Editing Charon (moon) (section)

==Physical characteristics==
{{main|Geology of Charon}}
{{see also|List of geological features on Charon}}

[[File:Charon, Earth & Moon size comparison.jpg|thumbnail|right|Size comparisons: [[Earth]], the [[Moon]], and Charon]]
Charon's diameter is {{convert|1212|km}}, just over half that of Pluto.<ref name="Stern_2015" /><ref name="Stern_2017" /> Larger than the dwarf planet [[Ceres (dwarf planet)|Ceres]], it is the twelfth-largest [[natural satellite]] in the [[Solar System]]. Charon is similar in size to [[Uranus]]'s moons [[Umbriel]] and [[Ariel (moon)|Ariel]]. Charon's slow rotation means that there should be little flattening or tidal distortion if Charon is sufficiently massive to be in [[hydrostatic equilibrium]]. Any deviation from a perfect sphere is too small to have been detected by observations by the ''New Horizons'' mission. This is in contrast to [[Iapetus (moon)|Iapetus]], a Saturnian moon similar in size to Charon but with a pronounced [[Spheroid#Oblate spheroids|oblate]]ness dating to early in its history. The lack of such oblateness in Charon could mean that it is currently in hydrostatic equilibrium, or simply that its orbit approached its current one early in its history, when it was still warm.<ref name="Nimmo2017"/>

Based on mass updates from observations made by ''New Horizons''<ref name="Stern_2017" /> the mass ratio of Charon to Pluto is 0.1218:1. This is much larger than the Moon to the Earth: 0.0123:1. Because of the high mass ratio, the [[barycenter]] is outside of the radius of Pluto, and the Pluto–Charon system has been referred to as a dwarf [[double planet]]. With four smaller satellites in orbit about the two larger worlds, the Pluto–Charon system has been considered in studies of the orbital stability of [[circumbinary planet]]s.<ref name="Sutherland_2019" />

=== Internal structure ===
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Two proposed models of Charon's interior
* A possible outcome of the ''hot start'' model, with two different levels of silicate 'fines,' or micron-sized particles<ref name="desch2017"/>
* A possible outcome of the ''cold start'' model<ref name="malamud2017"/>
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Charon's volume and mass allow calculation of its density, {{val|1.702|0.017|u=g/cm3}},<ref name="Stern_2017" /> from which it can be determined that Charon is slightly less dense than Pluto and suggesting a composition of 55% rock to 45% ice (±&nbsp;5%), whereas Pluto is about 70% rock. The difference is considerably lower than that of most suspected collisional satellites.{{which|date=January 2024}}

Following the ''New Horizons'' flyby, numerous discovered features on Charon's surface strongly indicated that Charon is differentiated, and may even have had a subsurface ocean early in its history. The past resurfacing observed on Charon's surface indicated that Charon's ancient subsurface ocean may have fed large-scale cryoeruptions on the surface, erasing many older features.<ref name="bagheri2022"/><ref name="desch2017"/><ref name="skyandtelescope2015"/> As a result, two broad competing views on the nature of Charon's interior arose: the so-called ''hot start'' model, where Charon's formation is rapid and involves a violent impact with Pluto, and the ''cold start'' model, where Charon's formation is more gradual and involves a less violent impact with Pluto.

According to the hot start model, Charon accreted rapidly (within ~{{val|e=4}} years) from the circumplanetary disc, resulting from a highly-disruptive giant impact scenario. This rapid time scale prevents the heat from accretion from radiating away during the formation process, leading to the partial melting of Charon's outer layers. However, Charon's crust failed to reach a melt fraction where complete differentiation occurs, leading to the crust retaining part of its silicate content upon freezing. A liquid subsurface ocean forms during or soon after Charon's accretion and persists for approximately 2 billion years before freezing, possibly driving cryovolcanic resurfacing of Vulcan Planitia. Radiogenic heat from Charon's core could then melt a second subsurface ocean composed of a [[eutectic]] water-ammonia mixture before it too freezes, possibly driving the formation of Kubrick Mons and other similar features. These freezing cycles could increase Charon's size by >20 km, leading to the formation of the complex tectonic features observed in Serenity Chasma and Oz Terra.<ref name="desch2017"/>

