=1 600w, https://images.theconversation.com/files/531170/original/file-20230609-27-zq3ly3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=397&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/531170/original/file-20230609-27-zq3ly3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=397&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/531170/original/file-20230609-27-zq3ly3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=499&fit=crop&dpr=1 754w, https://images.theconversation.com/files/531170/original/file-20230609-27-zq3ly3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=499&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/531170/original/file-20230609-27-zq3ly3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=499&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Enceladus, a tiny dot embedded in Saturn’s, E-ring.</span>
<span class="attribution"><span class="source">NASA/JPL/Space Science Institute</span></span>
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<h2>Canny collecting</h2>
<p>The team found Enceladus’s phosphorus by avoiding the cluttered data collected during the Cassini’s frantically quick zooms through the plumes. Instead, they scoured sparser data accumulated in a more leisurely fashion by Cassini’s Cosmic Dust Analyzer during 15 periods between 2004 and 2008 while Cassini was travelling within one of Saturn’s rings: the “<a href="https://solarsystem.nasa.gov/news/13021/put-a-ring-on-it/">E-ring</a>”. Enceladus travels along this hoop as it orbits.</p>
<p>The E-ring hoop is more than 2,000km thick. About 30% of the ice particles emitted in Enceladus’ plumes end up there, as demonstrated by a <a href="https://webbtelescope.org/contents/media/images/2023/112/01GYJ7H5VSDMPRWX0R0Z6R87EC">recent image from the James Webb Space Telescope</a>, which is the only proof we have that the plumes were still active five years after <a href="https://theconversation.com/cassini-crashes-its-time-for-a-new-mission-to-explore-the-possibility-of-life-on-saturns-moons-84016">the end of the Cassini mission</a>.</p>
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<span class="caption">Lower left: The plume from Enceladus, imaged at a range of more than a billion km by the James Webb (JWST) telescope, accompanied by an artist’s impression.</span>
<span class="attribution"><span class="source">NASA, ESA, CSA, STScI, Leah Hustak (STScI)</span></span>
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<p>Sorting through analyses of nearly a thousand ice particles, which are believed to represent frozen spray from Enceladus, the researchers found nine of them that contained phosphates. This may sound like a slim haul, but it is enough to demonstrate that Enceladus has more than enough dissolved phosphorus in its ocean to permit the functioning of life there.</p>
<p>Indeed, follow-up laboratory experiments suggest that the concentration of dissolved phosphorus in Enceladus’s ocean water may even be hundreds of times greater than in Earth’s oceans.</p>
<p>The team argue that their findings and associated modelling make it likely that any icy moon that grew further from the Sun than the Solar System’s “carbon dioxide snowline” – a location where temperatures during planetary formation were low enough for carbon dioxide to become ice – is likely to contain abundant phosphorus. This condition is met for icy moons at Saturn and beyond, but not at Jupiter. </p>
<p>Jupiter’s distance from the Sun places it beyond the “water-ice snowline” (where water becomes ice), but it is too close to the Sun, and hence too warm, to be beyond the carbon dioxide snowline.</p>
<p>So where does this leave Jupiter’s moon <a href="https://theconversation.com/new-water-plumes-from-jupiters-moon-europa-raise-hopes-of-detecting-microbial-life-66019">Europa</a>, a target for <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">missions</a> due to arrive about ten years from now? </p>
<p>This moon has been widely touted as potentially able to support a more flourishing biosphere than Enceladus because of its larger size and greater store of chemical energy in its rocky interior. The team behind the new study are reticent on this, but their modelling suggests a phosphate concentration in Europa’s internal ocean about a thousand times less than at Enceladus. </p>
<p>To me, that is not a gamechanger, and we should continue to expect Europa to be habitable. But it would be reassuring to find some proof of phosphorus there too.</p>
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Read more:
<a href="https://theconversation.com/the-chemistry-that-could-feed-life-within-saturns-moon-enceladus-study-gives-clue-ahead-of-flyby-49683">The chemistry that could feed life within Saturn's moon Enceladus: study gives clue ahead of flyby</a>
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<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>Five out of the six essential elements required for life on Earth were known to exist on Enceladus. Now the sixth and final one has been found too.David Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed 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/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/1030532018-11-06T11:41:10Z2018-11-06T11:41:10ZColonizing Mars means contaminating Mars – and never knowing for sure if it had its own native life<figure><img src="https://images.theconversation.com/files/242763/original/file-20181029-76411-ioau9b.jpg?ixlib=rb-1.1.0&rect=814%2C0%2C3775%2C2574&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Once people get there, Mars will be contaminated with Earth life.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/multimedia/imagegallery/image_feature_261.html">NASA/Pat Rawlings, SAIC</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>The closest place in the universe where extraterrestrial life might exist is Mars, and human beings are poised to attempt to colonize this planetary neighbor within the next decade. Before that happens, we need to recognize that a very real possibility exists that the first human steps on the Martian surface will lead to a collision between terrestrial life and biota native to Mars.</p>
<p>If the red planet is sterile, a human presence there would create no moral or ethical dilemmas on this front. But if life does exist on Mars, human explorers could easily lead to the extinction of Martian life. <a href="https://scholar.google.com/citations?user=KOrEwdkAAAAJ&hl=en&oi=ao">As an astronomer</a> who explores these questions in my book “<a href="https://press.princeton.edu/titles/11233.html">Life on Mars: What to Know Before We Go</a>,” I contend that we Earthlings need to understand this scenario and debate the possible outcomes of colonizing our neighboring planet in advance. Maybe missions that would carry humans to Mars need a timeout.</p>
<h2>Where life could be</h2>
<p>Life, scientists suggest, has some basic requirements. It could exist anywhere in the universe that has liquid water, a source of heat and energy, and copious amounts of a few essential elements, such as carbon, hydrogen, oxygen, nitrogen and potassium.</p>
<p>Mars qualifies, as do at least two other places in our solar system. Both <a href="https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/">Europa</a>, one of Jupiter’s large moons, and <a href="https://solarsystem.nasa.gov/moons/saturn-moons/enceladus/in-depth/">Enceladus</a>, one of Saturn’s large moons, appear to possess these prerequisites for hosting native biology.</p>
<p>I suggest that how scientists planned the exploratory missions to these two moons provides valuable background when considering how to explore Mars without risk of contamination.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/242558/original/file-20181026-7050-3k87rh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/242558/original/file-20181026-7050-3k87rh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/242558/original/file-20181026-7050-3k87rh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=685&fit=crop&dpr=1 600w, https://images.theconversation.com/files/242558/original/file-20181026-7050-3k87rh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=685&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/242558/original/file-20181026-7050-3k87rh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=685&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/242558/original/file-20181026-7050-3k87rh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=860&fit=crop&dpr=1 754w, https://images.theconversation.com/files/242558/original/file-20181026-7050-3k87rh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=860&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/242558/original/file-20181026-7050-3k87rh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=860&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Cassini shot this false-color image of jets erupting from the southern hemisphere of Enceladus on Nov. 27, 2005.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/cassini/media/saturn_sponge.html">NASA/JPL/Space Science Institute</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Below their thick layers of surface ice, both Europa and Enceladus have global oceans in which 4.5 billion years of churning of the primordial soup may have enabled life to develop and take root. NASA spacecraft have even imaged spectacular geysers ejecting plumes of water out into space from these subsurface oceans.</p>
<p>To find out if either moon has life, planetary scientists are actively developing the <a href="https://europa.nasa.gov/">Europa Clipper mission</a> for a 2020s launch. They also hope to plan future missions that will target Enceladus.</p>
<h2>Taking care to not contaminate</h2>
<p>Since the start of the space age, scientists have taken the threat of biological contamination of other worlds seriously. As early as 1959, NASA held meetings <a href="https://www.nasa.gov/connect/ebooks/when_biospheres_collide_detail.html">to debate the necessity of sterilizing spacecraft</a> that might be sent to other worlds. Since then, all planetary exploration missions have adhered to sterilization standards that balance their scientific goals with limitations of not damaging sensitive equipment, which could potentially lead to mission failures. Today, NASA protocols exist for the <a href="https://sma.nasa.gov/sma-disciplines/planetary-protection">protection of all solar system bodies</a>, including Mars.</p>
<p>Since avoiding the biological contamination of Europa and Enceladus is an extremely well-understood, high-priority requirement of all missions to the Jovian and Saturnian environments, their moons remain uncontaminated.</p>
<p>NASA’s <a href="https://solarsystem.nasa.gov/missions/galileo/overview/">Galileo mission explored Jupiter</a> and its moons from 1995 until 2003. Given Galileo’s orbit, the possibility existed that the spacecraft, once out of rocket propellant and subject to the whims of gravitational tugs from Jupiter and its many moons, could someday crash into and thereby contaminate Europa. </p>
<p>Such a collision might not occur until many millions of years from now. Nevertheless, though the risk was small, it was also real. NASA paid close attention to guidance from the <a href="https://www.nap.edu/initiative/committee-on-planetary-and-lunar-exploration">National Academies’ Committee on Planetary and Lunar Exploration</a>, which noted serious national and international objections to the possible accidental disposal of the Galileo spacecraft on Europa.</p>
<p>To completely eliminate any such risk, on Sept. 21, 2003, NASA used the last bit of fuel on the spacecraft to send it plunging into Jupiter’s atmosphere. At a speed of 30 miles per second, <a href="https://www.nasa.gov/vision/universe/solarsystem/galileo_final.html">Galileo vaporized within seconds</a>.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/xrGAQCq9BMU?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Cassini’s ‘Grand Finale’ ended with the spacecraft burning up in Saturn’s atmosphere.</span></figcaption>
</figure>
<p>Fourteen years later, NASA repeated this protect-the-moon scenario. The <a href="https://solarsystem.nasa.gov/missions/cassini/overview/">Cassini mission orbited and studied Saturn</a> and its moons from 2004 until 2017. On Sept. 15, 2017, when fuel had run low, on instructions from NASA Cassini’s operators deliberately <a href="https://saturn.jpl.nasa.gov/mission/about-the-mission/summary/">plunged the spacecraft into Saturn’s atmosphere</a>, where it disintegrated.</p>
<h2>But what about Mars?</h2>
<p>Mars is the target of <a href="https://mars.nasa.gov/#missions">seven active missions</a>, including two rovers, <a href="https://mars.nasa.gov/programmissions/missions/present/2003/">Opportunity</a> and <a href="https://mars.nasa.gov/msl/mission/mars-rover-curiosity-mission-updates/">Curiosity</a>. In addition, on Nov. 26 NASA’s <a href="https://mars.nasa.gov/insight/">InSight mission</a> is scheduled to land on Mars, where it will make measurements of Mars’ interior structure. Next, with planned 2020 launches, both ESA’s <a href="http://exploration.esa.int/mars/48088-mission-overview/">ExoMars rover</a> and NASA’s <a href="https://mars.nasa.gov/mars2020/">Mars 2020 rover</a> are designed to search for evidence of life on Mars.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/242772/original/file-20181029-76413-otea1r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/242772/original/file-20181029-76413-otea1r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/242772/original/file-20181029-76413-otea1r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=430&fit=crop&dpr=1 600w, https://images.theconversation.com/files/242772/original/file-20181029-76413-otea1r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=430&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/242772/original/file-20181029-76413-otea1r.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=430&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/242772/original/file-20181029-76413-otea1r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=540&fit=crop&dpr=1 754w, https://images.theconversation.com/files/242772/original/file-20181029-76413-otea1r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=540&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/242772/original/file-20181029-76413-otea1r.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=540&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 Curiosity rover was tested under clean conditions on Earth before launch to prevent microbial stowaways.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/msl/msl20100913.html">NASA/JPL-Caltech</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>The good news is that robotic rovers pose little risk of contamination to Mars, since all spacecraft designed to land on Mars are subject to <a href="https://www.nasa.gov/missions/solarsystem/mer_clean.html">strict sterilization procedures before launch</a>. This has been the case since NASA imposed “rigorous sterilization procedures” for the <a href="https://mars.nasa.gov/programmissions/missions/past/viking/">Viking Lander Capsules</a> in the 1970s, since they would directly contact the Martian surface. These rovers likely have an extremely low number of microbial stowaways.</p>
<p>Any terrestrial biota that do manage to hitch rides on the outside of those rovers would have a very hard time surviving the half-year journey from Earth to Mars. The vacuum of space combined with exposure to harsh X-rays, ultraviolet light and cosmic rays would <a href="https://www.nasa.gov/connect/ebooks/when_biospheres_collide_detail.html">almost certainly sterilize the outsides of any spacecraft</a> sent to Mars.</p>
<p>Any bacteria that sneaked rides inside one of the rovers might arrive at Mars alive. But if any escaped, the <a href="https://www.space.com/16903-mars-atmosphere-climate-weather.html">thin Martian atmosphere</a> would offer virtually no protection from high energy, sterilizing radiation from space. Those bacteria would likely be killed immediately. Because of this harsh environment, life on Mars, if it currently exists, almost certainly must be hiding beneath the planet’s surface. Since no rovers have explored caves or dug deep holes, we have not yet had the opportunity to come face-to-drill-bit with any possible Martian microbes.</p>
<p>Given that the exploration of Mars has so far been limited to unmanned vehicles, the planet likely remains free from terrestrial contamination.</p>
<p>But when Earth sends astronauts to Mars, they’ll travel with life support and energy supply systems, habitats, 3D printers, food and tools. None of these materials can be sterilized in the same ways systems associated with robotic spacecraft can. Human colonists will produce waste, try to grow food and use machines to extract water from the ground and atmosphere. Simply by living on Mars, human colonists will contaminate Mars.</p>
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<h2>Can’t turn back the clock after contamination</h2>
<p>Space researchers have developed a careful approach to robotic exploration of Mars and a hands-off attitude toward Europa and Enceladus. Why, then, are we collectively willing to overlook the risk to Martian life of human exploration and colonization of the red planet?</p>
<p>Contaminating Mars isn’t an unforeseen consequence. A quarter century ago, a National Research Council report entitled <a href="https://doi.org/10.17226/12305">“Biological Contamination of Mars: Issues and Recommendations”</a> asserted that missions carrying humans to Mars will inevitably contaminate the planet. </p>
<p>I believe it’s critical that every attempt be made to obtain evidence of any past or present life on Mars well in advance of future missions to Mars that include humans. What we discover could influence our collective decision whether to send colonists there at all.</p>
<p>Even if we ignore or don’t care about the risks a human presence would pose to Martian life, the issue of bringing Martian life back to Earth has serious societal, legal and international implications that deserve discussion before it’s too late. What risks might Martian life pose to our environment or our health? And does any one country or group have the right to risk back contamination if those Martian lifeforms could attack the DNA molecule and thereby put all of life on Earth at risk?</p>
<p>But players both public – NASA, United Arab Emirates’ <a href="https://government.ae/en/more/uae-future/2030-2117">Mars 2117 project</a> – and private – <a href="https://www.spacex.com/mars">SpaceX</a>, <a href="https://www.mars-one.com">Mars One</a>, <a href="https://www.blueorigin.com">Blue Origin</a> – already plan to transport colonists to build cities on Mars. And these missions will contaminate Mars. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/242775/original/file-20181029-76399-1ozr59w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/242775/original/file-20181029-76399-1ozr59w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/242775/original/file-20181029-76399-1ozr59w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=312&fit=crop&dpr=1 600w, https://images.theconversation.com/files/242775/original/file-20181029-76399-1ozr59w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=312&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/242775/original/file-20181029-76399-1ozr59w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=312&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/242775/original/file-20181029-76399-1ozr59w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=392&fit=crop&dpr=1 754w, https://images.theconversation.com/files/242775/original/file-20181029-76399-1ozr59w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=392&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/242775/original/file-20181029-76399-1ozr59w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=392&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Scientists hypothesize that dark narrow streaks were formed by briny liquid water – necessary for life – flowing down the walls of a crater on Mars.</span>
<span class="attribution"><a class="source" href="https://mars.nasa.gov/resources/7488/dark-recurring-streaks-on-walls-of-garni-crater/">NASA/JPL-Caltech/Univ. of Arizona</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p><a href="https://doi.org/10.1126/science.1165243">Some scientists believe they</a> <a href="https://doi.org/10.1126/science.aaq0131">have already uncovered</a> <a href="https://www.nasa.gov/press-release/nasa-finds-ancient-organic-material-mysterious-methane-on-mars">strong evidence for life on Mars</a>, both past and present. If life already exists on Mars, then Mars, for now at least, belongs to the Martians. Mars is their planet, and Martian life would be threatened by a human presence there.</p>
<p>Does humanity have an inalienable right to colonize Mars simply because we will soon be able to do so? We have the technology to use robots to determine whether Mars is inhabited. Do ethics demand that we use those tools to answer definitively whether Mars is inhabited or sterile before we put human footprints on the Martian surface?</p><img src="https://counter.theconversation.com/content/103053/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Weintraub 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>NASA’s InSight Mars lander touches down Nov. 26, part of a careful robotic approach to exploring the red planet. But human exploration of Mars will inevitably introduce Earth life. Are you OK with that?David Weintraub, Professor of Astronomy, Vanderbilt UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/869512017-11-06T16:09:16Z2017-11-06T16:09:16ZThe internal ocean of Saturn’s moon Enceladus could be old enough to have evolved life, finds study<figure><img src="https://images.theconversation.com/files/193408/original/file-20171106-1068-1bns8u4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Enceladus.</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p>We recently bade farewell to the <a href="https://saturn.jpl.nasa.gov/">Cassini spacecraft</a>, which after 13 years of faithfully orbiting Saturn and its moons <a href="https://theconversation.com/cassini-crashes-its-time-for-a-new-mission-to-explore-the-possibility-of-life-on-saturns-moons-84016">was directed to plunge into the giant planet’s atmosphere</a>. The reason for the “grand finale” was to guard against the possibility that Cassini might crash into one of Saturn’s moons – in particular Enceladus. </p>
