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Charon (Geology)

“Charon’s diameter is 1,208 kilometres (751 mi)—just over half that of Pluto,[26] and larger than the dwarf planet Ceres. Charon is sufficiently massive to have collapsed into a spheroid under its own gravity. Unlike Pluto’s surface, which is composed of nitrogen and methane ices, Charon’s surface appears to be dominated by the less volatile water ice. In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryogeysers and cryovolcanoes.[27][28]

Charon’s volume and mass allow calculation of its density, from which it can be determined that Charon is slightly less dense than Pluto and thus contains a larger proportion of ice relative to rock in its interior; the difference is considerably lower than that of most suspected collisional satellites. Before New Horizons’ flyby, there were two conflicting theories about Charon’s internal structure: some scientists thought Charon to be a differentiated body like Pluto, with a rocky core and an icy mantle, whereas others thought it would be uniform throughout. Evidence in support of the former position was found in 2007, when observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryogeysers. The fact that the ice was still in crystalline form suggested it had been deposited recently, because solar radiation would have degraded it to an amorphous state after roughly thirty thousand years.[27]

Photometric mapping of Charon’s surface shows a latitudinal trend in albedo, with a bright equator band and darker poles. The south polar region is apparently darker than the north.[29] The north polar region is dominated by a very large dark area informally dubbed “Mordor” by the New Horizons team.[30][31][32] Aside from Mordor, however, New Horizons imaged very few other impact craters on Charon and found a youthful surface, adding support to the above theory that Charon is geologically active and thus probably differentiated.[31] 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.[33]

https://en.wikipedia.org/wiki/Charon_%28moon%29#Geology