Editing Charon (moon) (section)

=== 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"/>