<p>With its curtain of geysers and internal ocean, Enceladus is unique. As a result, this small, icy moon is currently regarded as <a href="https://theconversation.com/nasa-saturn-moon-enceladus-is-able-to-host-life-its-time-for-a-new-mission-76102">a potential host for life</a>, and so no chance was taken that it might become contaminated by the Cassini spacecraft. Now new research, <a href="http://nature.com/articles/doi:10.1038/s41550-017-0289-8">published in Nature Astronomy</a>, suggests this ocean has existed within Enceladus for a very long time – possibly long enough to create the conditions to develop life.</p>
<p>The geysers are <a href="https://theconversation.com/icy-plumes-bursting-from-saturns-moon-enceladus-suggest-it-could-harbour-life-38673">plumes of salty water-ice</a> mixed with traces of carbon dioxide, ammonia, methane and other hydrocarbons that erupt along cracks in Enceladus’ south polar region. It was because of these geysers that scientists could work out that Enceladus must have an ocean below its icy crust and that the ocean is active (convecting). A subsequent observation that <a href="https://theconversation.com/nasa-saturn-moon-enceladus-is-able-to-host-life-its-time-for-a-new-mission-76102">hydrogen was present in the plumes</a> led to an additional conclusion, that hydrothermal activity – chemical reactions due to the interaction of water and rock – was taking place. But what scientists have failed to explain is what heat source could be powering this activity.</p>
<p>As more observations of the location of the plumes were made, the mystery of the missing heat source increased. The geysers are associated with features known as “tiger stripes” – a set of four, parallel depressions, about 100km long and 500m deep. The temperature of the stripes is higher than that of the rest of the icy crust, so it was assumed that they must be cracks in the ice. There are almost no impact craters in the tiger stripes region, so it must be very young, <a href="http://science.sciencemag.org/content/311/5766/1393">of the order of a million-years-old</a>. Any model that purported to explain the heat source had also to account for its focused nature – the ocean is global, but why is only the south polar region active?</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/193404/original/file-20171106-1061-kft5us.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/193404/original/file-20171106-1061-kft5us.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=253&fit=crop&dpr=1 600w, https://images.theconversation.com/files/193404/original/file-20171106-1061-kft5us.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=253&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/193404/original/file-20171106-1061-kft5us.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=253&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/193404/original/file-20171106-1061-kft5us.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=318&fit=crop&dpr=1 754w, https://images.theconversation.com/files/193404/original/file-20171106-1061-kft5us.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=318&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/193404/original/file-20171106-1061-kft5us.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=318&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 showing Cassini driving through geysers.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>For several years, scientists have favoured the explanation of “tidal heating” – a result of interactions between planetary-sized bodies. For instance tidal interaction with our own moon is responsible for the ebb and flow of water on Earth. Enceladus is in <a href="https://groups.csail.mit.edu/mac/users/wisdom/resonances.pdf">orbital resonance with the similar-sized moon Dione</a>, which affects the shape of Enceladus’ orbit around Saturn. The effect, however, is insufficient to account for the power required to keep the geysers active – calculated to be in the order of 5GW. This would be sufficient power for a city the size of Chicago.</p>
<h2>Porous core</h2>
<p>Researchers came a step closer to solving the puzzle when they looked at the internal structure of Enceladus. The moon has a density low enough to imply mainly ice with a small, rocky core. This observation has been known for many years, ever since the <a href="https://solarsystem.nasa.gov/missions/voyager2">Voyager 2 mission</a> took the first images of Enceladus and determined its radius, so allowing its volume to be calculated. The gravitational tug of Enceladus on Cassini allowed the moon’s mass to be estimated, <a href="https://www.ncbi.nlm.nih.gov/pubmed/24700854">giving a value for the density of the body</a>. Gravity measurements by Cassini showed that the core also had a low density which could be interpreted <a href="http://dx.doi.org/10.1016/j.icarus.2015.05.033">as the core being porous</a>, with the pores filled with ice.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/193406/original/file-20171106-1041-1tqy5c3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/193406/original/file-20171106-1041-1tqy5c3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=337&fit=crop&dpr=1 600w, https://images.theconversation.com/files/193406/original/file-20171106-1041-1tqy5c3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=337&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/193406/original/file-20171106-1041-1tqy5c3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=337&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/193406/original/file-20171106-1041-1tqy5c3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/193406/original/file-20171106-1041-1tqy5c3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/193406/original/file-20171106-1041-1tqy5c3.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">What was once thought to be a solid, rocky core may actually be porous.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>The new series of calculations fills the core’s pores with water, rather than ice, from which the authors show that tidal forces associated with the pore water are more than sufficient to explain how Enceladus’ heat is generated. The model is impressive because it is so thorough – considering not just the porosity of the core, but its permeability (how easily fluids can move through it) and how strong it is (will it shatter or flex as fluids run through it?). The researchers apply similar detail to the fluid, with consideration of its viscosity (how runny it is), temperature and composition, as well as its convective properties (how well can it transport heat). </p>
<p>Taking all these parameters together and assigning either known or conservatively assessed values to them results in a fearsome complex of equations. Fortunately, the authors (or, at least, their computer software) can solve the equations to produce an elegant model of heat flow within Enceladus. </p>
<p>The authors create a 3D picture of where and how heat from tidal movements within the pore spaces is transferred to the sub-surface ocean. They find that heat dissipation from the core is not homogeneous, but appears as a series of interlinked, narrow upwellings where temperatures are in excess of 363K (85°C), with hotspots mainly at the south pole. Because the heat sources are so focussed, there would be increased hydrothermal activity associated with them – explaining the hydrogen in the plumes.</p>
<p>The final exciting observation that comes from the model is that the amount of heat produced by the internal tide is sufficient to maintain Enceladus’ subsurface ocean for billions of years. Prior to this, it was thought that if the heat source for a global sub-surface ocean had been radioactive decay, the ocean would <a href="https://doi.org/10.1016/j.icarus.2007.11.010">freeze in a few million years</a>, which is why tidal forces were suggested as a potential heat source. But again, there were problems with such a model, requiring changes in Enceladus’ orbit – and even so, an ocean would be, at best, transient. </p>
<p>This immediately leads to another set of questions: what does this imply for life on Enceladus? A warm global ocean with a lifetime of several billion years would be a great place for life to get going - it only took about 640m years for life to evolve from microbe to mammal on Earth. Unfortunately, though, Enceladus itself may be quite young: a recent paper proposed that the moon<a href="https://theconversation.com/saturns-moons-may-be-younger-than-the-dinosaurs-so-could-life-really-exist-there-56860"> might only have formed about 100m years ago</a> – is that a sufficiently long interval for life to have got going? </p>
<p>Possibly – life seems to have got going on Earth within a few hundred million years of its formation under much more severe circumstances of impact bombardment. Although it took a further 3,500m years or so to get to the dramatic expansion of life. Maybe it is the future that is looking bright for Enceladus – if Enceladus’ ocean has the potential to last for billions of years, then could a similar evolutionary sequence to that on Earth take place in the darkened depths of an Enceladen ocean? Perhaps no future dwarf planet of the apes – but what price a mermaid?</p><img src="https://counter.theconversation.com/content/86951/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Monica Grady receives funding from the STFC, the UK Space Agency and the EU Horizon 2020 Programme. She is a Fellow of the Natural History Musuem and a Trustee of Lunar Mission One.</span></em></p>Scientists used to think that the ocean on Enceladus would be transient, perhaps freezing after a few million years. A new study suggests this isn’t the case.Monica Grady, Professor of Planetary and Space Sciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/840162017-09-15T12:01:07Z2017-09-15T12:01:07ZCassini crashes: it’s time for a new mission to explore the possibility of life on Saturn’s moons<figure><img src="https://images.theconversation.com/files/186100/original/file-20170914-21553-12tp9ok.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Cassini in front of The Lord of the Rings.</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p>NASA’s Cassini mission has made its “<a href="https://theconversation.com/mission-over-the-final-countdown-to-cassinis-fatal-plunge-into-saturn-83873">death plunge</a>” into the swirling clouds of Saturn after 20 years of exploring the planet and its moons. It’s been amazingly successful, making headlines with groundbreaking discoveries throughout its journey. But today the headlines are more like obituary notices, looking back at the mission’s <a href="https://theconversation.com/a-look-back-at-cassinis-incredible-mission-to-saturn-before-its-final-plunge-into-the-planet-83226">spectacular achievements</a>. </p>
<p>Cassini discovered <a href="https://theconversation.com/what-cassinis-mission-revealed-about-saturns-known-and-newly-discovered-moons-83430">new moons</a> around Saturn, found evidence for <a href="https://theconversation.com/the-chemistry-that-could-feed-life-within-saturns-moon-enceladus-study-gives-clue-ahead-of-flyby-49683">an ocean below the surface</a> of the moon Enceladus and even managed to land a probe on the satellite Titan (the Huygens probe). It also observed <a href="https://theconversation.com/the-beauty-and-mystery-of-saturns-rings-revealed-by-the-cassini-mission-83492">unusual features in the rings</a> of the planet and recorded an enormous, hurricane-like storm whirling around its north pole. Surely, we must now know everything about Saturn and its moons?</p>
<p>Fortunately, scientists are never satisfied, and the answer to one question usually leads to at least three new questions. The discoveries from Cassini and Huygens have resulted in a whole series of issues that require further investigation. Two of the main targets for future exploration are Titan and Enceladus.</p>
<h2>Signs of life</h2>
<p>Before Huygens <a href="https://www.nasa.gov/content/ten-years-ago-huygens-probe-lands-on-surface-of-titan">parachuted down onto Titan’s surface</a> in January 2005, all we knew about the moon was that it was cold (about 100K or -173.15°C) and had a thick atmosphere (mostly of nitrogen, but with traces of methane), which prevented us from seeing the surface. Huygens revealed networks of valleys and rivers cutting through hills to the shoreline of an inland sea. Subsequent observations by instruments on-board Cassini have given us a greatly expanded understanding of Titan’s landscape – with an entire <a href="https://planetarynames.wr.usgs.gov/Page/TITAN/target">gazetteer of named features</a>, from mountains to plains and oceans to ponds.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/186171/original/file-20170915-8093-wh4z8g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/186171/original/file-20170915-8093-wh4z8g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=255&fit=crop&dpr=1 600w, https://images.theconversation.com/files/186171/original/file-20170915-8093-wh4z8g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=255&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/186171/original/file-20170915-8093-wh4z8g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=255&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/186171/original/file-20170915-8093-wh4z8g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=321&fit=crop&dpr=1 754w, https://images.theconversation.com/files/186171/original/file-20170915-8093-wh4z8g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=321&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/186171/original/file-20170915-8093-wh4z8g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=321&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Titan.</span>
<span class="attribution"><span class="source">Imsofinite</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>We must now try to understand what they are, how they formed and how they change with the seasons. We need to learn about tides and ocean icebergs, to define a climate cycle and to determine the composition of the land masses – are they derived from basalt, the most common rock type in the solar system, or are they frozen ice and mud? Does Titan have a rocky core overlain directly by an icy mantle, or does it have an ocean below the surface? If so, is it made up of water? </p>
<p>This all matters because what we have learnt about Titan from Cassini and Huygens has confirmed that <a href="https://theconversation.com/saturns-moon-titan-may-harbour-simple-life-forms-and-reveal-how-organisms-first-formed-on-earth-81527">it has an active chemistry</a>, based on methane and ammonia. We know that these substances, when irradiated by the sun, result in interesting mixes of chemicals that are precursors to amino acids and other biologically important molecules. The freezing temperature of Titan’s surface precludes anything being alive – but how far below the surface do you have to go before the environment becomes sufficiently balmy for a cryophile to be comfortable? Without a dedicated mission to Titan, we will not find out.</p>
<p>Cassini’s exploration of Titan was always one of the main goals of the mission, with a few larger moons also scheduled for observation. But early in the mission, it became clear that Enceladus should be a prime target too. Anomalies in data observed as the spacecraft flew past Enceladus were subsequently verified as resulting from a <a href="http://www.igpp.ucla.edu/public/journal-club/2008.09-12.(Sep-Dec).Journal.Club.Papers/2008.10.29.Journal.Club.Papers/additional.reference.papers.for.Friday.seminar.Oct.31.2008/Dougherty.et.al.Science.2006.Identification%20of%20a%20Dynamic%20Atmosphere%20at%20Enceladus%20with%20the%20Cassini%20Magnetometer.1406.pdf">large plume of gas and dust</a> venting from the surface close to the south pole. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/186174/original/file-20170915-8065-t6lbre.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/186174/original/file-20170915-8065-t6lbre.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=371&fit=crop&dpr=1 600w, https://images.theconversation.com/files/186174/original/file-20170915-8065-t6lbre.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=371&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/186174/original/file-20170915-8065-t6lbre.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=371&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/186174/original/file-20170915-8065-t6lbre.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=466&fit=crop&dpr=1 754w, https://images.theconversation.com/files/186174/original/file-20170915-8065-t6lbre.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=466&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/186174/original/file-20170915-8065-t6lbre.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=466&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Plumes on Enceladus.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>This was further investigated by Cassini, flying past Enceladus at different altitudes – the closest of which was at 25km. The data it collected helped scientists resolve the plume into a series of vents through cracks in the surface. It became clear that, like <a href="https://theconversation.com/new-water-plumes-from-jupiters-moon-europa-raise-hopes-of-detecting-microbial-life-66019">Jupiter’s icy moon Europa</a>, Enceladus was home to an ocean below the icy crust. </p>
<p>Scientists also managed to identify <a href="https://theconversation.com/icy-plumes-bursting-from-saturns-moon-enceladus-suggest-it-could-harbour-life-38673">grains of dust</a>, water-rich ice and gases including methane, ammonia and carbon dioxide – plus traces of other <a href="https://theconversation.com/the-chemistry-that-could-feed-life-within-saturns-moon-enceladus-study-gives-clue-ahead-of-flyby-49683">organic molecules</a> – in the plume. This lead to to much speculation about the possibility of life in the ocean. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/186173/original/file-20170915-8125-yrxu74.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/186173/original/file-20170915-8125-yrxu74.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/186173/original/file-20170915-8125-yrxu74.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/186173/original/file-20170915-8125-yrxu74.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/186173/original/file-20170915-8125-yrxu74.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/186173/original/file-20170915-8125-yrxu74.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/186173/original/file-20170915-8125-yrxu74.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">Enceladus.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>Like Titan, Enceladus is now recognised as one of the solar system’s most likely locations for extraterrestrial life. A recent report of <a href="http://science.sciencemag.org/content/356/6334/155">hydrogen in Enceladus’ plume</a> has given that recognition even greater prominence. That’s because hydrogen is expected to be released as a byproduct of reactions between water and rock. Scientists believe that ocean water on Enceladus collides with rock, becomes heated, reacts chemically and rises up in the ocean via “hydrothermal vents”. That happens in the Earth’s oceans, too. And here, the chemically charged water around these vents supports a rich ecology of microbes and other life forms.</p>
<h2>A single mission?</h2>
<p>Follow-up missions to Saturn, Titan and Enceladus have all been proposed to both the European Space Agency and NASA, but none has yet been accepted and taken forward to the planning stage.</p>
<p>There might be a case for combining a mission to Titan with a mission to Enceladus to investigate the opportunities for life close to Saturn. I am not sure what such a mission would look like. Certainly there should be a spacecraft orbiting Titan and Enceladus, to gather additional information prior to launching vessels to the surface of both moons. Would these vessels be “penetrators” so they could pierce the ice of Enceladus? Or should they be balloons to float down through the atmosphere of Titan? I don’t know, but a possible name for the mission could be EnTiRE – Enceladus and Titan Research and Exploration. You heard it here first…</p>
<p>Of course, one should not forget the “Lord of the Rings” itself. There are still many unknown aspects of the giant planet that would be worth going back to investigate, such as the composition and formation of its unusual rings. And what about that huge, hexagonal-shaped hurricane at the north pole? Is there something comparable at the south pole? How do such weather systems form – and, more to the point, given the <a href="https://theconversation.com/hurricanes-may-be-getting-more-severe-do-we-need-a-whole-new-category-to-describe-them-83782">current catastrophic hurricanes</a> experienced recently on Earth, can we learn anything from Saturn about our own atmospheric disturbances? </p>
<p>Given the timescale for mission planning, alongside the time it takes to get to the Saturnian system, it will realistically be at least another 20 years before Cassini’s successor arrives. Until then, we will have to rely on the next generation of Earth-based telescopes to help us explore Saturn and its fascinating satellites.</p><img src="https://counter.theconversation.com/content/84016/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Monica Grady receives funding from the STFC. She is a Fellow of the Natural History Museum and a Trustee of Lunar Mission One.</span></em></p>As Cassini’s titanic mission comes to an end, we need to start thinking ahead. A combined mission to explore Saturn’s moons Titan and Enceladus would be a good place to start.Monica Grady, Professor of Planetary and Space Sciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/766702017-04-26T08:56:29Z2017-04-26T08:56:29ZBittersweet feeling as Cassini mission embarks on its ‘grand finale’ ahead of death plunge<figure><img src="https://images.theconversation.com/files/166657/original/file-20170425-12650-1k2x71.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Saturn eclipsing the sun, seen from behind by the Cassini orbiter. Earth is the small dot between the rings on the upper, left-hand side.
