tag:theconversation.com,2011:/africa/topics/charon-20544/articlesCharon – The Conversation2016-09-14T17:01:29Ztag:theconversation.com,2011:article/651942016-09-14T17:01:29Z2016-09-14T17:01:29ZMordor mystery: scientists solve puzzle of the strange, dark north pole on Pluto’s moon Charon<figure><img src="https://images.theconversation.com/files/137256/original/image-20160909-13383-8s9jru.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Charon's north pole, imaged by New Horizons.</span> <span class="attribution"><a class="source" href="http://pluto.jhuapl.edu/Multimedia/Science-Photos/image.php?page=6&gallery_id=2&image_id=323">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a></span></figcaption></figure><p>Had <a href="https://theconversation.com/stunning-crystal-clear-images-of-pluto-but-what-do-they-mean-47517">Pluto itself</a> not proved to be so spectacular when NASA’s <a href="http://pluto.jhuapl.edu/">New Horizons</a> probe flew past last year, there can be no doubt that its large moon <a href="https://theconversation.com/images-of-plutos-moon-charon-show-huge-fractures-and-hints-of-icy-lava-flows-48533">Charon</a> would have won more admirers.</p>
<p>The remarkable moon has a mysterious dark-red stain over its north pole, called “Mordor Macula” by the New Horizons team – where Macula means “dark spot” and Mordor refers to the “black land” in Tolkien’s The Lord of the Rings. While many bodies in the solar system have polar caps or hoods of some sort, these are typically bright, due to reflective ice or frost of some kind, rather than dark. So what’s going on at Charon? A new study, <a href="http://nature.com/articles/doi:10.1038/nature19340">published in Nature</a>, has proposed an answer.</p>
<p>One of Charon’s most interesting features is a vast chasm system, which cuts across the middle of Charon’s Pluto-facing hemisphere and reaches a maximum depth of 7.5km. This speaks of an era of upheaval when slabs of Charon’s icy crust were ruptured, tilted and then partially flooded by a type of lava produced by icy volcanism.</p>
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<figcaption>
<span class="caption">Charon’s vast equatorial chasms. The most obvious ones are unofficially named Macross Chasma (on the left) and Serenity Chasma (on the right).</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<p>Charon’s surface is mostly dirty, grey water-ice, and the red stain around the north pole looks to be a thin film, coating but not burying the underlying topographic features such as craters.</p>
<p>Mordor Macula resembles the red-stained areas of Pluto. This led to early suggestions that they share a similar origin. However, Pluto’s red stuff is understood to be tarry molecules called <a href="http://www.planetary.org/blogs/guest-blogs/2015/0722-what-in-the-worlds-are-tholins.html">tholins</a>, which form in the atmosphere when sunlight strikes molecules and makes them sufficiently reactive to link together. This forms haze particles, which eventually settle to the ground. Unlike Pluto, however, Charon has no trace of an atmosphere, so how can tholins form there?</p>
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<figcaption>
<span class="caption">Pluto’s hazy atmosphere, made visible by scattered sunlight seen by New Horizons.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<h2>Molecules of Mordor Macula</h2>
<p>The researchers behind the new study think they have the answer. The New Horizons probe detected two sorts of molecules <a href="http://www.nasa.gov/feature/science-papers-reveal-new-aspects-of-pluto-and-its-moons">leaking away from Pluto</a>: methane and nitrogen. Although nitrogen is the most abundant gas in Pluto’s atmosphere, methane is lighter and is being lost about 500 times faster. Tholins and their precursors are far too heavy, and cannot escape Pluto in the same way, so Charon’s tholins must be somehow made from methane arriving from Pluto.</p>
<p>Charon orbits very close to Pluto, and quite a lot of Pluto’s escaped methane falls onto its surface. If a methane molecule strikes the night side of Charon it will stick, particularly near the winter pole where the temperature is lowest (less than 30 degrees above absolute zero). This is because at lower temperatures surface molecules vibrate more slowly, and atmospheric molecules travel more slowly, so collisions are gentle. However, if a methane molecule hits a sunlit part of the surface, it will tend to bounce off and fly away into space because of Charon’s weak gravity (which has long since allowed it to lose any atmosphere that it may once have possessed). </p>
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<figcaption>
<span class="caption">Charon (left) and Pluto (right) at the same scale. Charon is 1,212 km in diameter, Pluto 2,370 km.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<p>Ultraviolet sunlight shining on methane molecules coating Charon’s surface could stimulate photochemical reactions to link them into progressively longer molecular chains, to form tholins. However, the “Catch 22” is that a methane molecule on the cold night-side surface ought to be able to break free in the daytime, when the temperature rises to the giddy heights of -220°C – allowing it to float away into space and become permanently lost. And as there is no sunlight at night, methane molecules can’t link together (making them too massive to float away) before the dawn of a new day.</p>
<p>In a surprising result the new study shows that, even by night, Charon’s poles receive enough ultraviolet light to allow methane molecules to link together. This is because interplanetary dust in the region scatters sunlight in all directions, including onto the night side of Charon. Once a molecule is a chain of two or three methanes linked together (with or without the occasional nitrogen for good measure) it is probably heavy enough to stay bound to the surface even during the daytime. Normal daytime sunlight can then take over to complete the process of forming tholins.</p>