In contrast, the cold start model argues that a large subsurface ocean early in Charon's history is not necessary to explain Charon's surface features, and instead proposes that Charon may have been homogeneous and more porous at formation. According to the cold start model, as Charon's interior begins to warm due to radiogenic heating and heating from [[serpentinization]], a phase of contraction begins, largely driven by compaction in Charon's interior. Approximately 100–200 million years after formation, enough heat builds up to where a subsurface ocean melts, leading to rapid differentiation, further contraction, and the hydration of core rocks. Despite this melting, a pristine crust of amorphous water ice on Charon remains. After this period, differentiation continues, but the core can no longer absorb more water, and thus freezing at the base of Charon's mantle begins. This freezing drives a period of expansion until Charon's core becomes warm enough to begin compaction, starting a final period of contraction. Serenity Chasma may have formed from the expansion episode, whilst the final contraction episode may have given rise to the arcuate ridges observed in Mordor Macula.<ref name="malamud2017"/>

=== Surface ===
{{See also|Kubrick Mons}}
[[File:CharonFeatureMap Annotated.png|thumb|A map of Charon with [[International Astronomical Union|IAU]] names|center|upright=3]]
[[File:Pluto & Charon - Mountains Craters and Plains.jpg|thumb|Comparison between Pluto's [[Sputnik Planitia]] and Charon's informally named [[Vulcan Planitia]]]]
Unlike Pluto's surface, which is composed of [[nitrogen]] and [[methane]] ices, Charon's surface appears to be dominated by the less [[Volatile (astrogeology)|volatile]] water ice.

In 2007, observations by the [[Gemini Observatory]] detected patches of ammonia hydrates and water crystals on the surface of Charon that suggested the presence of active [[Geyser#Cryogeysers|cryogeysers]] and [[cryovolcano]]es. The fact that the ice was still in crystalline form suggested it may have been deposited recently, as it was expected that solar radiation would have degraded it to an [[Amorphous ice|amorphous]] state after roughly thirty thousand years.<ref name="ice"/><ref name="cook2007" /> However, following new data from the ''New Horizons'' flyby, no active cryovolcanoes or geysers were detected. Later research has also called into question the cryovolcanic origin for the crystalline water ice and ammonia features, with some researchers instead proposing that ammonia may be replenished passively from underground material.<ref name="holler2017"/>

Photometric mapping of Charon's surface shows a latitudinal trend in [[albedo]], with a bright equatorial band and darker poles. The north polar region is dominated by a very large dark area informally dubbed "[[Mordor Macula|Mordor]]" by the ''New Horizons'' team.<ref name="theweekmordor"/><ref name="NBC1"/><ref name="NYTmountains"/> The favored explanation for this feature is that it is formed by condensation of gases that escaped from [[Atmosphere of Pluto|Pluto's atmosphere]]. In winter, the temperature is −258&nbsp;°C, and these gases, which include nitrogen, carbon monoxide, and methane, condense into their solid forms; when these ices are subjected to solar radiation, they chemically react to form various reddish [[tholin]]s. Later, when the area is again heated by the Sun as Charon's seasons change, the temperature at the pole rises to −213&nbsp;°C, resulting in the volatiles sublimating and escaping Charon, leaving only the tholins behind. Over millions of years, the residual tholin builds up thick layers, obscuring the icy crust.<ref name="physorgmordor"/> In addition to Mordor, ''New Horizons'' found evidence of extensive past geology that suggests that Charon is probably differentiated;<ref name="NBC1" /> in particular, the southern hemisphere has fewer craters than the northern and is considerably less rugged, suggesting that a massive resurfacing event—perhaps prompted by the partial or complete freezing of an internal ocean—occurred at some point in the past and removed many of the earlier craters.<ref name="skyandtelescope2015"/>

Charon has a system of extensive [[graben]]s and scarps, such as [[List of geological features on Charon|Serenity Chasma]], which extend as an equatorial belt for at least {{Cvt|1000|km||abbr=|-1}}. Argo Chasma potentially reaches as deep as {{Cvt|9|km||abbr=|0}}, with cliffs that may rival [[Verona Rupes]] on [[Miranda (moon)|Miranda]] for the title of the tallest cliff in the Solar System.<ref name="nasa2017"/>

=== Hypothesized exosphere ===
[[File:PIA20375-PlutoMoon-Charon-NightSide-20150717.jpg|thumb|Charon's night side seen by ''New Horizons'']]
In contrast to Pluto, Charon has no significant atmosphere.<ref name="Stern_2015"/> There has been speculation about an extremely thin [[exosphere]] surrounding the moon contributing to the formation of dark regions such as Mordor Macula. The strong seasons experienced by Pluto and Charon could provide brief periods of exosphere formation as methane sublimates on Charon, interspersed by centuries of dormancy.<ref name="Teolis2022">{{cite journal |last1=Teolis |first1=Ben |last2=Raut |first2=Ujjwal |last3=Kammer |first3=Joshua A. |title=Extreme Exospheric Dynamics at Charon: Implications for the Red Spot |date=15 April 2022 |journal=Geophysical Research Letters |volume=49 |issue=8 |pages=e97580 |doi=10.1029/2021GL097580 |doi-access=free |bibcode=2022GeoRL..4997580T }}</ref>