</span> <span class="attribution"><span class="source">NASA/JPL/Space Science Institute</span></span></figcaption></figure><p>It’s the slow beginning of what will be a violent end for the Cassini mission. At 10.00 BST on April 26, the spacecraft became the first ever to dive between Saturn and its spectacular innermost ring. This marks the start of its “grand finale” consisting of 22 daring orbits – enabling a new era of science. Then, on September 15, it will crash into the planet’s atmosphere and burn up. It is sure to be a sad but proud moment for those of us who have worked on the mission.</p>
<p>Cassini was <a href="https://www.nasa.gov/feature/jpl/cassini-completes-final-and-fateful-titan-flyby">initially set on its path to destruction</a> on April 22 when it flew by Saturn’s moon Titan. The spacecraft has used Titan not just as a fascinating scientific target in itself, but also to provide the gravitational slingshots needed to tweak, crank and change its orbit. It is Titan that gave it the impetus to penetrate the 2,400km gap between the rings and the planet every week before the mission ends.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/166647/original/file-20170425-25594-1a7v5wn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/166647/original/file-20170425-25594-1a7v5wn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/166647/original/file-20170425-25594-1a7v5wn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=337&fit=crop&dpr=1 600w, https://images.theconversation.com/files/166647/original/file-20170425-25594-1a7v5wn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=337&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/166647/original/file-20170425-25594-1a7v5wn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=337&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/166647/original/file-20170425-25594-1a7v5wn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/166647/original/file-20170425-25594-1a7v5wn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/166647/original/file-20170425-25594-1a7v5wn.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">Grand finale orbits.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<p>During the next few months, the instrument teams have several new observations to make in this region. These include understanding a new radiation belt <a href="https://saturn.jpl.nasa.gov/resources/1851/">discovered inside the rings early in the mission</a> and taking close-up pictures of the rings and other features. It will also image Saturn’s cloud tops at close range, weigh its ring system (which will indicate just how old it is), sample the atmosphere of the planet and its rings, and measure Saturn’s internal structure.</p>
<h2>Hazardous journey</h2>
<p>But there are risks. The ring plane includes a lot of particles. These vary from micron-sized grains (a millimetre is equal to 1,000 microns) to mansion-sized chunks – and they could all collide with the spacecraft. Although Cassini’s path in the gap between Saturn and the rings has fewer particles than in the visible rings, we don’t know exactly how few.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/166663/original/file-20170425-12658-sqmwvz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/166663/original/file-20170425-12658-sqmwvz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=360&fit=crop&dpr=1 600w, https://images.theconversation.com/files/166663/original/file-20170425-12658-sqmwvz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=360&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/166663/original/file-20170425-12658-sqmwvz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=360&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/166663/original/file-20170425-12658-sqmwvz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=452&fit=crop&dpr=1 754w, https://images.theconversation.com/files/166663/original/file-20170425-12658-sqmwvz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=452&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/166663/original/file-20170425-12658-sqmwvz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=452&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 the particles in Saturn’s rings.</span>
<span class="attribution"><span class="source">Judy Schmidt/Flickr</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>However, it is thought that the risks of a significant dust hit in the ring plane are low. At the large relative speed – a maximum of 124,000km per hour – a hit could be a significant problem for the spacecraft. So, as Cassini flies through the ring plane, its <a href="http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Communicating_from_space_gaining_a_grip_on_antennas">high-gain antenna</a> will be oriented to provide a shield for the rest of the spacecraft and its instruments. This was successfully done early in the mission when it was first inserted into orbit around Saturn, crossing outside the main rings.</p>
<p>The only protruding features beyond the shield are a 10-metre electric field antennae and an 11-metre magnetometer boom (used to make magnetic observations). These should make it possible to monitor dust impacts, each of which makes a tiny plume of plasma (loose and electrically charged particles) as the spacecraft and booms are impacted. Hopefully, no large impact will disable the spacecraft.</p>
<p>Indeed, the best computer models show that <a href="https://saturn.jpl.nasa.gov/faq/#end_of_mission">this should not happen</a>. But even if it does, the spacecraft will eventually crash into Saturn – as it’s supposed to. That is important. Cassini must not crash onto interesting targets in the quest to find extraterrestrial lifeforms – particularly the moons Enceladus and Titan. These are subject to “planetary protection” which means we cannot put probes there which could carry contamination from Earth in the form of microbes.</p>
<p>The spacecraft will be useful until the very moment it burns up – it will be sending back data continuously. These last measurements of the composition of Saturn’s atmosphere, rotation rate and interior structure will be totally unique.</p>
<h2>Personal highlights</h2>
<p>I’ve been privileged to work on Cassini for 28 years. We proposed our instrument, the electron spectrometer, which measures the energy and direction of electrons, in 1989, and were selected to develop it in 1990. We then secured funding, and built, tested and calibrated the instrument before the launch in 1997. </p>
<p>The <a href="https://www.nasa.gov/image-feature/oct-15-1997-launch-of-cassini-to-saturn">launch itself</a> was a tense time. The powerful Titan IV rocket lifted off near dawn in October 1997, went behind a cloud from our vantage point and then thankfully emerged serenely on its way to Saturn, reaching the planet on July 1 2004. I remember nervously watching in the middle of the night as Cassini burned its engines for an excruciating 96 minutes to slow the spacecraft into Saturn orbit. Luckily it all went smoothly.</p>
<p>Cassini has operated at Saturn for almost half a Saturn year. It’s made <a href="https://theconversation.com/water-weather-new-worlds-cassini-mission-revealed-saturns-secrets-76195">many discoveries</a> – in fact, it has rewritten the textbooks on the Saturn system. We now know that Saturn has 45 more moons than previously thought – placing the total now at 62.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/166667/original/file-20170425-22270-1t26u25.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/166667/original/file-20170425-22270-1t26u25.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=615&fit=crop&dpr=1 600w, https://images.theconversation.com/files/166667/original/file-20170425-22270-1t26u25.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=615&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/166667/original/file-20170425-22270-1t26u25.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=615&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/166667/original/file-20170425-22270-1t26u25.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=773&fit=crop&dpr=1 754w, https://images.theconversation.com/files/166667/original/file-20170425-22270-1t26u25.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=773&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/166667/original/file-20170425-22270-1t26u25.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=773&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Titan seen by Cassini.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>At Titan, Cassini <a href="https://phys.org/news/2014-06-nasa-titan-potential-prebiotic-chemistry.html">found prebiotic chemistry</a> – which means the moon is essentially a frozen version of what the Earth was several billion years ago. Cassini also discovered that the moon has <a href="https://www.nasa.gov/feature/jpl/cassini-explores-a-methane-sea-on-titan">three large seas and a number of smaller lakes</a> made of liquid methane. </p>
<p>At Enceladus, Cassini discovered water-rich plumes from a subsurface ocean and found that the ocean is salty. It recently also confirmed that chemical reactions between water and rock on the moon can provide enough energy in the water <a href="https://theconversation.com/nasa-saturn-moon-enceladus-is-able-to-host-life-its-time-for-a-new-mission-76102">to feed microbial life</a>. There have been many other incredible discoveries about the other icy moons, the atmosphere and the magnetosphere.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=218&fit=crop&dpr=1 600w, https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=218&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=218&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=274&fit=crop&dpr=1 754w, https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=274&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=274&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Enceladus’s south polar plumes, as seen by Cassini, November 30, 2010.</span>
<span class="attribution"><a class="source" href="https://photojournal.jpl.nasa.gov/catalog/PIA17184">NASA/JPL-Caltech/Space Science Institute</a></span>
</figcaption>
</figure>
<p>Some of the <a href="https://saturn.jpl.nasa.gov/news/2385/cassini-spacecraft-reveals-evidence-of-tholin-formation-at-high-altitudes-in-titans-atmosphere/">key discoveries we made</a> with the electron spectrometer include large amounts of hydrocarbons and nitriles (a form of organic compound) in Titan’s atmosphere. These fall through the atmosphere and may form the dunes seen on Titan’s surface. We also uncovered Saturn’s ring atmosphere and ionosphere, and found weak atmospheres at the moons Rhea and Dione. An important discovery was that of water clusters and charged dust particles in plumes at Enceladus, helping us to understand that it has a subsurface ocean.</p>
<p>Needless to say, it has truly been an honour to be a part of the Cassini mission and a privilege to work with colleagues, postdocs and students through the years. As the mission heads towards its end, the overwhelming feeling is that this has been one of humankind’s most spectacular voyages of discovery. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/xrGAQCq9BMU?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Daring dive.</span></figcaption>
</figure>
<p>But there are tantalising discoveries yet to be made. Enceladus has just recently joined Mars and Jupiter’s moon Europa in the top three likely locations for life beyond Earth in our solar system. I’m <a href="https://theconversation.com/decades-of-attempts-show-how-hard-it-is-to-land-on-mars-heres-how-we-plan-to-succeed-in-2021-69734">involved in the continued search for life on Mars</a> with the ExoMars 2020 rover and at Jupiter’s icy moons with the <a href="http://sci.esa.int/juice/">JUICE mission</a>. NASA also plans the Europa Clipper mission. Now, Cassini has shown us that we must go back to Titan and Enceladus, too.</p><img src="https://counter.theconversation.com/content/76670/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Coates receives funding from STFC and UKSA. </span></em></p>One of the most successful space exploration missions of all time still has a lot left to uncover.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/761022017-04-13T17:58:40Z2017-04-13T17:58:40ZNASA: Saturn moon Enceladus is able to host life – it’s time for a new mission<figure><img src="https://images.theconversation.com/files/270778/original/file-20190424-121220-1jq7yk5.jpg?ixlib=rb-1.1.0&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>Ever since <a href="https://theconversation.com/the-chemistry-that-could-feed-life-within-saturns-moon-enceladus-study-gives-clue-ahead-of-flyby-49683">studies started suggesting</a> that chemical reactions between water and rock on Saturn’s moon Enceladus could provide enough energy in the water to feed microbial life, scientists have been searching for proof that the right sort of reactions really do occur.</p>
<p>And during its last dive through the icy plumes that Enceladus erupts into space in October 2015, the Cassini spacecraft has finally managed to find it – in the form of molecular hydrogen. The finding, <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aai8703">published in Science</a>, means the moon can now be considered highly likely to be suitable to host microbial life. In fact, the results should undermine the last strong objection from those who argue it could not.</p>
<p>Enceladus is a small (502km in diameter) moon with an icy surface, a rocky interior and an ocean of liquid water sandwiched between the two. Cassini <a href="https://saturn.jpl.nasa.gov/the-journey/timeline/#theres-something-strange-about-enceladus">discovered back in 2005</a> that Enceladus is venting water into space, in the form of plumes of ice crystals escaping from cracks in the surface. For a decade, Enceladus was the only icy moon where this was known to happen, but plumes have recently been <a href="https://theconversation.com/new-water-plumes-from-jupiters-moon-europa-raise-hopes-of-detecting-microbial-life-66019">found on Europa, too</a>, a larger icy moon of Jupiter.</p>
<p>Cassini’s discovery led to it being re-tasked to fly through Enceladus’s plumes. There, in addition to water, it was able to identify traces of methane, ammonia, carbon monoxide, carbon dioxide, simple organic molecules and salts. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/164866/original/image-20170411-26751-9b1hsj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/164866/original/image-20170411-26751-9b1hsj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=243&fit=crop&dpr=1 600w, https://images.theconversation.com/files/164866/original/image-20170411-26751-9b1hsj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=243&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/164866/original/image-20170411-26751-9b1hsj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=243&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/164866/original/image-20170411-26751-9b1hsj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=305&fit=crop&dpr=1 754w, https://images.theconversation.com/files/164866/original/image-20170411-26751-9b1hsj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=305&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/164866/original/image-20170411-26751-9b1hsj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=305&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Cutaway view inside Enceladus, showing where hot water and rock interact below the ice.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<p>Eventually, in <a href="https://theconversation.com/icy-plumes-bursting-from-saturns-moon-enceladus-suggest-it-could-harbour-life-38673">March 2015</a>, it detected microscopic particles of silica. By then, the composition of the plumes showed almost every sign that ocean water had reacted chemically with heated rock – altering the minerals of the rocky silicate seabed while the water became rich in chemicals.</p>
<p>Presumably, the ocean water is drawn into the rock, becomes heated, reacts chemically, and escapes back up to the ocean via “hydrothermal vents”. These exist on the floor of the Earth’s oceans, too, where the chemically charged water supports a rich ecology of microbes and other, more complex, life forms – requiring no sunlight.</p>
<p>The only missing evidence of water-rock chemical reactions in Enceladus was molecules of hydrogen, which should be released as a byproduct of the water-rock reactions. Searching for hydrogen was <a href="https://theconversation.com/the-chemistry-that-could-feed-life-within-saturns-moon-enceladus-study-gives-clue-ahead-of-flyby-49683">a key goal of Cassini’s final and closest dive</a> through the plumes.</p>
<figure>
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</figure>
<p>The new study unveils how hydrogen was detected during the frantic half-minute when Cassini was about 120km above the surface of Enceladus, whizzing through a plume at 8.5km per second. This was achieved by operating the mass spectrometer (an instrument which knocks electrons off chemical substances and sorts them based on their mass-to-charge ratio) in a special mode. It admitted plume material directly into the instrument’s detection chamber to avoid the possibility of hydrogen being generated by plume-water reacting with the metallic components of the instrument itself.</p>
<h2>The astrobiology</h2>
<p>Hydrogen is of immense significance, because its presence along with hot water and rock would enable simple microbes to make a living. When dissolved carbon dioxide reacts with dissolved hydrogen, it produces methane and water. This chemical reaction releases energy that organisms can use to drive their metabolism. There are many kinds of “methanogenic” organisms at deep sea hydrothermal vents on Earth that do this. Now that we know Enceladus has <em>all</em> the necessary ingredients for this to happen, we are lacking only the proof of life itself.</p>
<p>For that we will need a purpose-built mission, such as the <a href="https://en.wikipedia.org/wiki/Enceladus_Life_Finder">Enceladus Life Finder (ELF)</a>. This would collect and analyse any complex organic molecules in the plumes. It is hard to imagine a more important goal for solar system exploration than establishing whether a habitable environment, such as the warm bottom of Enceladus’s ocean, actually <em>does</em> host life.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=218&fit=crop&dpr=1 600w, https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=218&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=218&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=274&fit=crop&dpr=1 754w, https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=274&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/164865/original/image-20170411-26720-1avikn7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=274&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Enceladus’s south polar plumes, as seen by Cassini November 30 2010.</span>
<span class="attribution"><a class="source" href="https://photojournal.jpl.nasa.gov/catalog/PIA17184">NASA/JPL-Caltech/Space Science Institute</a></span>