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<img alt="" src="https://images.theconversation.com/files/137292/original/image-20160911-13345-18fsxlt.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/137292/original/image-20160911-13345-18fsxlt.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=180&fit=crop&dpr=1 600w, https://images.theconversation.com/files/137292/original/image-20160911-13345-18fsxlt.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=180&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/137292/original/image-20160911-13345-18fsxlt.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=180&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/137292/original/image-20160911-13345-18fsxlt.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=226&fit=crop&dpr=1 754w, https://images.theconversation.com/files/137292/original/image-20160911-13345-18fsxlt.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=226&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/137292/original/image-20160911-13345-18fsxlt.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=226&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">An example of a tholin molecule. This is a molecule proposed to form in the atmosphere of Saturn’s moon Titan. Tholins on Charon probably have less nitrogen.</span>
<span class="attribution"><span class="source">after Ehrenfreund et al., 1996</span></span>
</figcaption>
</figure>
<p>But what about the south pole? Could Mordor Macula’s location at Charon’s north pole, where the temperature is low enough for cold-trapping of methane, be just a fluke? Apparently not. Although it was winter in Charon’s southern hemisphere when New Horizon made its flyby, keeping its south pole in darkness, the researchers were able to study the south pole by “Pluto shine” (sunlight reflected off Pluto). The spatial resolution of these data is poor, but it is sufficient to demonstrate that Charon’s south pole has similarly dark red material at its surface. So there is not just one Mordor on Charon, but two.</p><img src="https://counter.theconversation.com/content/65194/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>Trapped gas could be tainting the north pole of Pluto’s moon Charon dark red.David Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/562612016-03-17T18:01:55Z2016-03-17T18:01:55ZPicture of Pluto further refined by months of New Horizons data<figure><img src="https://images.theconversation.com/files/115335/original/image-20160316-30211-mjo9qb.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">New Horizons continues to help unravel the icy dwarf planet's secrets.</span> <span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>When the New Horizons spacecraft made its <a href="https://theconversation.com/a-team-members-view-of-all-the-work-on-earth-it-took-to-get-new-horizons-to-pluto-44696">flyby of Pluto</a> on July 14, 2015, there was worldwide celebration that we’d finally gotten <a href="https://theconversation.com/in-focus-new-horizons-crisp-images-shed-new-light-on-the-origins-of-pluto-and-its-moons-44451">our first detailed look</a> at this completely new type of planet in the outer reaches of our solar system.</p>
<p>But for those of us on the <a href="http://pluto.jhuapl.edu/Mission/The-Team.php">New Horizons science team</a>, that day and those first images were only the beginning. Since then, I’ve been watching with amazement as the New Horizons spacecraft has transmitted spectacular images back that reveal surprises all over the place. We’ve been making discovery after discovery about the dwarf ice planet Pluto and its moon Charon, and this is likely to continue as we get more data back from the spacecraft. Here’s a summary of just a few of our scientific results to date. </p>
<h2>What do we see on Pluto’s surface?</h2>
<p>Perhaps one of the biggest surprises that was obvious from the very first images was that Pluto has a <a href="http://dx.doi.org/10.1126/science.aad7055">surface that is incredibly diverse</a>. </p>
<p>Some surface areas, such as those that are heavily cratered from asteroid impacts, seem to date back to just after Pluto formed, about 4.5 billion years ago. Other regions show evidence of geological activity that may have lasted throughout Pluto’s billions of years of history. Enormous ice volcanoes (cryovolcanoes) must have taken a large fraction of Pluto’s history to form. These volcanoes are driven by warm underground liquids, such as, perhaps, water and ammonia, instead of liquid rock-magma that we have on Earth, and their rough, crusty surface is made of stuff that has erupted from deep within Pluto’s interior. </p>
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<figcaption>
<span class="caption">Pluto’s Sputnik Planum captured hearts here on Earth.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
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<p>Other areas, such as the informally named Sputnik Planum – the heart-shaped, Texas-sized nitrogen ice glacier – show no evidence of asteroid impacts at all, suggesting continual surface activity, such as convection of ices from underground. This surface can’t be more than 10 million years old – a blink of the eye on a geological time scale! </p>
<p>Pluto is geologically active! I doubt there’s a single person on Earth who would have expected to see that!</p>
<h2>What’s Pluto made of?</h2>
<p>The diverse chemical compositions we’ve seen on Pluto are giving us some important clues to understanding <a href="http://dx.doi.org/10.1126/science.aad9189">Pluto’s geological history and climate</a>.</p>
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<figcaption>
<span class="caption">Pluto’s rugged, icy cratered plains.</span>
<span class="attribution"><a class="source" href="http://pluto.jhuapl.edu/Multimedia/Science-Photos/image.php?page=2&gallery_id=2&image_id=385">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
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<p>The high-resolution images from the New Horizons cameras show diverse ice reservoirs across Pluto’s surface. By studying the reflected spectra from the surface, we’ve identified several different types of ices: in particular, nitrogen, methane and carbon monoxide. The locations and characteristics of these ice reservoirs mean that there have been long epochs of ice transport across the dwarf planet’s surface.</p>