Pluto does have a thin but significant atmosphere, which Charon's gravitation might pull toward Charon's surface. The gas, specifically nitrogen, is mostly caught in the combined center of gravity between the two bodies before reaching Charon, but any gas that does reach Charon is held closely against the surface. The gas is mostly made up of ions of nitrogen, but the amounts are negligible compared to the total of Pluto's atmosphere.<ref>{{Cite journal |last1=Tucker |first1=O. J. |last2=Johnson |first2=R. E. |last3=Young |first3=L. A. |date=January 15, 2015 |title=Gas transfer in the Pluto–Charon system: A Charon atmosphere |url=https://www.sciencedirect.com/science/article/pii/S0019103514002462 |journal=Icarus |series=Special Issue: The Pluto System |language=en |volume=246 |pages=291–297 |bibcode=2015Icar..246..291T |doi=10.1016/j.icarus.2014.05.002 |issn=0019-1035}}</ref>

The many spectral signatures of ice formations on the surface of Charon have led some to believe that the ice formations could supply an atmosphere, but atmosphere supplying formations have not been confirmed yet. Many scientists theorize that these ice formations could be concealed out of direct sight, either in deep craters or beneath Charon's surface. Charon's relatively low gravity, due to its low mass, causes any atmosphere that might be present to rapidly escape the surface into space.<ref>{{Cite book |last=Stern |first=S. Alan |last2=Tholen |first2=David J. |last3=Tholen |first3=D. J. |url=http://worldcat.org/oclc/963785129 |title=Pluto and Charon. |date=2017 |publisher=University of Arizona Press |isbn=978-0-8165-3613-9 |oclc=963785129}}</ref> Even through stellar occultation, which is used to probe the atmosphere of stellar bodies, scientists cannot confirm an existing atmosphere; this was tested in 1986 while attempting to perform stellar occultation testing on Pluto. Charon also acts as a protector for Pluto's atmosphere, blocking the solar wind that would normally collide with Pluto and damage its atmosphere. Since Charon blocks these solar winds, its own atmosphere is diminished, instead of Pluto's. This effect is also a potential explanation for Charon's lack of atmosphere; the solar winds remove gases faster than they can accumulate.<ref>{{Cite web |date=January 12, 2017 |title=Charon protects Pluto's atmosphere from solar wind |url=https://www.spaceflightinsider.com/missions/solar-system/charon-protects-pluto-atmosphere-solar-wind/ |archive-url=https://web.archive.org/web/20170112194023/http://www.spaceflightinsider.com/missions/solar-system/charon-protects-pluto-atmosphere-solar-wind/ |url-status=dead |archive-date=January 12, 2017 |access-date=May 13, 2022 |website=SpaceFlight Insider |language=en-US }}</ref> It is still possible for Charon to have an atmosphere, as Pluto transfers some of its atmospheric gas to Charon, from where it tends to escape into space. Assuming Charon's density is 1.71 g/cm<sup>3</sup>, it would have a surface gravity of 0.6 of Pluto's. It also has a higher mean molecular weight than Pluto and a lower exobase surface temperature, so that the gases in its atmosphere would not escape as rapidly from Charon as they do from Pluto.<ref>{{Cite journal |last1=Elliot |first1=J. L. |last2=Young |first2=L. A. |year=1991 |title=Does Charon have an Atmosphere? |journal=Lunar and Planetary Science Conference |volume=22 |page=347 |bibcode=1991LPI....22..347E}}</ref>

There has been significant proof of CO<sub>2</sub> gas and H<sub>2</sub>O vapor on the surface of Charon, but these vapors are not sufficient for a viable atmosphere due to their low vapor pressures. Pluto's surface has abundant ice formations, but these are volatile, as they are made up of volatile substances like methane. These volatile ice structures cause a good deal of geological activity, keeping its atmosphere constant, while Charon's ice structures are mainly made up of water and carbon dioxide, much less volatile substances that can stay dormant and not affect the atmosphere much.<ref>{{Cite book |last1=Spohn |first1=Tilman |title=Encyclopedia of the solar system |last2=Breuer |first2=Doris |last3=Johnson |first3=Torrence V. |date=2014 |publisher=Elsevier |isbn=978-0-12-415845-0 |edition=3d |location=Amsterdam Boston |page=909–924 |chapter=Pluto |chapter-url=https://www.sciencedirect.com/topics/earth-and-planetary-sciences/charon}}</ref>