</figcaption>
</figure>
<p>Enceladus is a long way from Earth. If we were able to prove that it hosts life, it would be highly likely that such life had originated there, independently of life on Earth. That would be a crucial discovery. It would provide evidence to suggest that our galaxy is teeming with life, because if life began independently on two different bodies in our solar system, then surely it also got going on many of the potentially habitable planets that we are now finding <a href="https://theconversation.com/solar-system-with-seven-earth-like-planets-found-around-nearby-star-heres-what-they-could-be-like-73394">around other stars</a>.</p>
<p>Enceladus is a tiny world, and the amount of available energy and nutrients is small. Few scientists therefore expect it to host an ecosystem consisting of more than simple microbes. The much larger <a href="https://www.jpl.nasa.gov/missions/europa-clipper/">Europa</a>, if it has life too, is a better prospect.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/164911/original/image-20170411-26726-17eo7zs.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/164911/original/image-20170411-26726-17eo7zs.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=253&fit=crop&dpr=1 600w, https://images.theconversation.com/files/164911/original/image-20170411-26726-17eo7zs.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=253&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/164911/original/image-20170411-26726-17eo7zs.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=253&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/164911/original/image-20170411-26726-17eo7zs.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=318&fit=crop&dpr=1 754w, https://images.theconversation.com/files/164911/original/image-20170411-26726-17eo7zs.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=318&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/164911/original/image-20170411-26726-17eo7zs.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=318&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">How Cassini will end, on September 15, 2017.</span>
<span class="attribution"><span class="source">NASA/Jet Propulsion Laboratory-Caltech</span></span>
</figcaption>
</figure>
<p>However, to protect Enceladus from the slightest risk of contamination by any terrestrial microbes that accidentally hitched a ride on Cassini, the craft will not be allowed to become a derelict object that might eventually crash onto its surface. Instead, the mission is facing its “<a href="https://saturn.jpl.nasa.gov/the-journey/timeline/#the-grand-finale">grand finale</a>”, a series of 22 orbits in which it will pass spectacularly <em>between</em> Saturn and its innermost ring. This will end with Cassini burning up in Saturn’s atmosphere.</p><img src="https://counter.theconversation.com/content/76102/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Rothery 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 receives funding from the UK Space Agency and the Science & Technology Facilities Council for work related to Mercury and the European Space Agency's Mercury orbiter BepiColombo. He is Educator on the Open University/FutureLearn Moons MOOC <a href="https://www.futurelearn.com/courses/moons">https://www.futurelearn.com/courses/moons</a></span></em></p>New findings make it hard to imagine a more important goal for solar system exploration than searching for microbial life in Enceladus’ internal sea.David Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/735082017-02-23T19:23:44Z2017-02-23T19:23:44ZThe search for extraterrestrial life in the water worlds close to home<figure><img src="https://images.theconversation.com/files/158021/original/image-20170223-32094-bd6imx.jpg?ixlib=rb-1.1.0&rect=308%2C222%2C1089%2C769&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A likely candidate for life: Saturn's icy moon Enceladus.</span> <span class="attribution"><a class="source" href="https://saturn.jpl.nasa.gov/resources/7597/?category=images">NASA/JPL-Caltech/Space Science Institute</a></span></figcaption></figure><p>The discovery of <a href="https://theconversation.com/seven-earth-sized-planets-discovered-orbiting-a-nearby-star-73352">seven exoplanets around a star</a> 40 light years from our Sun has raised the possibility that they could <a href="https://exoplanets.nasa.gov/news/1419/nasa-telescope-reveals-largest-batch-of-earth-size-habitable-zone-planets-around-single-star/">harbour life</a>. </p>
<p>Why? Because the astronomers who made the discovery believe some of the planets may have liquid water. And on Earth, wherever there is liquid water, there is life. </p>
<p>But we believe we can look much closer to Earth for potential candidates for evidence of extraterrestrial life, as we state this month in the <a href="https://doi.org/10.1017/S1473550416000483">International Journal of Astrobiology</a>.</p>
<p>Recent discoveries by the NASA Voyager and Cassini space missions infer the presence of liquid oceans beneath a sea ice crust on some of the moons of Jupiter and Saturn. </p>
<p>These provide the most likely sites for finding extraterrestial life in our solar system.</p>
<h2>Just like on Earth</h2>
<p>The independent scientist <a href="http://www.jameslovelock.org/">James Lovelock</a>, best known for developing the Gaia hypothesis, was contracted to NASA in the 1960s to develop atmospheric and planetary sensors for the Viking probes subsequently deployed to Mars in 1975.</p>
<p>Following a precursory Earth-based assessment, Lovelock theorised that the red planet was likely devoid of life because of atmospheric chemical equilibrium. In contrast, Earth’s atmosphere is in dynamic flux due to the biological activity that takes place on the surface. </p>
<p>Notwithstanding the continued ambiguity as to whether or not life is, or ever has been, present on Mars, Lovelock set a powerful precedent for the emerging field of astrobiology – the comparative approach with Earth in the search for extra-terrestrial life. </p>
<h2>Energy and life</h2>
<p>In our endeavour to answer the question of whether we are alone in the Universe, we have a solitary clue: “follow the energy”. </p>
<p>Earth is our only point of reference, and life on Earth requires energy – thermal energy for melting water and chemical energy for maintaining life. That’s it. Just two forms of energy define the cosmic imperative for life as we know it. </p>
<p>But ironically, we do not know when, where or how life originated on Earth. </p>
<p>What we do know is that the oldest and most abundant life forms on the planet are microorganisms. Biological adaptation is not restricted by structural simplicity, because microbes occupy every conceivable ecological niche on Earth. </p>
<p>If we accept the simple prokaryotic cell as being life’s universal blueprint, then ET is either an amalgamation of microbes or is still a microbe.</p>
<h2>Follow the energy = follow the water</h2>
<p>The mandate to “follow the energy” is synonymous with “follow the water”. The recent discovery of evidence of <a href="https://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars">liquid water on the surface of Mars</a> is therefore intriguing, but there is a lot more of it on Jupiter’s <a href="http://solarsystem.nasa.gov/planets/europa">Europa</a> and Saturn’s <a href="http://solarsystem.nasa.gov/planets/enceladus">Enceladus</a>. </p>
<p>These moons are compelling targets for astrobiology because of the <a href="https://theconversation.com/new-water-plumes-from-jupiters-moon-europa-raise-hopes-of-detecting-microbial-life-66019">inferred presence of oceans</a> beneath a sea ice crust that have persisted over geological time scales. </p>
<p>A new interpretation of data collected by the Cassini spacecraft suggests that the <a href="https://www.nasa.gov/press-release/cassini-finds-global-ocean-in-saturns-moon-enceladus">ocean beneath the ice on Enceladus</a> is not just confined to the south polar region. Like <a href="https://www.nasa.gov/feature/jpl/europas-ocean-may-have-an-earthlike-chemical-balance">Europa</a>, it is global. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/158024/original/image-20170223-32121-11mcs1n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/158024/original/image-20170223-32121-11mcs1n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/158024/original/image-20170223-32121-11mcs1n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/158024/original/image-20170223-32121-11mcs1n.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/158024/original/image-20170223-32121-11mcs1n.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/158024/original/image-20170223-32121-11mcs1n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/158024/original/image-20170223-32121-11mcs1n.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/158024/original/image-20170223-32121-11mcs1n.jpg?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 rendering of Saturn’s moon Enceladus shows possible hydrothermal activity that may take place on and under the seafloor of the moon’s subsurface ocean.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<p>It now also appears that <a href="http://www.jpl.nasa.gov/news/news.php?release=2014-300">Europa’s ice shell comprises a mobile, plate tectonic-like system</a> that overlies warm convecting ice and a salty seawater reservoir that is 30-35 times the volume of Earth’s ocean. </p>
<p>Should more water equate to more life? Not necessarily. There are many biological constraints on habitability in extreme environments. </p>
<p>Life as we know it appears to be absent on the surface of Europa and Enceladus because of ionising radiation and extremely low temperatures. Photosynthesis as we know it is also very unlikely to occur under ice that is kilometres thick.</p>
<p>Hydrothermal vents, a habitat for deep-sea ecosystems on Earth, may or may not exist on the moons.</p>
<p>So is this the end of comparison with Earth and end of story? Actually no, because it’s feasible that microorganisms that currently inhabit sea ice on Earth could also inhabit the ice water-interface and ice fissures on Europa or Enceladus. </p>
<h2>Life in extreme conditions</h2>
<p>The molecular basis for adaptation is not completely understood, but <a href="https://theconversation.com/au/topics/extremophiles-5967">extremophiles</a> (organisms that live in extreme conditions) must tolerate steep gradients in temperature, salinity, acidity and inorganic nutrients, as well as dissolved gas and light signatures. </p>
<p>Stress-related fissures in the ice shells of Europa and Enceladus are complex, and our understanding of their topography is based on theoretical modelling. But fissures appear to actively exchange liquid from the subsurface oceans to the ice exteriors. </p>
<p>The physiological demands on any microbial organisms would be exceptional, but these features could harbour small-scale, biologically permissive domains. Even brief periods of photosynthesis might be possible. </p>
<p>Extremophiles are relevant reference organisms because they adapt to multiple stressors in ways we don’t completely understand.</p>
<p>Life on these moons may be possible, but how likely is it? The comparative approach calls for an understanding of how these microbes respond to multiple stressors and the limits to which they can be pushed. </p>
<p>But the search for extra-terrestrial life is impeded because we lack a framework linking the capacity for adaptation with environmental variability. Future research and exploration to these moons will benefit from experimental work that defines life’s limits in the sea ice ecosystem. </p>
<p>Ultimately, we need to characterise theoretical biological limits that are distinct from the limits imposed on Earth-based analogues.</p><img src="https://counter.theconversation.com/content/73508/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Martin receives funding from the Australian Research Council's Special Research Initiative for Antarctic Gateway Partnership (Project ID SR140300001). </span></em></p><p class="fine-print"><em><span>Andrew McMinn receives funding from Australian Antarctic Science Grants, Australian Research Council. </span></em></p>There has been much excitement this week about the possibility of water – and life – on some newly discovered exoplanets. But we can look closer to home for evidence of ET.Andrew Martin, Sea ice microbial physiologist, University of TasmaniaAndrew McMinn, Professor of Antarctic Science, University of TasmaniaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/660192016-09-26T21:56:40Z2016-09-26T21:56:40ZNew water plumes from Jupiter’s moon Europa raise hopes of detecting microbial life<figure><img src="https://images.theconversation.com/files/139328/original/image-20160926-31840-1djtnt2.jpg?ixlib=rb-1.1.0&rect=0%2C98%2C983%2C788&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Composite image of suspected water vapour plumes erupting at the seven o’clock position off the limb of Jupiter’s moon Europa.</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p>The Hubble Space Telescope <a href="http://www.nasa.gov/press-release/nasa-s-hubble-spots-possible-water-plumes-erupting-on-jupiters-moon-europa">has obtained evidence</a> of Jupiter’s moon Europa erupting plumes of water vapour, NASA reports. Hubble <a href="http://www.planetary.org/blogs/guest-blogs/2013/1212-fletcher-the-plumes-of-europa.html">first caught a glimpse</a> of the jets in December 2012, but now more plumes have been revealed using a different technique. </p>
<p>The discovery is exciting, as Europa, with its large subsurface ocean, is one of the best <a href="https://theconversation.com/the-moon-was-a-first-step-mars-will-test-our-capabilities-but-europa-is-the-prize-37253">candidates for microbial life</a> in the solar system – despite its surface temperature being a frigid -160°C. And plumes mean that samples of microbes could be collected during a flyby, without having to land on the surface.</p>
<p>Europa is Jupiter’s innermost large, icy moon (only slightly smaller than our own moon) and has about 100km of ice and liquid water surrounding its rocky interior.</p>
<h2>Galileo discovery</h2>
<p>Scientists started getting really interested in Europa following observations by NASA’s Galileo probe of Jupiter and its moons <a href="http://www.space.com/18632-galileo-spacecraft.html">from orbit between 1995 and 2003</a>. These showed that Europa’s surface ice is marked by multiple generations of parallel ridges and grooves, except where this pattern has been shattered by the surface breaking apart into randomly disposed slabs (known as “chaos regions”). This can be explained if Europa’s icy shell floats on a layer of liquid water. Each grooved ridge could mark a crack that opened and closed with the changing tide during Europa’s 3.5 day orbit of Jupiter – the ridges being formed from slush that was squeezed to the surface each time the crack closed. The chaos regions would represent slabs of the ice shell that broke apart as a region of the ocean became melted from below, powered by a release of tidal heat through the ocean floor – now frozen into a new configuration.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/139112/original/image-20160925-13539-1w7b1dd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/139112/original/image-20160925-13539-1w7b1dd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=481&fit=crop&dpr=1 600w, https://images.theconversation.com/files/139112/original/image-20160925-13539-1w7b1dd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=481&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/139112/original/image-20160925-13539-1w7b1dd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=481&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/139112/original/image-20160925-13539-1w7b1dd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=605&fit=crop&dpr=1 754w, https://images.theconversation.com/files/139112/original/image-20160925-13539-1w7b1dd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=605&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/139112/original/image-20160925-13539-1w7b1dd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=605&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A 17km wide Galileo image of ‘ball of string’ terrain on Europa, where multiple generations of cracks in the ice shell have opened and closed.</span>
<span class="attribution"><span class="source">NASA/JPL/ASU</span></span>
</figcaption>
</figure>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/139111/original/image-20160925-13542-mqen80.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/139111/original/image-20160925-13542-mqen80.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/139111/original/image-20160925-13542-mqen80.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=461&fit=crop&dpr=1 600w, https://images.theconversation.com/files/139111/original/image-20160925-13542-mqen80.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=461&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/139111/original/image-20160925-13542-mqen80.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=461&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/139111/original/image-20160925-13542-mqen80.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=579&fit=crop&dpr=1 754w, https://images.theconversation.com/files/139111/original/image-20160925-13542-mqen80.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=579&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/139111/original/image-20160925-13542-mqen80.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=579&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 region of Europa known as Conamara Chaos, imaged by Galileo.</span>
<span class="attribution"><span class="source">NASA/JPL/University of Arizona</span></span>
</figcaption>
</figure>
<p>Microbes could live near the floor of Europa’s internal ocean, feeding off the chemical energy supplied where hot water (“hydrothermal”) vents discharge chemicals dissolved out of the tidally-heated rocky interior. This is the kind of setting where life on Earth is thought to have begun. Microbes whose ancestors had developed a taste for sunlight might now be found surviving by photosynthesis (like plants) in the water drawn up and (mostly) forced down again every time a tidal crack opened. However, although Galileo searched as hard as it was able (hampered by a failed antenna) it found no signs of present-day activity, such as water venting to space.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/W1xUyJfd4dY?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Alien Ocean: NASA’s video about Europa’s ocean and its habitability.</span></figcaption>
</figure>
<h2>Europa versus Enceladus</h2>