<p>A darker veneer on top of the ices is probably tholin material – organic compounds processed by solar radiation. These are produced slowly in Pluto’s atmosphere and gently rain down, even now, onto the surface. An enormous darker region, informally named Cthulhu Regio, has a meters-thick layer of this organic tholin material that has built up over billions of years. Frozen water is one of the strongest solids at the low temperatures we see on Pluto. We believe that the ice mountains that extend several miles above the surface are made of water ice – the biggest ice cubes in the solar system. </p>
<p>Charon, too, had some major surprises in store for us. Pluto’s largest moon has an extended equatorial region of smooth plains that may also be due to material that erupted from Charon’s interior via ice volcanoes that then flowed over its surface about four billion years ago. We suspect that was when Charon’s subsurface water ocean froze, causing global fractures as the moon expanded in size (water expands when it freezes).</p>
<p>Charon has dark poles that may be related to volatile gases that escaped from Pluto’s atmosphere only to be captured by the moon’s cold poles. These gases trigger chemical reactions on the surface that, we believe, produce the darker color of the poles. </p>
<h2>How does Pluto’s atmosphere work?</h2>
<p>The <a href="http://www.boulder.swri.edu/palice/">spacecraft’s Alice instrument</a> made observations of sunlight passing <a href="http://dx.doi.org/10.1126/science.aad8866">through Pluto’s atmosphere</a>. We see absorption features that indicate an atmosphere made up of nitrogen (like Earth’s) with methane, acetylene and ethylene as minor constituents.</p>
<p>Pluto’s small size and low gravity cause it to hold onto its atmosphere much more weakly than larger planets like the Earth (which has 16 times stronger gravity than Pluto). Prior to the New Horizons’ encounter, we expected this would produce an atmosphere that was greatly extended and rapidly escaping to space. But it turned out the upper atmosphere is much colder than we thought it would be and so more compact – the atmosphere does not extend nearly as far into space as we expected and the escape rate of atmospheric gases is extremely slow. But why the atmosphere is so cold is still a complete mystery.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/115334/original/image-20160316-30231-etsjv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/115334/original/image-20160316-30231-etsjv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/115334/original/image-20160316-30231-etsjv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=256&fit=crop&dpr=1 600w, https://images.theconversation.com/files/115334/original/image-20160316-30231-etsjv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=256&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/115334/original/image-20160316-30231-etsjv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=256&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/115334/original/image-20160316-30231-etsjv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=322&fit=crop&dpr=1 754w, https://images.theconversation.com/files/115334/original/image-20160316-30231-etsjv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=322&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/115334/original/image-20160316-30231-etsjv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=322&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Pluto’s haze is a photochemical smog.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>As an atmospheric scientist, I found the most amazing discovery to be the brilliant, light blue, globally extensive haze that we can see because large numbers of small atmospheric particles scatter sunlight. This haze extends hundreds of kilometers into space, and embedded within it are over 20 very thin, but far brighter, layers. We suspect the thin layers are produced by some type of atmospheric wave that causes localized regions of condensation of some as-yet-unknown gas. The largest moon of Saturn, Titan, shows similar layering of haze in its upper atmosphere. So there may be some interesting comparative planetary studies that come out of the analysis of the Pluto data.</p>
<h2>Where did Pluto’s moons come from?</h2>
<p>The origin of <a href="http://dx.doi.org/10.1126/science.aae0030">Pluto’s five moons</a> has been a long-standing question. But the flyby observations have given us some critical data that we needed in order to develop convincing explanations.</p>
<p>We believe that Charon is just about as old as Pluto, having formed when Pluto was very young. An impact between Pluto and another large Kuiper Belt object early in their history ejected an enormous amount of debris into orbit around Pluto. As time went on, this orbital debris coalesced into Charon.</p>
<p>Previous speculation was that the four smaller moons are actually asteroids captured by Pluto’s gravity as they passed too near the dwarf planet. But the New Horizons observations showed that these four moons have an unusually high reflectivity – much different than the extremely dark materials that we see on asteroids in the outer solar system. This has led to a compelling argument that the smaller moons are also made of debris from the same impact that formed Charon.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/115333/original/image-20160316-30241-1ca6h5c.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/115333/original/image-20160316-30241-1ca6h5c.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/115333/original/image-20160316-30241-1ca6h5c.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=329&fit=crop&dpr=1 600w, https://images.theconversation.com/files/115333/original/image-20160316-30241-1ca6h5c.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=329&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/115333/original/image-20160316-30241-1ca6h5c.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=329&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/115333/original/image-20160316-30241-1ca6h5c.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=413&fit=crop&dpr=1 754w, https://images.theconversation.com/files/115333/original/image-20160316-30241-1ca6h5c.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=413&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/115333/original/image-20160316-30241-1ca6h5c.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=413&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Pluto’s atmosphere is buffeted by protons and electrons streaming from the sun as the solar wind.</span>
