Editing Charon (moon) (section)

=== 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>