<p>Soon after Galileo’s findings, Europa’s crown as the most active icy moon was <a href="https://theconversation.com/the-chemistry-that-could-feed-life-within-saturns-moon-enceladus-study-gives-clue-ahead-of-flyby-49683">snatched away by Enceladus</a>, a much smaller (500km diameter) moon of Saturn, in 2005. The Cassini probe discovered jets (or plumes) of ice crystals venting to space from cracks near Enceladus’s south pole. In the decade that followed, it became clear that Enceladus has a <a href="http://www.jpl.nasa.gov/infographics/infographic.view.php?id=11308">global internal ocean</a> between its icy surface shell and a rocky interior. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/139113/original/image-20160925-13529-1gj1k5l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/139113/original/image-20160925-13529-1gj1k5l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/139113/original/image-20160925-13529-1gj1k5l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=371&fit=crop&dpr=1 600w, https://images.theconversation.com/files/139113/original/image-20160925-13529-1gj1k5l.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=371&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/139113/original/image-20160925-13529-1gj1k5l.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=371&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/139113/original/image-20160925-13529-1gj1k5l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=466&fit=crop&dpr=1 754w, https://images.theconversation.com/files/139113/original/image-20160925-13529-1gj1k5l.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=466&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/139113/original/image-20160925-13529-1gj1k5l.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=466&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Jets erupting from cracks near the south pole of Enceladus.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/Space Science Institute</span></span>
</figcaption>
</figure>
<p>Cassini was even able to fly through the plumes, finding them to be chemically rich and bearing signs of the kind of water-rock interaction which could feed micriobial life. This raised hopes that such fly-throughs could one day find signs of microbial life (including maybe even a few ejected microbes themselves), without the need for a technically-challenging landing on the surface. </p>
<figure class="align-center ">
<img alt="" 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 and the rocky interior.</span>
<span class="attribution"><a class="source" href="http://www.nasa.gov/topics/solarsystem/features/pia16826.html">Edited by the author from a NASA source</a></span>
</figcaption>
</figure>
<p>It wasn’t until December 2012 that the Hubble Space Telescope at last detected evidence for an <a href="http://www.planetary.org/blogs/guest-blogs/2013/1212-fletcher-the-plumes-of-europa.html">active plume erupting from Europa</a>, in the form of a faint ultraviolet glow caused by atoms of hydrogen and oxygen rising up to 200km above Europa’s south pole. This is probably water molecules broken apart in the harsh space environment.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/139219/original/image-20160926-31849-5x69iv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/139219/original/image-20160926-31849-5x69iv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/139219/original/image-20160926-31849-5x69iv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/139219/original/image-20160926-31849-5x69iv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/139219/original/image-20160926-31849-5x69iv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/139219/original/image-20160926-31849-5x69iv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/139219/original/image-20160926-31849-5x69iv.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">The location of ionized oxygen and hydrogen (strongly pixellated) detected by the Hubble Space Telescope in December 2012, superimposed on a Galileo image of Europa. Until now, this was the only evidence of active plumes.</span>
<span class="attribution"><span class="source">NASA/ESA/L. Roth/SWRI/University of Cologne</span></span>
</figcaption>
</figure>
<p>However, subsequent efforts failed to find any repeat evidence, until now. The latest evidence was obtained with an enterprising new technique, using Jupiter as a light source to reveal plumes as Europa passes across its face. The Hubble Space Telescope was used to collect ultraviolet images during ten transits of Europa across Jupiter in 2014, which on three occasions revealed plumes erupting above the edge of Europa’s disc. This shows that the plumes are intermittent. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/139330/original/image-20160926-31875-1ghiuo.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/139330/original/image-20160926-31875-1ghiuo.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=200&fit=crop&dpr=1 600w, https://images.theconversation.com/files/139330/original/image-20160926-31875-1ghiuo.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=200&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/139330/original/image-20160926-31875-1ghiuo.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=200&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/139330/original/image-20160926-31875-1ghiuo.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=251&fit=crop&dpr=1 754w, https://images.theconversation.com/files/139330/original/image-20160926-31875-1ghiuo.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=251&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/139330/original/image-20160926-31875-1ghiuo.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=251&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The plumes of Europa seen as it passed across the face of Jupiter on three dates in 2014.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>However, sometimes three or four distinct plumes can be made out, and given that the technique can see only those plumes erupting near the visible edge of the disc, there may be many more undetectable plumes in front of or behind the disc.</p>
<p>Hopefully it won’t be too long before a mission designed to detect microbial life from the plumes of Europa or Enceladus sets off to find out. It certainly seems like a worthwhile endeavour – especially after the emergence of this latest evidence.</p><img src="https://counter.theconversation.com/content/66019/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Rothery 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 receives funding from the UK Space Agency and the Science & Technology Facilities Council for work related to Mercury and the European Space Agency's Mercury orbiter BepiColombo. He is Educator on the Open University/FutureLearn Moons MOOC <a href="https://www.futurelearn.com/courses/moons">https://www.futurelearn.com/courses/moons</a></span></em></p>There’s now strong evidence that Europa might be a worthwhile place to search for life.David Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/634172016-08-03T11:35:47Z2016-08-03T11:35:47ZSpace submarines will allow us to explore the seas of icy moons<figure><img src="https://images.theconversation.com/files/132778/original/image-20160802-17177-wta4kg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's impression of a cryobot and submarine in the ice on Jupiter's Europa</span> <span class="attribution"><span class="source">NASA/JPL</span></span></figcaption></figure><p>One of the most profound and exciting breakthroughs in planetary science in the last two decades has been the discovery of <a href="http://www.jpl.nasa.gov/news/news.php?feature=4635">liquid methane lakes</a> on the surface of Saturn’s largest moon Titan, and <a href="http://solarsystem.nasa.gov/europa/overview.cfm">liquid oceans</a> under the icy surfaces of many of the giant gas planets’ other moons. Thrillingly, these some of these “waters” <a href="https://theconversation.com/the-moon-was-a-first-step-mars-will-test-our-capabilities-but-europa-is-the-prize-37253">may actually harbour life</a>.</p>
<p>Unfortunately, we don’t know much about them. Probes such as Juno and Cassini can only get so close. Also, subsurface oceans can only be sensed indirectly. The European Space Agency’s <a href="http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Huygens_spacecraft">Huygens</a> probe did land on Titan in 2005, but on a solid surface rather than on liquid. So how can we explore these seas? </p>
<p>An exciting idea being explored is developing submarines to send through space to the moons. Over the next two years, NASA is devoting half a million dollars to researching the prospect of <a href="https://www.nasa.gov/content/titan-submarine-exploring-the-depths-of-kraken">sending such a vehicle to Titan</a>. But there are <a href="http://www.kiss.caltech.edu/workshops/titan2010/presentations/aharonson.pdf">other studies out there, too</a> – with targets including Jupiter’s Europa and Ganymede, and Saturn’s Enceladus. But are such missions actually <a href="http://dx.doi.org/10.1016/j.cryogenics.2015.09.009">within our technological reach</a>?</p>
<h2>The challenges of a Titan submarine</h2>
<p><a href="https://saturn.jpl.nasa.gov/resources/725/">Kraken Mare</a> is thought to be the largest sea on Titan with an area of 400,000 square kilometres – larger than Earth’s Caspian Sea. But it’s not made of water – we have good evidence that this is instead a <a href="http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Profile_of_a_methane_sea_on_Titan">lake of methane</a>, ethane and nitrogen.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/132758/original/image-20160802-17177-14aq5vd.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/132758/original/image-20160802-17177-14aq5vd.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=366&fit=crop&dpr=1 600w, https://images.theconversation.com/files/132758/original/image-20160802-17177-14aq5vd.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=366&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/132758/original/image-20160802-17177-14aq5vd.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=366&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/132758/original/image-20160802-17177-14aq5vd.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=460&fit=crop&dpr=1 754w, https://images.theconversation.com/files/132758/original/image-20160802-17177-14aq5vd.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=460&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/132758/original/image-20160802-17177-14aq5vd.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=460&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Cassini radar image of the northern region of Kracken Mare on Titan showing the large island of Mayda Insula.</span>
<span class="attribution"><span class="source">NASA/Jet Propulsion Laboratory-Caltech/Agenzia Spaziale Italiana.</span></span>
</figcaption>
</figure>
<p>So what would a submarine on Titan look like? It turns out that a design like a traditional submarine, with a high “aspect ratio” (ten times as long as it is wide), would minimise drag and could fit inside a launch vehicle. Most deep space missions operate autonomously and a submarine would be no different. However, they would have to go to the surface for periods of time. Radio and microwave signals get absorbed very quickly in oceans, so to send a signal back to Earth the antenna would have to be above the surface. </p>
<p>Another issue is electrical power – this obviously cannot be provided by solar panels as it is on many spacecraft. As part of a <a href="http://dx.doi.org/10.1016/j.cryogenics.2015.09.009">recent study</a>, engineers investigated various alternatives, including compact nuclear reactors and fuel cells, but concluded these were too heavy. Instead, they proposed that electricity could be generated from the radioactive decay of plutonium – a technique <a href="http://mars.nasa.gov/mars2020/files/mep/MMRTG_FactSheet_update_10-2-13.pdf">similar to that powering Cassini</a>.</p>
<p>Some of the shallow shorelines of Kraken Mare are only 30-40 metres deep but it is thought to be 150 metres at its deepest. As you dive down beneath the surface the pressure increases because of the weight of the liquid above. On Earth, you can feel this in your ears when swimming underwater. Liquid methane is about half as dense as water and gravity on Titan is about seven times weaker than Earth, similar to our moon. So submarines diving down 150 meters on Titan don’t need to withstand the same pressure as they would on Earth.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/NnKxbdpLP5E?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Credit: NASA/NIAC.</span></figcaption>
</figure>
<p>A huge difficulty with these missions is to package the submarine into a system that can be launched on a rocket, survive in deep space during the roughly seven-year cruise to Titan, and then make it through a hypersonic descent to the ocean. It turns out that spaceplanes, such as the <a href="https://www.theguardian.com/science/2010/dec/01/space-vehicle-earth">X-37</a>, are ideal and would work well when descending into Titan’s thick hydrocarbon atmosphere. The spaceplane would launch from Earth on top of a rocket with the submarine inside. Once at the Saturnian system, the spaceplane would then land on Kraken Mare and deploy the submarine.</p>
<p>But perhaps the hardest thing will be to control the temperature inside the submarine – even though the sea is a somewhat frigid -180°C, the radioactive decay of the plutonium produces a lot of heat that needs to be dissipated.</p>
<h2>Descending to the depths of subsurface oceans</h2>
<p>Some tens of kilometres below the icy surface of Europa, meanwhile, we have good evidence that there could be a liquid salt water ocean. In fact, there could be subsurface liquid water oceans on a number of the moons of Jupiter, Saturn, and possibly Uranus and Neptune. As water is a prerequisite for life on Earth, this raises the exciting notion that these moons may be habitable. This is why many planetary scientists are interested in ideas to explore under the ice – either with submarines or <a href="http://www.jpl.nasa.gov/missions/europa-mission/">ice penetrating radar</a>.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/132757/original/image-20160802-17187-1swnp9i.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/132757/original/image-20160802-17187-1swnp9i.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=874&fit=crop&dpr=1 600w, https://images.theconversation.com/files/132757/original/image-20160802-17187-1swnp9i.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=874&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/132757/original/image-20160802-17187-1swnp9i.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=874&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/132757/original/image-20160802-17187-1swnp9i.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1098&fit=crop&dpr=1 754w, https://images.theconversation.com/files/132757/original/image-20160802-17187-1swnp9i.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1098&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/132757/original/image-20160802-17187-1swnp9i.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1098&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Travel to Europa.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<p>However, getting a submarine through at least 5km of ice makes putting a submarine on Titan look very easy. Cryobots – robotic devices that penetrate ice by <a href="http://www.jpl.nasa.gov/releases/2002/release_2002_6.html">melting it, allowing gravity to pull the robot downwards</a> – have been proposed as a way to <a href="http://www.jpl.nasa.gov/news/news.php?release=2013-077">deliver a submarine</a> into Europa’s oceans.</p>
<p>But energy is needed both to heat up the ice and then melt it -– a typical power station would be able to provide this in about five minutes. But it’s not going to be practical to send a power station to Europa. And with the amount of power available to most spacecraft, it would take a cryobot about eight years to get through the ice.</p>
<p>One way this problem can be solved is to use a compact nuclear fission reactor, which will do the job in about six weeks. But such a nuclear reactor wouldn’t fit into the cryobot. One problem solved, another is created. To get around this, <a href="http://www.igsoc.org:8080/annals/55/65/a65A200.pdf">one idea</a> involves leaving the reactor on the surface and sending the electrical power to the descending cryobot as light along a fibre-optic cable. Once the cryobot has reached the ocean it would deploy a submarine to take measurements. Communications with the cryobot could be achieved by sound waves in the ocean (think whales talking to each other) and then sent back up the connection to the surface vehicle for transmission to Earth.</p>
<p>Amazingly, these ideas have actually been <a href="https://www.newscientist.com/article/dn1786-ice-melting-robot-passes-arctic-test/">tested in Antarctica</a>. But one significant challenge is that, as the water melts, sediments build up ahead of the probe. Another is that the cryobot and submarine would have to undergo expensive extreme <a href="https://planetaryprotection.arc.nasa.gov/about">sterilisation</a> to avoid introducing any contamination to an environment that may harbour life.</p>
<p>So there are big hurdles to clear. But NASA does appear committed. It’s mission concept could possibly executed in the mid 2040s. And after Titan, who knows, we may even be hunting for <a href="https://en.wikipedia.org/wiki/Hydrothermal_vent">hydrothermal vents</a> on Europa.</p><img src="https://counter.theconversation.com/content/63417/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chris Arridge receives funding from the Science and Technology Facilities Council (STFC), the Royal Society and the Royal Astronomical Society. He also provides scientific advice to STFC and the UK Space Agency on solar system exploration.</span></em></p>We could be exploring the oceans of Jupiter’s and Saturn’s icy moons in a couple of decades. Here’s what we need to work out.Chris Arridge, Research Fellow/Lecturer, Lancaster UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/535832016-01-26T15:22:16Z2016-01-26T15:22:16ZExciting missions that could unlock secrets of the solar system in 2016<figure><img src="https://images.theconversation.com/files/109045/original/image-20160122-437-1nh62go.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Juno in front of Jupiter.</span> <span class="attribution"><span class="source">NASA/JPL</span></span></figcaption></figure><p>From the mystery of methane on Mars to how Jupiter formed and whether there is microbial life on Saturn’s moon Enceladus, there are many questions about our solar system waiting to be answered this year.</p>
<p>For planetary scientists, 2016 is a year of grand finales, anniversaries and planning major new missions. Let’s take a look at some of the most exciting possibilities. </p>
<h2>Mars</h2>