<span class="attribution"><span class="source">H.A. Weaver et al. / Science (2016)</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>How does Pluto interact with its space environment?</h2>
<p>As the New Horizons spacecraft approached Pluto, there was some concern that a small amount of debris might still remain in orbit around the dwarf planet. A collision between New Horizons and even one particle of debris the size of a grain of sand could cause considerable damage to, or possibly destroy, the spacecraft.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/115341/original/image-20160316-30234-1ehh95s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/115341/original/image-20160316-30234-1ehh95s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/115341/original/image-20160316-30234-1ehh95s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=603&fit=crop&dpr=1 600w, https://images.theconversation.com/files/115341/original/image-20160316-30234-1ehh95s.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=603&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/115341/original/image-20160316-30234-1ehh95s.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=603&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/115341/original/image-20160316-30234-1ehh95s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=758&fit=crop&dpr=1 754w, https://images.theconversation.com/files/115341/original/image-20160316-30234-1ehh95s.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=758&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/115341/original/image-20160316-30234-1ehh95s.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=758&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 Student Dust Counter is one part of the science payload New Horizons’ been carrying for a decade.</span>
<span class="attribution"><a class="source" href="http://pluto.jhuapl.edu/Mission/Spacecraft/Payload.php">NASA/New Horizons</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>But the student-built Dust Counter, which measures small micrometer size dust particles in space, detected only a single particle during the flyby – and it was much too small to cause spacecraft damage. This means Pluto’s environment is now largely devoid of debris – all of it likely swept up by the moons early in the system’s history.</p>
<p>The <a href="http://dx.doi.org/10.1126/science.aad9045">New Horizons spacecraft also carried instruments</a> to study what happens to the <a href="http://solarscience.msfc.nasa.gov/SolarWind.shtml">solar wind</a> when it encounters Pluto’s atmosphere. The detailed way in which solar wind particles from the sun interact with a planet’s atmosphere provides important clues about the nature of that atmosphere, particularly how far it extends into space and the escape rate of atmospheric gases. The interaction region between Pluto and the solar wind was observed to be much smaller than expected, only about 12 Pluto diameters across. This means that the atmosphere is smaller than expected, and so these results confirm the Alice observations that the upper atmosphere is much colder than expected. </p>
<h2>So much more yet to come</h2>
<p>These are just a few of the many exciting, and unexpected, results from the New Horizons flyby of Pluto and Charon. The discoveries we’ve already made will mean that textbooks on planetary science must be rewritten. And yet this sampling of the New Horizons results is just from the tip of an ice mountain of data that we’ll be analyzing and writing papers about for many years, perhaps decades. The data are so rich in things we’ve never seen before that I’m sure there are many more surprises yet to come.</p><img src="https://counter.theconversation.com/content/56261/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Mike Summers receives funding from NASA. </span></em></p>After last summer’s Pluto flyby, the New Horizons spacecraft started sending data back to Earth – at 2 kilobits per second. Here’s some of what scientists have learned so far from that rich, slow cache.Mike Summers, Professor of Planetary Science and Astronomy, George Mason UniversityLicensed 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/485332015-10-06T05:29:38Z2015-10-06T05:29:38ZImages of Pluto’s moon Charon show huge fractures and hints of icy ‘lava flows’<figure><img src="https://images.theconversation.com/files/97098/original/image-20151002-23101-3houuq.jpg?ixlib=rb-1.1.0&rect=14%2C2%2C1859%2C670&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Charon has a huge fracture system, unlike anything seen on Pluto.</span> <span class="attribution"><a class="source" href="http://pluto.jhuapl.edu/Multimedia/Science-Photos/pics/Charon-Neutral-Bright-Release.jpg">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a></span></figcaption></figure><p>Since the <a href="https://theconversation.com/new-horizons-is-an-old-spacecraft-but-it-will-transform-our-knowledge-of-pluto-44524">New Horizons probe</a> flew through the Pluto system <a href="https://theconversation.com/new-horizons-finally-gets-up-close-with-pluto-for-15-minutes-44603">on July 14</a>, most of the attention has been on Pluto itself. But it also has a comparatively large and – as we now know – fascinating moon called Charon. </p>
<p>The vast haul of data from New Horizons is now being transmitted to Earth at a slow rate because the signal is so weak. Yet some amazingly <a href="https://www.nasa.gov/feature/pluto-s-big-moon-charon-reveals-a-colorful-and-violent-history">detailed images</a> have been released, helping researchers trace the moon’s violent history through its ridges and craters.</p>
<p>Charon is usually pronounced “Shairon” or even “Sharon”, though “Kairon” would be a more faithful rendering of the name of the <a href="http://www.theoi.com/Khthonios/Kharon.html">ferryman of the classical underworld</a> after whom it is named. Charon and Pluto are locked in a tidal embrace that matches their rotation rates to their orbital period about each other, so that the same side of Charon always faces Pluto, and vice versa.</p>