<p>In July 1976, NASA’s Viking Landers were the first probes to successfully reach the Martian surface. An immensely successful series of Mars landers followed. </p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/109043/original/image-20160122-403-qx3blr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/109043/original/image-20160122-403-qx3blr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=900&fit=crop&dpr=1 600w, https://images.theconversation.com/files/109043/original/image-20160122-403-qx3blr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=900&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/109043/original/image-20160122-403-qx3blr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=900&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/109043/original/image-20160122-403-qx3blr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1132&fit=crop&dpr=1 754w, https://images.theconversation.com/files/109043/original/image-20160122-403-qx3blr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1132&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/109043/original/image-20160122-403-qx3blr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1132&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The ExoMars 2016 Trace Gas Orbiter being moved.</span>
<span class="attribution"><a class="source" href="http://exploration.esa.int/jump.cfm?oid=57174">TsENKI/ESA</a></span>
</figcaption>
</figure>
<p>NASA’s Curiosity Rover is the latest. In 2016, Curiosity will pick its way through its current position in the Bagnold dunes – a band along the north-western flank of a 5.5km high mountain inside Gale Crater called Mount Sharp – and continue its drive up the mountain to reach iron-oxide layers (rust is a kind of iron oxide) and a clay-rich horizon – both believed to have formed from the reaction between water and Mars’ crust. We know that lakes filled the Gale Crater around 3.8 billion years ago. Curiosity will photograph, drill and analyse to find out more about Mars’ past warm and wet environment.</p>
<p>Europe is also aiming for Mars. ESA’s <a href="http://exploration.esa.int/mars/46475-trace-gas-orbiter/">ExoMars Trace Gas Orbiter</a> will launch on a Russian rocket in March and get there in October. It will follow up Curiosity’s discovery of traces of methane in Mars’ atmosphere, which could be a result of cosmic dust, geological processes or even past microbial life. ExoMars will test for current geological processes that might be releasing the methane. </p>
<p>It will also drop a simple, small lander to the surface. If it performs as planned then it is likely that a more ambitious ExoMars Rover – a two-metre drill on wheels with science instrumentation designed to test for traces of ancient life – will launch between 2018 and 2020. If ExoMars goes ahead as planned then the €150m or so needed to complete it could lead to postponements of other possible projects such as a mission to return material from Mars’ moon Phobos.</p>
<h2>Moon</h2>
<p>The moon also has a landmark anniversary: it is 50 years since the first successful robotic landing on its surface by the Soviet Union’s Luna 9. The Russians have always maintained a keen interest in the moon. Meanwhile, ESA has offered European countries <a href="http://www.bbc.co.uk/news/science-environment-34504067">a plan to collaborate with RosCosmos</a>, the Russian Space Agency, to start a new lunar exploration programme – with an initial focus on sampling the previously unstudied South Pole Aitken region. But budgets are limited. A decision at the European Council of Ministers in November about space priorities will be difficult, with projects targeting the moon, Mars’ moon Phobos and Mars all competing for funds.</p>
<p>Meanwhile, US company <a href="https://www.astrobotic.com/">Astrobotic Technology</a> hopes to be the first private lunar lander and rover mission in 2016. </p>
<h2>Outer solar system</h2>
<p>The <a href="https://www.nasa.gov/mission_pages/juno/main/index.html">Juno NASA spacecraft</a> will reach Jupiter in July. Juno will orbit Jupiter 32 times for a year helping us to answer questions about how the planet formed, how much water exists inside its atmosphere and how its mighty magnetosphere works.</p>
<p>Meanwhile, the <a href="http://saturn.jpl.nasa.gov/">Cassini mission</a> will begin a gradual grand finale in September, orbiting between Saturn and its outermost ring while flying past the moons Titan and Enceladus before crashing into Saturn in 2017. This will provide a last opportunity to analyse the water-rich geysers on Enceladus. In 2015, <a href="https://theconversation.com/the-chemistry-that-could-feed-life-within-saturns-moon-enceladus-study-gives-clue-ahead-of-flyby-49683">researchers even suggested</a> that certain chemical reactions inside its internal ocean may provide enough energy to feed microbial life. The study predicted that these would create molecular hydrogen that should be detectable in the plumes. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/109042/original/image-20160122-437-brzfjw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/109042/original/image-20160122-437-brzfjw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=707&fit=crop&dpr=1 600w, https://images.theconversation.com/files/109042/original/image-20160122-437-brzfjw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=707&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/109042/original/image-20160122-437-brzfjw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=707&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/109042/original/image-20160122-437-brzfjw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=888&fit=crop&dpr=1 754w, https://images.theconversation.com/files/109042/original/image-20160122-437-brzfjw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=888&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/109042/original/image-20160122-437-brzfjw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=888&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Enceladus craters and complex fractured terrains.</span>
<span class="attribution"><a class="source" href="http://www.esa.int/spaceinimages/Images/2007/03/Enceladus_craters_and_complex_fractured_terrains">ESA</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>An even more exciting candidate for life in our solar system is Jupiter’s moon Europa, which has a fractured crust of ice thought to overly an ocean which might harbour life. It would be nice if Europe got involved in exploring this, perhaps by contributing a “penetrator”, a light probe designed to bury itself on a body’s surface, to a planned <a href="http://www.jpl.nasa.gov/missions/europa-mission/">NASA fly by mission</a>. </p>
<p>A tiny member of the outer solar system is <a href="https://theconversation.com/explainer-what-philae-did-in-its-60-hours-on-comet-67p-34289">comet 67p/Churyumov-Gerasimenko</a>. In 2015, the Rosetta mission achieved a remarkable feat by landing on the 4km-wide cometary nucleus. Rosetta will be crashed into the comet in September – look out for some spectacular close-up images.</p>
<p>A NASA mission called <a href="http://www.asteroidmission.org/">Osiris Rex</a> to sample a carbonaceous asteroid called Bennu will be launched in September on a seven-year round trip. The mission could help us better understand the materials that make up our planets. Fittingly it comes on the tenth anniversary of the Stardust mission which returned samples from <a href="http://www.ncbi.nlm.nih.gov/pubmed/15205524">Jupiter Family Comet 81P/Wild2</a> and changed our view of what comets are made of. </p>
<p>So there’s a lot to look forward to. But aside from European, Russian and US missions, let’s not forget that China will steadily continue to build a space station and plans to go on to Mars and the <a href="https://theconversation.com/chinas-plan-to-be-first-to-far-side-of-the-moon-could-unveil-inner-lunar-secrets-53253">far side of the Moon</a>. India hopes to launch its first astronauts into orbit, and the Japanese Hayabusa2 mission will continue its journey to return samples from an asteroid. </p>
<p>It is impossible to determine what the greatest highlight of space science will be in 2016. But it might not even come from the glamour of space missions. Each year, 50,000 tonnes of cosmic dust and material from bodies in the solar system comes to planet Earth – and that can lead to unexpectedly great discoveries.</p><img src="https://counter.theconversation.com/content/53583/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>John Bridges receives funding from STFC, UKSA</span></em></p>Missions including ExoMars, Juno and Rosetta could make some major discoveries in 2016.John Bridges, Professor of Planetary Science, University of LeicesterLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/525312015-12-23T15:51:33Z2015-12-23T15:51:33ZThe magical Solar System discoveries we made in 2015<figure><img src="https://images.theconversation.com/files/106971/original/image-20151223-27875-10fe10l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Comet 67P/Churyumov-Gerasimenko, as seen from Rosetta.</span> <span class="attribution"><a class="source" href="http://www.esa.int/spaceinimages/Images/2015/11/Comet_on_17_November_2015_-_NAVCAM">ESA/Rosetta/NAVCAM</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>It has been a busy year for Solar System exploration – and particularly our galactic neighbourhood’s small icy bodies. Comets, asteroids, <a href="http://pluto.jhuapl.edu/Participate/learn/What-We-Know.php?link=The-Kuiper-Belt">Kuiper Belt Objects</a> and planetary satellites have all been in the news – from stunning images of comet <a href="http://sci.esa.int/rosetta/14615-comet-67p/">67P Churyumov-Gerasimenko</a> at the start of the year, to the recent close-up of Saturn’s moon, Enceladus, via Ceres and Pluto. </p>
<p>Early January was a continuation of the stream of data from <a href="http://www.esa.int/Our_Activities/Space_Science/Rosetta">Rosetta</a>, as comet 67P drew closer to the sun. Images were released of jets emanating from the <a href="http://www.esa.int/spaceinimages/Images/2015/01/Comet_activity_22_November_2014">sun-facing surface</a>, from which it could be seen that sublimation of water-ice increased during the daytime, and died down at night. But because the dark surface of the comet retained some heat, the comet was not completely inactive at night – it was possible that fluid might exist for very short periods, leading to sub-surface hydrous activity.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/106984/original/image-20151223-27880-1m1k3oh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/106984/original/image-20151223-27880-1m1k3oh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/106984/original/image-20151223-27880-1m1k3oh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/106984/original/image-20151223-27880-1m1k3oh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/106984/original/image-20151223-27880-1m1k3oh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/106984/original/image-20151223-27880-1m1k3oh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/106984/original/image-20151223-27880-1m1k3oh.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">Activity on Comet 67P.</span>
<span class="attribution"><a class="source" href="http://www.esa.int/spaceinimages/Images/2015/01/Comet_activity_22_November_2014">ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/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>One of the other significant results from Rosetta was recognition from magnetic measurements that the two lobes of the comet had been separate bodies, presumably brought together by collision. </p>
<p>Closest approach to the sun was in mid-August, a few weeks after the <a href="http://blogs.esa.int/rosetta/tag/philae-2/">Philae lander</a> signalled that it had woken up after its enforced hibernation. Unfortunately, communication between Rosetta and Philae could not be established reliably, leaving a certain amount of frustration that additional data could not be acquired from the surface. </p>
<h2>Welcome to Pluto (and Charon)</h2>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/106974/original/image-20151223-27863-s0qf68.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/106974/original/image-20151223-27863-s0qf68.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/106974/original/image-20151223-27863-s0qf68.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/106974/original/image-20151223-27863-s0qf68.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/106974/original/image-20151223-27863-s0qf68.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/106974/original/image-20151223-27863-s0qf68.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/106974/original/image-20151223-27863-s0qf68.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">Composite image of Pluto and Charon.</span>
<span class="attribution"><a class="source" href="http://www.nasa.gov/image-feature/charon-and-pluto-strikingly-different-worlds">NASA/JHUAPL/SwRI</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>July brought us a close-up of Pluto. The images have been tremendous, turning Pluto from a fuzzy grey blob into a fascinating body of mountains, plains and valleys. Pluto’s closest, and largest, moon, Charon was also captured by the New Horizon mission cameras in similar detail. <a href="https://www.nasa.gov/image-feature/charon-and-pluto-strikingly-different-worlds">The two worlds are very different</a> in their characteristics. Parts of Pluto’s icy surface are crumpled into <a href="https://www.nasa.gov/image-feature/plutos-incredible-diversity-of-surface-reflectivities-and-geological-landforms">mountains and ridges</a>. The “heart” of Pluto, around 1500km across, is a flat and featureless plain, presumably resurfaced relatively recently, showing that Pluto is more active than anticipated.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/106975/original/image-20151223-27851-10v23di.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/106975/original/image-20151223-27851-10v23di.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=541&fit=crop&dpr=1 600w, https://images.theconversation.com/files/106975/original/image-20151223-27851-10v23di.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=541&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/106975/original/image-20151223-27851-10v23di.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=541&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/106975/original/image-20151223-27851-10v23di.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=679&fit=crop&dpr=1 754w, https://images.theconversation.com/files/106975/original/image-20151223-27851-10v23di.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=679&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/106975/original/image-20151223-27851-10v23di.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=679&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Dark areas on Pluto.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/plutos-incredible-diversity-of-surface-reflectivities-and-geological-landforms">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Charon, with approximately half the diameter of Pluto, is also a <a href="https://www.nasa.gov/feature/pluto-s-big-moon-charon-reveals-a-colorful-and-violent-history">world of contrasts</a>. The southern hemisphere is flat, relatively smooth and low-lying, broken mainly by impact craters, while the northern regions are fractured with much more topography. Separating the two halves of the moon is an extensive system of canyons, perhaps akin to the Valles Marineris on Mars, probably caused by tectonic stress. Mission scientists have suggested that the southern region is younger than the northern, and has been resurfaced by cryovolcanism – instead of lava, cryovolcanos eject substances such as water, methane or ammonia – implying a frozen ocean below Charon’s crust.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/106976/original/image-20151223-27880-1vw2axp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/106976/original/image-20151223-27880-1vw2axp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/106976/original/image-20151223-27880-1vw2axp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/106976/original/image-20151223-27880-1vw2axp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/106976/original/image-20151223-27880-1vw2axp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/106976/original/image-20151223-27880-1vw2axp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/106976/original/image-20151223-27880-1vw2axp.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">Charon.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/feature/pluto-s-big-moon-charon-reveals-a-colorful-and-violent-history">NASA/JHUAPL/SwRI</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>It should be remembered that the wealth of data now returning to Earth from the New Horizon’s spacecraft was acquired during a fly-by of Pluto that lasted a mere 15 minutes. Imagine what could be learnt with an orbiter.</p>
<h2>A new dawn</h2>
<p><a href="http://dawn.jpl.nasa.gov">The Dawn mission</a> to Asteroid (1) Ceres was a bright spot – literally – in the planetary exploration calendar. At the start of the year, the spacecraft began to orbit the dwarf planet, and images revealed several patches of <a href="http://dawn.jpl.nasa.gov/multimedia/images/image-detail.html?id=PIA20180">highly reflective material</a>. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/106979/original/image-20151223-27894-z2s7i6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/106979/original/image-20151223-27894-z2s7i6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/106979/original/image-20151223-27894-z2s7i6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/106979/original/image-20151223-27894-z2s7i6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/106979/original/image-20151223-27894-z2s7i6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/106979/original/image-20151223-27894-z2s7i6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/106979/original/image-20151223-27894-z2s7i6.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">Ceres from Dawn.</span>
<span class="attribution"><a class="source" href="http://dawn.jpl.nasa.gov/multimedia/images/image-detail.html?id=PIA20180">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>At first it was suggested that the patches may be of water-ice. This would be unexpected, since the surface of Ceres is too warm for water-ice to be stable at the surface. It is now thought that the bright spots might be salts remaining <a href="https://theconversation.com/dawn-breaks-over-distant-ceres-and-perhaps-reveals-signs-of-habitability-38967">following evaporation (or sublimation of water)</a>.</p>
<h2>A ball of mud</h2>
<p>Ceres is currently thought to be like a ball of compressed mud, possibly with a layer of water-rich slurry below a thin crust of mud – not an attractive description for the largest of the asteroids, but one which clearly shows that the minor planet has had a lengthy history of aqueous activity. </p>
<p>This is important because, like comets, asteroids have played a significant role in the delivery of water and other volatile compounds to Earth. The Dawn spacecraft spent most of 2015 orbiting Ceres at gradually decreasing altitudes – settling, at the beginning of December, in its lowest orbit about 400km above the surface. </p>
<p>Images have shown that Ceres is crossed by troughs and grooves reminiscent of those present on other (larger) <a href="http://dawn.jpl.nasa.gov/multimedia/images/image-detail.html?id=PIA20186">planetary bodies</a>. Some of the features are impact-related, but some seem to have been produced by stress fracturing of the crust, another example of Ceres’ puzzling and sometimes contradictory history.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/106981/original/image-20151223-27858-psoq2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/106981/original/image-20151223-27858-psoq2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/106981/original/image-20151223-27858-psoq2w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/106981/original/image-20151223-27858-psoq2w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/106981/original/image-20151223-27858-psoq2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/106981/original/image-20151223-27858-psoq2w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/106981/original/image-20151223-27858-psoq2w.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">Ceres: time for your close-up.</span>
<span class="attribution"><a class="source" href="http://dawn.jpl.nasa.gov/multimedia/images/image-detail.html?id=PIA20186">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>The oceans of Enceladus</h2>
<p>Although the <a href="http://saturn.jpl.nasa.gov">Cassini mission</a> has been operational within the Saturnian system for over a decade, and is coming to the end of its life, it is still returning exciting data from the icy moon, Enceladus. In October, the spacecraft took a dive through the plume at the south pole, flying only about 50km above the surface. Then, only last week, Cassini completed its final close encounter with Enceladus, giving us pictures of <a href="http://www.ciclops.org/view_media/41440/Frozen-Fractures">frozen fractures and ridges</a>. We know that Enceladus has a sub-surface global salty ocean – placing it with Jupiter’s moon, Europa, as a possible host to an ocean-floor ecosystem.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/106985/original/image-20151223-27890-fagymy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/106985/original/image-20151223-27890-fagymy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/106985/original/image-20151223-27890-fagymy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/106985/original/image-20151223-27890-fagymy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/106985/original/image-20151223-27890-fagymy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/106985/original/image-20151223-27890-fagymy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/106985/original/image-20151223-27890-fagymy.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">The surface of Enceladus.</span>
<span class="attribution"><a class="source" href="http://photojournal.jpl.nasa.gov/catalog/PIA17209">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>
<h2>What next?</h2>
<p>The amazing images that have come from the missions to icy bodies have generated a great increase in public interest in Solar System exploration. A quick trawl through social media sites such as Twitter shows how avidly the missions are followed and results discussed, almost in real time. If 2015 was a Golden Age for the exploration of small icy bodies, we can only hope that among the legacies of the missions is a generation of students enthused to continue investigation of our neighbourhood. </p>
<p>But what may be in store for small icy bodies in 2016? Work has only just started for the cameras on board Dawn, as the spacecraft starts its mapping of Ceres. There will be more images and information from Pluto and its satellites, especially Charon. Cassini has made its final close fly-by of Enceladus, and the images will be returned throughout the coming year. Rosetta will watch 67P’s activity die down as the comet moves further and further away from the sun. But before we say a complete goodbye to comet Churyumov-Gerasimenko, maybe, just maybe, we will hear from Philae …</p><img src="https://counter.theconversation.com/content/52531/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Monica Grady receives funding from the STFC and is a Trustee of Lunar Mission One.</span></em></p>Prepare to be amazed …Monica Grady, Professor of Planetary and Space Sciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/496832015-10-27T16:03:28Z2015-10-27T16:03:28ZThe chemistry that could feed life within Saturn’s moon Enceladus: study gives clue ahead of flyby<figure><img src="https://images.theconversation.com/files/99556/original/image-20151024-27625-1r81j2d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Will Cassini find evidence of microbial life in the plumes from Enceladus? A new study has made it a whole lot easier.</span> <span class="attribution"><a class="source" href="http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=3852">NASA/JPL-Caltech/Space Science Institute </a></span></figcaption></figure><p>Saturn’s 502km-diameter icy moon Enceladus has fascinated scientists since it was <a href="http://voyager.jpl.nasa.gov/image/saturn.html">first seen up close</a> by NASA’s Voyager probes in the 1980s. The moon is venting plumes of ice particles into space including traces of methane, carbon and simple organic compounds – making it a <a href="https://theconversation.com/icy-plumes-bursting-from-saturns-moon-enceladus-suggest-it-could-harbour-life-38673">good candidate for harbouring life</a>. </p>
<p>Now laboratory experiments suggest that chemical reactions between the water of Enceladus’s internal ocean and its rocky core are likely to provide enough energy in the water to feed microbial life – a process similar to that near hot “hydrothermal” vents on the Earth’s ocean floor. The study, <a href="http://nature.com/articles/doi:10.1038/ncomms9604">published in Nature Communications</a>, predicts that the reactions should create molecular hydrogen that should be detectable in the plumes, which the Cassini probe could corroborate following its flyby on October 28.</p>
<h2>Plumes from surface fissures</h2>
<p>Enceladus’ plumes emerge from fissures near its south pole and <a href="http://saturn.jpl.nasa.gov/science/moons/enceladus/enceladusdiscoverytimeline/">were discovered</a> in 2005 by the Cassini probe after it had just arrived in orbit about Saturn.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/99570/original/image-20151025-27619-1tizdej.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/99570/original/image-20151025-27619-1tizdej.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/99570/original/image-20151025-27619-1tizdej.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=454&fit=crop&dpr=1 600w, https://images.theconversation.com/files/99570/original/image-20151025-27619-1tizdej.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=454&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/99570/original/image-20151025-27619-1tizdej.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=454&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/99570/original/image-20151025-27619-1tizdej.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=571&fit=crop&dpr=1 754w, https://images.theconversation.com/files/99570/original/image-20151025-27619-1tizdej.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=571&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/99570/original/image-20151025-27619-1tizdej.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=571&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Perspective view along one of the south polar fissures from which Enceladus’s plumes erupt, about 20km across.</span>
<span class="attribution"><span class="source">NASA/JPL/Space Science Institute/Universities Space Research Association/Lunar & Planetary Institute</span></span>
</figcaption>
</figure>
<p>As a result, Cassini was re-tasked to fly through these plumes several times, measuring their composition <a href="http://saturn.jpl.nasa.gov/spacecraft/cassiniorbiterinstruments/instrumentscassiniinms/">using instruments</a> originally designed to sample cosmic dust and the outer atmosphere of Saturn’s much larger moon Titan. As well as ice particles it documented traces of methane, ammonia, carbon monoxide, carbon dioxide, simple organic compounds and salts. Microscopic silica “nanoparticles”, about ten nanometres (a hundred thousandth of a millimetre) in size, were also found.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/99840/original/image-20151027-4985-1jqupvv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/99840/original/image-20151027-4985-1jqupvv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=755&fit=crop&dpr=1 600w, https://images.theconversation.com/files/99840/original/image-20151027-4985-1jqupvv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=755&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/99840/original/image-20151027-4985-1jqupvv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=755&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/99840/original/image-20151027-4985-1jqupvv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=948&fit=crop&dpr=1 754w, https://images.theconversation.com/files/99840/original/image-20151027-4985-1jqupvv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=948&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/99840/original/image-20151027-4985-1jqupvv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=948&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 depiction of Enceladus’s subsurface ocean and plumes.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<p>It also <a href="https://www.nasa.gov/press-release/cassini-finds-global-ocean-in-saturns-moon-enceladus">recently became clear</a> that the source of the plumes was not merely a vast underground pod of liquid water at the south pole, but an internal ocean of global extent, sandwiched between the icy crust and the rocky core.</p>
<p>The power to drive the plumes must come from heat generated by the varying tidal stresses experienced by Enceladus as it orbits Saturn. The tell-tale chemistry of the plumes shows interaction between the ocean water and the moon’s rocky core, which is therefore where most of the heat must be generated. </p>
<p>The heat is transferred to the internal ocean by so-called “<a href="http://www.whoi.edu/page.do?pid=83497&cid=112904&cl=74915&tid=5142">hydrothermal circulation</a>”. In this process, cold water is drawn down into the rock, where it becomes heated and then escapes back through the ocean floor, enriched in chemicals leached from the rock.</p>
<h2>Water-rock reactions</h2>
<p>The new study casts more light on what’s going on. The team conducted experiments in which they flushed saline water of compositions thought to exist inside Enceladus through heated powdered rock samples at a pressure equivalent to that within Enceladus. </p>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/99558/original/image-20151024-27625-1lj7rk1.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/99558/original/image-20151024-27625-1lj7rk1.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=552&fit=crop&dpr=1 600w, https://images.theconversation.com/files/99558/original/image-20151024-27625-1lj7rk1.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=552&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/99558/original/image-20151024-27625-1lj7rk1.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=552&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/99558/original/image-20151024-27625-1lj7rk1.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=693&fit=crop&dpr=1 754w, https://images.theconversation.com/files/99558/original/image-20151024-27625-1lj7rk1.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=693&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/99558/original/image-20151024-27625-1lj7rk1.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=693&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 rendering depicting possible hydrothermal activity at the floor of Enceladus’s subsurface ocean.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<p>Their chemical analysis of the resulting hydrothermally altered water across a range of reaction temperatures from 50 to 350°C is a good fit to what is actually documented in the plumes. Notably they found that methane and carbon dioxide can be present at the same time (as observed at Enceladus). They also found that ammonia will survive without being converted to nitrogen, thereby possibly explaining why no nitrogen has been observed in Enceladus’ plumes.</p>
<p>The silica nanoparticles in Enceladus’s plumes originate as particles known as colloids flushed out in the hot water escaping from the altered rock. Moreover, they can best be explained if the rock is similar in composition to <a href="http://www.meteorite.fr/en/classification/carbonaceous.htm">carbonaceous chondrites</a>, the most primitive class of meteorites. This suggests that the rocky core of Enceladus formed very early in solar system’s history, and never melted to form a more chemically evolved type of rock.</p>
<p>One thing predicted by these experiments but not yet found at Enceladus is molecular hydrogen, which is an expected byproduct of water-rock interactions. Fortunately the presence of molecular hydrogen in the ocean water can be tested with results from <a href="http://saturn.jpl.nasa.gov/mission/flybys/enceladus20151028/">Cassini’s deepest-ever flight</a> through Enceladus’s plumes at a height of only 49km above the surface on October 28.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/QuAgTEunHFA?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">NASA animation of Cassini’s flyby of Enceladus October 28, 2015.</span></figcaption>
</figure>
<h2>Methanogenic life?</h2>
<p>Enceladus’s plumes makes it a member of a small but distinguished club of moons known to be active. In particular, the reactions between water and hot rock described in the new study, charges the emerging hot water with chemical energy that can be exploited by microbial life.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/99812/original/image-20151027-4974-x3zwqo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/99812/original/image-20151027-4974-x3zwqo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/99812/original/image-20151027-4974-x3zwqo.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/99812/original/image-20151027-4974-x3zwqo.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/99812/original/image-20151027-4974-x3zwqo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/99812/original/image-20151027-4974-x3zwqo.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/99812/original/image-20151027-4974-x3zwqo.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">Ocean-floor hydrothermal vents on Earth. The white ‘smoke’ is chemicals precipitating as the hot water emerging from the vents mixes with the ocean water.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Hydrothermal_vent">NOAA/wikimedia</a></span>
</figcaption>
</figure>
<p>This happens at <a href="http://oceanservice.noaa.gov/facts/vents.html">hydrothermal vents</a> on Earth’s ocean floor, where microbes get their energy from mediating the chemical reaction that turns dissolved carbon dioxide plus hydrogen into biomass plus methane plus water. This metabolic pathway is described as <a href="http://toxics.usgs.gov/definitions/methanogenesis.html">methanogensis</a> and doesn’t require sunlight. In fact a whole ecosystem can exist supported by methanogenic microbes, rather than by the photosynthetic plants with which we are familiar.</p>
<p>Hydrothermal vents are intriguing when it comes to life. Researchers are investigating them as a possible source of the very <a href="http://london-nerc-dtp.org/the-origin-of-life-in-alkaline-hydrothermal-vents/">origin of life</a> here on Earth. One team is currently trying to copy what they do by building an <a href="https://www.leverhulme.ac.uk/awards-made/awards-focus/origin-life-reactor">origin of life reactor</a>.</p>
<p>Whether there is actually any life, microbial or even possibly more advanced, clustered around the hydrothermal vents on the floor of Enceladus’s ocean, is likely to remain a mystery for a while. Even though Enceladus obligingly sprays samples out into space where they can be collected without the expense and technical challenges of landing and finding a way below the ice, Cassini’s instruments were not built to detect and analyse direct tracers for life. However, if it finds molecular hydrogen in the plumes we will know that at least conditions inside Enceladus are highly likely to be suitable for life. </p>
<p>Our next chance to find better evidence depends on NASA funding the Enceladus Life Finder, the <a href="http://www.space.com/30419-alien-life-search-enceladus-mission.html">ELF mission</a>, or a similar mission for further development.</p><img src="https://counter.theconversation.com/content/49683/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Rothery is author of Moons: A Very Short Introduction (Oxford University Press, 2015) and Planets: A Very Short Introduction (Oxford University Press, 2010). He receives funding from the UK Space Agency and the Science & Technology Facilities Council for work related to Mercury and the European Space Agency's Mercury orbiter BepiColombo</span></em></p>Study pinpoints some of the chemical signs of microbial life on Enceladus. The study could in part be corroborated by Cassini’s flyby of the moon.David Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/386732015-03-11T19:35:43Z2015-03-11T19:35:43ZIcy plumes bursting from Saturn’s moon Enceladus suggest it could harbour life<figure><img src="https://images.theconversation.com/files/74519/original/image-20150311-24188-hpeflk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">With geysers bursting through an icy crust, Enceladus is a tiny moon with a big personality.</span> <span class="attribution"><span class="source">Hsiang-Wen Hsu et al/Nature</span>, <span class="license">Author provided</span></span></figcaption></figure><p>The <a href="http://saturn.jpl.nasa.gov/mission/introduction/">Cassini</a> mission that has investigated Saturn since 2004 has revealed much about the giant planet and its many moons. Perhaps most tantalising is the discovery that the moon Enceladus is the source of strong geysers ejecting plumes of water and ice.</p>
<p>A new <a href="http://www.google.com/url?q=http%3A%2F%2Fdx.doi.org%2F10.1038%2Fnature14262&sa=D&sntz=1&usg=AFQjCNGdOviBn9Rlh1do4S58gDVPgMfNDA">study of Cassini data</a> published in Nature by Hsiang-Wen Hsu and colleagues reveals these plumes are laced with grains of sand. This indicates that hydrothermal activity may be at work in Enceladus’ sub-surface ocean, and propels this tiny moon into the extremely exclusive club of locations that could harbour life. </p>