<p>Our prior understanding is that Charon’s surface is mostly water-ice, tainted by ammonia and lacking the more exotic methane, carbon monoxide and nitrogen ices that so <a href="https://theconversation.com/stunning-crystal-clear-images-of-pluto-but-what-do-they-mean-47517">spectacularly coat parts of Pluto</a>. Charon is a darker, drabber world than Pluto when you see an identically processed image of each (below).</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/97101/original/image-20151002-23109-1ckl0c1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/97101/original/image-20151002-23109-1ckl0c1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/97101/original/image-20151002-23109-1ckl0c1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/97101/original/image-20151002-23109-1ckl0c1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/97101/original/image-20151002-23109-1ckl0c1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/97101/original/image-20151002-23109-1ckl0c1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/97101/original/image-20151002-23109-1ckl0c1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/97101/original/image-20151002-23109-1ckl0c1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Pluto (lower right) and Charon (upper left) identically processed.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<h2>Surface secrets</h2>
<p>But there is still plenty of variety across the face of Charon if you look for it. The image below is an enhanced colour view of Charon’s Pluto-facing hemisphere, brightened up relative to the previous dark view, showing details as small as 2.9 km. The brown smudge over its north pole might be discoloured by tiny particles of tar that have somehow drifted across from the top of Pluto’s atmosphere, though if so we can’t yet explain their concentration around Charon’s pole.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/97099/original/image-20151002-23109-141ruwe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/97099/original/image-20151002-23109-141ruwe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/97099/original/image-20151002-23109-141ruwe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/97099/original/image-20151002-23109-141ruwe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/97099/original/image-20151002-23109-141ruwe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/97099/original/image-20151002-23109-141ruwe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/97099/original/image-20151002-23109-141ruwe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/97099/original/image-20151002-23109-141ruwe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Full globe of Charon.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<p>As a geologist, what interests me most is Charon’s surface history. The north is more heavily cratered than the south, which tells us that the terrain in the north is older, whereas the south has been subjected to some kind of resurfacing process that has either erased or buried the older craters. While we can tell that Charon has been resurfaced more recently than parts of Pluto, we don’t know the exact mechanism for this. It could have to do with the tidal-heating interaction between Pluto and Charon, but few people expected such apparently recent tidal heating. </p>
<p>If this were our own moon, we could use density of craters to work out a surface age in billions of years, because we know the rate at which impacts have occurred to form those craters. But because we don’t know the rate of crater formation on Pluto, all we can say is that Charon’s more densely cratered surfaces are older than the less densely cratered areas.</p>
<p>Perhaps the most impressive aspect of Charon is the system of fractures that stretch right across the globe. The broad fracture in the right-centre of the globe in the previous view has been provisionally named Serenity Chasma, and is seen in more detail in the view below, which includes a mosaic of high-resolution images showing features as small as 0.8 km. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/97100/original/image-20151002-23072-kw9f8m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/97100/original/image-20151002-23072-kw9f8m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/97100/original/image-20151002-23072-kw9f8m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=787&fit=crop&dpr=1 600w, https://images.theconversation.com/files/97100/original/image-20151002-23072-kw9f8m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=787&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/97100/original/image-20151002-23072-kw9f8m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=787&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/97100/original/image-20151002-23072-kw9f8m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=989&fit=crop&dpr=1 754w, https://images.theconversation.com/files/97100/original/image-20151002-23072-kw9f8m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=989&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/97100/original/image-20151002-23072-kw9f8m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=989&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Part of Charon in the highest resolution yet available.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<p>There’s a clue here as to what might have gone on. The ground surface on the south of Serenity Chasma has been heaved up and tilted away from the rift, and the old tilted surface disappears below a smooth younger surface with fewer craters than the northern terrain. So, maybe, the original surface was arched up over the rift and became split as the rift opened, and something smooth came flooding out. That could easily be “lava flows” made of an ammonia-water mixture erupted from Charon’s interior, in a process called “<a href="http://www.wired.com/tag/cryovolcanism/">cryovolcanism</a>” to distinguish it from the much hotter volcanism on Earth. Some much narrower cracks crossing those plains also demonstrate a later phase of fracturing.</p>
<h2>Charon - the movie</h2>
<p>The New Horizons team has used images obtained from different viewpoints to make a stereoscopic model of parts of Charon’s surface, from which they constructed the artificial fly-over video below. It starts high above the north pole, swoops down to fly along the length of Serenity Chasma, and ends with a view across the cryovolcanic plains towards a moated mountain in the distance.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/lrMBzJcvtt0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">NASA’s fly-over movie of Charon, made from real images.</span></figcaption>