<p>The club’s only current member is Earth, of course – although it’s very possible that <a href="https://theconversation.com/the-moon-was-a-first-step-mars-will-test-our-capabilities-but-europa-is-the-prize-37253">Europa</a>, one of Jupiter’s moons, is, like Enceladus, also a candidate. What they have in common is that they host liquid oceans of salty water that exists in contact with a rocky, silicate seabed from which the oceans can absorb complex minerals and elements.</p>
<h2>A bit of a geyser</h2>
<p>With a diameter of just 500km Enceladus is nevertheless the sixth largest of Saturn’s more than 60 moons, orbiting at a distance of just two planet-widths. Cassini has shown that Enceladus is the source of huge geysers of neutral water-rich gas and ice grains erupting at a rate of 100-300kg per second. This makes Enceladus the second most active object, after Jupiter’s moon Io which ejects one tonne per second of sulphur-rich material. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/74535/original/image-20150311-24212-1djlagy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/74535/original/image-20150311-24212-1djlagy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/74535/original/image-20150311-24212-1djlagy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=704&fit=crop&dpr=1 600w, https://images.theconversation.com/files/74535/original/image-20150311-24212-1djlagy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=704&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/74535/original/image-20150311-24212-1djlagy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=704&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/74535/original/image-20150311-24212-1djlagy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=885&fit=crop&dpr=1 754w, https://images.theconversation.com/files/74535/original/image-20150311-24212-1djlagy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=885&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/74535/original/image-20150311-24212-1djlagy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=885&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">How the plumes are formed, from beneath Enceladus’ surface.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>Gravity measurements have shown that there is at least a local and <a href="http://www.sciencemag.org/content/344/6179/78.short">possibly a global ocean</a> under Enceladus’ icy crust, and some of the emitted grains are <a href="http://www.nature.com/nature/journal/v459/n7250/full/nature08046.html">rich in sodium salt</a>, which indicates the presence of a salty ocean. Now we also discover that some are silicate-rich, and analysis shows that these may have been produced close to hydrothermal vents at temperatures above 90°C. This raises the interesting comparison with <a href="http://www.lostcity.washington.edu/science/biology/newlife.html">hydrothermal vents on Earth</a>, which may have played a role in the origin of life on our planet.</p>
<h2>The recipe for life</h2>
<p>For life as we know it to exist, four key ingredients are important: liquid water; the right chemistry involving the elements carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur; a source of heat; and enough time for life to develop. While we know these conditions exist on Earth, planetary research throughout the solar system shows that it may exist on other objects too, and the details from this paper pushes Enceladus towards the top of the list.</p>
<p>We know liquid water oceans exist on several objects in our solar system. These include Earth with its surface oceans, and Jupiter’s moons Europa, Ganymede and Callisto, and Saturn’s moons Titan and Enceladus where the oceans are below the surface. Water has also played a vital role in Mars’ history: Geronimo Villanueva and colleagues recently showed that there may have been enough water on Mars to cover the planet in <a href="http://www.sciencemag.org/content/early/2015/03/04/science.aaa3630">an ocean 137 metres deep</a> around 3.8 billion years ago –- about the time when life was starting on Earth. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/74534/original/image-20150311-24212-1runpow.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/74534/original/image-20150311-24212-1runpow.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/74534/original/image-20150311-24212-1runpow.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=350&fit=crop&dpr=1 600w, https://images.theconversation.com/files/74534/original/image-20150311-24212-1runpow.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=350&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/74534/original/image-20150311-24212-1runpow.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=350&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/74534/original/image-20150311-24212-1runpow.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=439&fit=crop&dpr=1 754w, https://images.theconversation.com/files/74534/original/image-20150311-24212-1runpow.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=439&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/74534/original/image-20150311-24212-1runpow.jpg?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">The view from Cassini towards the geyser region of Enceladus.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>There may also be water on the dwarf planets <a href="https://theconversation.com/dawn-eases-into-orbit-around-the-dwarf-planet-ceres-38398">Ceres</a> and Pluto, Neptune’s moon Triton, and several other objects in the solar system – but only further investigation will tell. Two other objects have lakes and oceans, but not of water. Titan has lakes of methane and ethane, for example – the only extraterrestrial object we know of with liquid on the surface – and volcanic Io has a <a href="http://www.sciencemag.org/content/332/6034/1186.short">subsurface ocean of liquid magma</a>.</p>
<h2>A shortlist for extraterrestrial life</h2>
<p>So where are the best places to look for life in our solar system? The short list now seems to be Mars, Europa and Enceladus. At Mars the most likely time for life to have existed is 3.8 billion years ago when water was present, so the <a href="http://exploration.esa.int/mars/45084-exomars-rover/">ESA-Russia ExoMars rover</a> due for launch in 2018 will focus on drilling 2m below the present surface’s harsh oxidising and radiation-rich environment to search for buried evidence from the past. It carries our PanCam instrument which will provide context for the mission.</p>
<p>As for right now, Mars may be a less good candidate for life. Following a catastrophic collision about 3.8 billion years ago the planet underwent massive climate change, volcanic activity stopped, and the planet’s magnetic field disappeared. But the recent confirmation by Curiosity of the presence of methane is tantalising. At Europa, <a href="http://sci.esa.int/juice/">ESA’s JUICE mission</a> and the proposed <a href="http://www.jpl.nasa.gov/missions/europa-clipper/">NASA Europa Clipper</a> may bring more clues in the 2030s, but further missions to Enceladus have yet to make it past the proposal stage.</p>
<p>Nevertheless, this leaves Europa and Enceladus as prime sites where conditions may be suitable for life to exist now – but who knows which other solar system objects could be the next to join the club.</p><img src="https://counter.theconversation.com/content/38673/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Coates receives funding from STFC and the UK Space Agency.</span></em></p>Saturn’s tiny moon Enceladus joins Europa and Mars as possible locations for life beyond Earth.Andrew Coates, Professor of Physics, Head of Planetary Science at the Mullard Space Science Laboratory, UCLLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/250692014-04-03T19:46:54Z2014-04-03T19:46:54ZWaterworld? Cassini spots the motion of Enceladus’s ocean<figure><img src="https://images.theconversation.com/files/45494/original/vygk58s7-1396496991.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">One of Saturn's many moons, Enceladus seems to have a large body of water hiding under its icy crust.</span> <span class="attribution"><span class="source">NASA</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>An ocean of water has been found underneath the icy crust of <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Object=Enceladus">Enceladus</a>, Saturn’s sixth largest moon, according to observations of the <a href="http://www.nasa.gov/mission_pages/cassini/main/">Cassini</a> spacecraft <a href="http://www.sciencemag.org/">published in Science today</a>.</p>
<p>This result has come from painstaking charting of the movements of the spacecraft, monitored here on Earth. From measuring small changes in the flight path as Cassini sailed by Enceladus, the scientists here detected a large gravity anomaly close to its southern pole. </p>
<p>Similar to how the gravity fields of <a href="http://www.nasa.gov/audience/foreducators/k-4/features/F_Measuring_Gravity_With_Grace.html">Earth</a> and the <a href="http://www.nasa.gov/mission_pages/grail/main/index.html">moon</a> have been measured, the spacecraft would move slightly closer to Enceladus when flying over a denser area, and slightly further away when the ground underneath was less dense.</p>
<p>Coupled with the information that has been gathered about the rest of this body, a model has now been constructed of the moon’s interior that includes a large ocean in line with this anomaly.</p>
<p>This is another significant piece in the puzzle of this strange icy moon. Enceladus is barely <a href="http://www.nasa.gov/images/content/126470main_pia07724-browse.jpg">the size of the UK</a> but, on examination by the Cassini spacecraft, has been shown to be an <a href="http://www.bbc.co.uk/science/space/solarsystem/solar_system_highlights/cryovolcano">active little world</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/45272/original/fygtydnp-1396332883.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/45272/original/fygtydnp-1396332883.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/45272/original/fygtydnp-1396332883.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=371&fit=crop&dpr=1 600w, https://images.theconversation.com/files/45272/original/fygtydnp-1396332883.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=371&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/45272/original/fygtydnp-1396332883.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=371&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/45272/original/fygtydnp-1396332883.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=466&fit=crop&dpr=1 754w, https://images.theconversation.com/files/45272/original/fygtydnp-1396332883.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=466&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/45272/original/fygtydnp-1396332883.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=466&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Enceladus’s water vapour jets, emitted from the southern polar region.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech and Space Science Institute</span></span>
</figcaption>
</figure>
<h2>Water water everywhere</h2>
<p>We’ve suspected there is something going on in that southern pole region for some time. One of the first major results of the Cassini mission was the observation of “<a href="http://www.nasa.gov/mission_pages/cassini/media/cassini-083005.html">tiger stripes</a>” – four long and recently active fissures that Cassini has spent time studying.</p>
<p>The stripes (which, for your daily dose of trivia, are known as Baghdad, Damascus, Cairo and Alexandria after cities in the book <a href="http://www.bartleby.com/16/">One Thousand and One Nights</a>) are darker compared to the rest of Enceladus’s surface, thought to be because of the larger ice crystals formed there.</p>
<p>As well as the stripes close to the southern pole, the region puts out a <a href="http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20101130/">lot of heat</a>. Cassini’s measurements with its infrared spectrometer instrument show that up 15.8GW (gigawatts) of power is flowing from its icy crust here.</p>
<p>This amount of heat is thought to be too much to be sustainable though – so we could be observing a current event.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/45273/original/2f4bfxkf-1396332884.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/45273/original/2f4bfxkf-1396332884.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/45273/original/2f4bfxkf-1396332884.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=601&fit=crop&dpr=1 600w, https://images.theconversation.com/files/45273/original/2f4bfxkf-1396332884.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=601&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/45273/original/2f4bfxkf-1396332884.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=601&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/45273/original/2f4bfxkf-1396332884.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=755&fit=crop&dpr=1 754w, https://images.theconversation.com/files/45273/original/2f4bfxkf-1396332884.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=755&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/45273/original/2f4bfxkf-1396332884.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=755&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 possible interior of Enceladus based on Cassini gravity investigation, which suggests an ice outer shell and a low density, rocky core with a regional water ocean sandwiched at high southern latitudes.</span>
<span class="attribution"><span class="source">Image courtesy of NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<p>Enceladus is the latest in a series of icy moons that we’ve suspect are hiding more beneath their surfaces. There’s evidence from magnetic measurements for there to be a global ocean under the ice of <a href="http://science1.nasa.gov/science-news/science-at-nasa/2000/ast10jan_1/">Jupiter’s moon Europa</a>, and similar large regional oceans are suspected for Ganymede and Titan.</p>
<p>Why is this observation so interesting? In the case of Europa, and now Enceladus the models suggest that their oceans extend down to the rocks below, such as the buried Antarctic seas of <a href="http://www.extremetech.com/extreme/160667-3500-species-discovered-in-lake-vostok-underneath-miles-of-ice-in-conditions-similar-to-jupiters-europa">Vostok</a> and <a href="http://www.bbc.com/news/science-environment-20850360">Ellesworth</a>. </p>
<p>This is pretty crucial as that means it will be warm down there, and the oceans will have easy access to a heap of minerals and nutrients from the rocks. Who knows what could be living off these nutrients.</p>
<p>Hence, the evidence adds up to a large and active body of water under Enceladus’s southern polar region. But it is going to be a long time before we can get out there and verify if this ocean is there, if ever. </p>
<h2>Retirement is nigh</h2>
<p>Cassini took seven years to travel to the giant ringed planet, arriving in 2004. I know what you’re all thinking; Cassini has been touring around Saturn and its moons for 10 years now – so how come it’s not seen this gravity anomaly before?</p>
<p>That answer, in part, has been that there’s been just too much for Cassini to study! The exploration of Saturn and its moons has turned out to be more akin to a massive sweet shop than many had expected. Scientists working on the mission really didn’t know what to scoff first.</p>
<p>From the <a href="http://www.nasa.gov/mission_pages/cassini/multimedia/pia16481.html">seasons</a> and <a href="http://www.bbc.com/news/science-environment-26622586">lakes</a> on Titan, to the <a href="http://saturn.jpl.nasa.gov/science/moons/recentdiscoveries/">new moons</a> and <a href="http://burro.astr.cwru.edu/Academics/Astr221/SolarSys/Rings/dynamics.html">dynamics of Saturn’s rings</a> this mission has given endless delights and surprises to the Cassini scientists.</p>
<p>The key difficulty with the gravity measurements of Enceladus is that when flying past the moon, Cassini’s antenna has to be pointed at Earth to track its movements. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/45274/original/rs3kjmkr-1396332887.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/45274/original/rs3kjmkr-1396332887.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/45274/original/rs3kjmkr-1396332887.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/45274/original/rs3kjmkr-1396332887.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/45274/original/rs3kjmkr-1396332887.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/45274/original/rs3kjmkr-1396332887.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/45274/original/rs3kjmkr-1396332887.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/45274/original/rs3kjmkr-1396332887.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=603&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">One of NASA’s Deep Space Network antennas used in Enceladus’s gravity field determination.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<p>To do this most of the other instruments on board, such as the infrared spectrometer and high-resolution camera, don’t point at the surface – so other discoveries could be missed. There have only been three flybys where gravity measurements could be made, the last one was in <a href="http://saturn.jpl.nasa.gov/mission/flybys/enceladus20120306/">March 2012</a>.</p>
<p>Cassini’s mission has been in its solstice phase since 2008, after the primary mission expired. Its plutonium power source still has enough energy to power the craft until 2017, but there are worries that the Cassini mission may be brought to an abrupt halt before then. </p>
<p>NASA (who runs the mission, which was a joint NASA, <a href="http://www.esa.int/ESA">European Space Agency</a> and <a href="http://www.asi.it/en">Italian Space Agency</a> venture) is looking to make cuts, and there’s concern that <a href="http://www.wired.com/2013/11/end-of-cassini-science/">Cassini will lose out</a> to the Mars science laboratory, <a href="http://mars.jpl.nasa.gov/msl/">Curiosity</a>. </p>
<p>Whether it happens sooner, rather than later, the fate of Cassini is sealed. Plans are afoot to direct it to <a href="http://www.spaceanswers.com/space-exploration/1501/why-will-cassini-impact-saturn-in-2017/">plummet into Saturn</a>, to prevent it crashing into the potential enclaves of life on Titan and Enceladus. </p>
<p>We can only hope that decisions here on Earth means it can be left to make many more discoveries before that fateful day. </p><img src="https://counter.theconversation.com/content/25069/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Helen Maynard-Casely 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>An ocean of water has been found underneath the icy crust of Enceladus, Saturn’s sixth largest moon, according to observations of the Cassini spacecraft published in Science today. This result has come…Helen Maynard-Casely, Instrument Scientist, Australian Nuclear Science and Technology OrganisationLicensed as Creative Commons – attribution, no derivatives.