</figure>
<h2>Cousin moons</h2>
<p>Are there other worlds out there like Charon? Well, maybe some of the <a href="http://voyager.jpl.nasa.gov/science/uranus_moons.html">moons of Uranus</a>, which have ammonia mixed with their water-ice too. At 1207km in diameter, Charon is a little larger than <a href="http://www.bbc.co.uk/science/space/solarsystem/moons/ariel_%28moon%29">Ariel</a> but considerably smaller than <a href="http://www.space.com/28837-living-on-titania-uranus-moon-explained-infographic.html">Titania</a>. All three have about 1.6 to 1.7 times the density of water, so they must contain about the same proportion of rock in their interiors. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/97134/original/image-20151004-23063-1t981tj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/97134/original/image-20151004-23063-1t981tj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=307&fit=crop&dpr=1 600w, https://images.theconversation.com/files/97134/original/image-20151004-23063-1t981tj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=307&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/97134/original/image-20151004-23063-1t981tj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=307&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/97134/original/image-20151004-23063-1t981tj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=386&fit=crop&dpr=1 754w, https://images.theconversation.com/files/97134/original/image-20151004-23063-1t981tj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=386&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/97134/original/image-20151004-23063-1t981tj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=386&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Voyager 2 views of two icy fractured moons of Uranus that are most similar to Charon. Left: Ariel, 1155 km diameter. Right: Titania, 1576 km diameter.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<p>During the only <a href="http://voyager.jpl.nasa.gov/science/uranus.html">fly-by of Uranus</a> (by Voyager 2 in 1986) we found that the globes of both Ariel and Titania are crossed by giant fracture systems and there are signs of cryovolcanic resurfacing on Ariel. The trouble is that we saw Uranus’s moons much less well than we have now seen Charon, and the Voyager images, which seemed staggeringly detailed 30 years ago, are now frustratingly limited. </p>
<p>But Charon shows us that Ariel and Titania are likely to be more complicated than they seem. Maybe it’s time for another probe to explore the wonders around Uranus.</p><img src="https://counter.theconversation.com/content/48533/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Rothery is author of Planets: A Very Short Introduction (Oxford University Press, 2010) and Moons: A Very Short Introduction (Oxford University Press, 2015). 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>Pluto’s moon Charon seems to have had a violent past, with icy volcanoes leaving huge fractures.David Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/475172015-09-18T11:26:47Z2015-09-18T11:26:47ZStunning, crystal-clear images of Pluto – but what do they mean?<figure><img src="https://images.theconversation.com/files/95261/original/image-20150917-7534-11t5a81.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The best shot yet.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/feature/pluto-wows-in-spectacular-new-backlit-panorama">NASA/JHUAPL/SwRI</a></span></figcaption></figure><p>The more we find out about Pluto, the more perplexing it seems. For several weeks after its <a href="https://theconversation.com/live-blog-new-horizons-flyby-of-pluto-44670">July 14 fly-by</a>, NASA’s probe <a href="https://theconversation.com/new-horizons-is-an-old-spacecraft-but-it-will-transform-our-knowledge-of-pluto-44524">New Horizons</a> was too busy doing science to transmit data to Earth. During that time we had to content ourselves with the few <a href="https://theconversation.com/historic-close-ups-of-pluto-and-its-moon-charon-present-puzzle-for-scientists-44615http://example.com/">“taster” images</a> that were beamed back immediately after it passed Pluto. </p>
<p>However the probe has now begun the year-long process of transmitting its vast haul of fly-by data, including images that are crisper and more reliable because they preserve original details that we couldn’t see in the compressed versions. A selection of <a href="http://pluto.jhuapl.edu/Multimedia/Science-Photos/">new images</a> is released every Thursday – with the latest revealing Pluto’s haze layers in stunning detail.</p>
<h2>Crazy haze</h2>
<p>The picture above is my favourite. It was captured just 15 minutes after closest approach, looking back sunwards across part of the southern edge of the bright area informally known as Tombaugh Regio. </p>
<p>The water-ice mountains adjoining and projecting up through it are wonderfully shadowed, and look at the incredibly complex haze layers that you can see above the distant horizon. These are probably a product of the solar ultraviolet light stripping hydrogen from molecules of methane, which then link together to form long chains of <a href="http://www.planetary.org/blogs/guest-blogs/2015/0722-what-in-the-worlds-are-tholins.html">tar-like substances</a> called tholins. These form tiny particles, and those that settle to the ground help to give it a reddish stain, but why there are so many layers in the atmosphere, we don’t know.</p>
<p>Below is a spectacular simulated view (compiled from several individual images) showing the Tombaugh Regio area to the north and the dark, heavily-cratered terrain to its south. These craters are prominent because of their bright rims, which could be nitrogen frost. What catches my attention in particular on this image are the curved and <a href="http://www.lpi.usra.edu/education/explore/shaping_the_planets/tectonism.shtml">branching fractures</a> that sweep across the terrain in the lower left and slice through several of the smaller craters, providing evidence of an episode of geological activity, maybe even within the past billion years.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/94676/original/image-20150914-4698-p0kjfw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/94676/original/image-20150914-4698-p0kjfw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/94676/original/image-20150914-4698-p0kjfw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=389&fit=crop&dpr=1 600w, https://images.theconversation.com/files/94676/original/image-20150914-4698-p0kjfw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=389&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/94676/original/image-20150914-4698-p0kjfw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=389&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/94676/original/image-20150914-4698-p0kjfw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=489&fit=crop&dpr=1 754w, https://images.theconversation.com/files/94676/original/image-20150914-4698-p0kjfw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=489&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/94676/original/image-20150914-4698-p0kjfw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=489&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Simulated view looking north-east from 1,800km above Pluto’s equator. Tombaugh Regio dominates the upper right.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<p>You can see the northern ends of a couple of these fractures in the lower left of the next image. More importantly, it shows a relationship between Pluto’s dominant landscape-forming material (water-ice) and the more <a href="https://www.nasa.gov/press-release/nasa-s-new-horizons-discovers-frozen-plains-in-the-heart-of-pluto-s-heart">volatile ices</a> (such as frozen nitrogen, methane and carbon monoxide) that make up the pale terrain of Tombaugh Regio. At the very low temperatures on Pluto’s surface (-225°) water-ice is as strong as rock on Earth.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/94685/original/image-20150914-4695-18jjpux.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/94685/original/image-20150914-4695-18jjpux.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/94685/original/image-20150914-4695-18jjpux.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=469&fit=crop&dpr=1 600w, https://images.theconversation.com/files/94685/original/image-20150914-4695-18jjpux.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=469&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/94685/original/image-20150914-4695-18jjpux.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=469&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/94685/original/image-20150914-4695-18jjpux.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=590&fit=crop&dpr=1 754w, https://images.theconversation.com/files/94685/original/image-20150914-4695-18jjpux.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=590&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/94685/original/image-20150914-4695-18jjpux.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=590&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Detail of part of the western edge of Tombaugh Regio. Image is 470km across.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics La boratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<p>The landscape in the left third of this image is composed of water-ice. The very flat terrain in the right third, which is part of Tombaugh Regio, is weaker and cannot be frozen water. You can see it divided into slabs or cells. Some on the <a href="https://theconversation.com/a-team-members-view-of-all-the-work-on-earth-it-took-to-get-new-horizons-to-pluto-44696">New Horizons team</a> have suggested it might be traces of convection currents driven by heat from Pluto’s interior.</p>
<p>There’s a rugged mountain peak of water-ice, more than a kilometre high, piercing the exotic ice in the lower right. The middle third of this image reveals for the first time that all the water-ice around this edge of Tombaugh Regio has been disrupted into a jumble of giant, jagged blocks, of which that mountain is just a large example. As yet we have no idea how this happened, but it suggests that each cell in the interior of Tombaugh Regio might be sitting over its own block of disrupted water-ice.</p>
<h2>Layers of ice</h2>
<p>The next image is from a little further south-east around the edge of Tombaugh Regio. Here the water-ice terrain has been stained very dark, perhaps by particles of tar settling out from the nitrogen atmosphere. There’s a similar pattern of giant jumbled blocks, but there’s a new twist here. If you look carefully you can make out fine patterns of lines, in both the dark terrain and in some of the bright terrain.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/94686/original/image-20150914-4678-e2rbo0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/94686/original/image-20150914-4678-e2rbo0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/94686/original/image-20150914-4678-e2rbo0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=541&fit=crop&dpr=1 600w, https://images.theconversation.com/files/94686/original/image-20150914-4678-e2rbo0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=541&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/94686/original/image-20150914-4678-e2rbo0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=541&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/94686/original/image-20150914-4678-e2rbo0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=679&fit=crop&dpr=1 754w, https://images.theconversation.com/files/94686/original/image-20150914-4678-e2rbo0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=679&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/94686/original/image-20150914-4678-e2rbo0.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"></a>
<figcaption>
<span class="caption">View, 350km wide, including a dark area at the south-western edge of Tombaugh Regio.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<p>One suggestion from the New Horizons team is that these are dunes composed of wind-blown ice grains, preserved from a time when Pluto’s atmosphere was denser. The present atmosphere is mostly nitrogen with a little carbon monoxide and methane, and there are signs that it is currently freezing out onto the surface as <a href="https://theconversation.com/pluto-and-its-collision-course-place-in-our-solar-system-43404">Pluto’s eccentric orbit</a> carries it further from the sun. I’m sceptical about the dunes, and wonder if instead the pattern records <a href="http://www.atacamaphoto.com/atacama/atacama45.htm">ablation of ice-like penitentes</a> on Earth – when snow formations at high altitudes are removed by melting or evaporation.</p>
<p>Next a view covering the northern edge of Tombaugh Regio. At the upper left, note the glacier-like flow of the weak exotic ice into the rugged water-ice terrain. The part of Tombaugh Regio on the right cannot be floored by weak ice; it is rugged terrain, no different to that seen elsewhere except that its surface is especially bright. This is probably nitrogen snow covering water-ice terrain, and there are signs that nitrogen-ice flows glacier-like from the bright rigged terrain into the flat area. Why here? How recent? We don’t know.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/95259/original/image-20150917-7507-gotyhe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/95259/original/image-20150917-7507-gotyhe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/95259/original/image-20150917-7507-gotyhe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=429&fit=crop&dpr=1 600w, https://images.theconversation.com/files/95259/original/image-20150917-7507-gotyhe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=429&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/95259/original/image-20150917-7507-gotyhe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=429&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/95259/original/image-20150917-7507-gotyhe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=539&fit=crop&dpr=1 754w, https://images.theconversation.com/files/95259/original/image-20150917-7507-gotyhe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=539&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/95259/original/image-20150917-7507-gotyhe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=539&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Pluto’s Tombaugh Regio.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<p>This next image, like the top one was taken just 15 minutes after the closest approach to Pluto. The mountains on the left are up to 3,500 meters high. The layers of haze in Pluto’s atmosphere are also visible.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/95263/original/image-20150917-7521-1o9pg86.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/95263/original/image-20150917-7521-1o9pg86.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=386&fit=crop&dpr=1 600w, https://images.theconversation.com/files/95263/original/image-20150917-7521-1o9pg86.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=386&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/95263/original/image-20150917-7521-1o9pg86.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=386&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/95263/original/image-20150917-7521-1o9pg86.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=485&fit=crop&dpr=1 754w, https://images.theconversation.com/files/95263/original/image-20150917-7521-1o9pg86.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=485&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/95263/original/image-20150917-7521-1o9pg86.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=485&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Area including the informally named icy plain Sputnik Planum, which is part of Tombaugh Regio.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/sites/default/files/thumbnails/image/nh-apluto-mountains-plains-9-17-15_0.png">NASA/JHUAPL/SwRI</a></span>
</figcaption>
</figure>
<h2>Charon</h2>
<p>And finally, Pluto’s large moon Charon. It’s not been getting much attention, but for me it too is a fascinating world. New Horizons found no trace of an atmosphere and the surface is mostly water-ice but it boasts the only dark polar cap in the solar system. Did tar particles from Pluto’s atmosphere somehow make it across to Charon? </p>
<p>Elsewhere there are plenty of impact craters, though probably fewer than on Pluto’s water-ice terrains, which poses a problem of why and how Charon has been resurfaced more recently than much of Pluto. Maybe there is some twist to the <a href="http://www.open.edu/openlearn/science-maths-technology/science/physics-and-astronomy/icy-bodies-europa-and-elsewhere/content-section-1.4">tidal-heating interaction</a> between Pluto and Charon that we have yet to fathom. Charon also boasts a globe-spanning system of kilometres-deep fractures, grander than anything seen on Pluto so far.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/94694/original/image-20150914-31151-ca21m4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/94694/original/image-20150914-31151-ca21m4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/94694/original/image-20150914-31151-ca21m4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/94694/original/image-20150914-31151-ca21m4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/94694/original/image-20150914-31151-ca21m4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/94694/original/image-20150914-31151-ca21m4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/94694/original/image-20150914-31151-ca21m4.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 best global view of Pluto’s largest moon Charon (1,200km diameter). The dark north polar cap can be seen at the top.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<h2>Challenges ahead</h2>
<p>While the images tell us a lot about Pluto and Charon, they also raise a number of new questions. We simply don’t know what controls the localised nitrogen snowfall, the intricate haze layers or the old fractures that cut through the craters. We don’t know why Pluto’s surface is so diverse or its atmosphere so complex, or how much of this is driven by tidal interactions between Pluto and Charon. The list goes on. Scientists will be using New Horizons data to try to answer these questions for decades to come.</p><img src="https://counter.theconversation.com/content/47517/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Rothery is author of Planets: A Very Short Introduction (Oxford University Press, 2010) and Moons: A Very Short Introduction (Oxford University Press, 2015). 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>From a mysterious haze to strange nitrogen snowfall, the latest pictures of Pluto pose many new questions.David Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.