With half the diameter and one-eighth the mass of Pluto, Charon is a very large moon in comparison to its parent body.

[22] The reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be essential ingredients of life.

[26] On June 22, 1978, he had been examining highly magnified images of Pluto on photographic plates taken with the telescope two months prior.

The discovery of Charon allowed astronomers to calculate accurately the mass of the Plutonian system, and mutual occultations revealed their sizes.

[9] Simulation work published in 2005 by Robin Canup suggested that Charon could have been formed by a collision around 4.5 billion years ago, much like Earth and the Moon.

This is in contrast to Iapetus, a Saturnian moon similar in size to Charon but with a pronounced oblateness dating to early in its history.

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.

According to the hot start model, Charon accreted rapidly (within ~104 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.

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.

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.

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.

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 cryogeysers and cryovolcanoes.

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 state after roughly thirty thousand years.

[47] 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" by the New Horizons team.

[51] In addition to Mordor, New Horizons found evidence of extensive past geology that suggests that Charon is probably differentiated;[49] 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.

[44] Charon has a system of extensive grabens and scarps, such as Serenity Chasma, which extend as an equatorial belt for at least 1,000 km (620 mi).

[12] There has been speculation about an extremely thin exosphere surrounding the moon contributing to the formation of dark regions such as Mordor Macula.

Many scientists theorize that these ice formations could be concealed out of direct sight, either in deep craters or beneath Charon's surface.

This effect is also a potential explanation for Charon's lack of atmosphere; the solar winds remove gases faster than they can accumulate.

[30] Although ground-based observation is very challenging, a group of amateur astronomers in Italy used a 14-inch telescope in 2008 to successfully resolve Charon in an image of Pluto.

Charon's discoverer James Christy and the children of Clyde Tombaugh were guests at the Johns Hopkins Applied Physics Laboratory during the New Horizons closest approach.

Under this proposal, Charon would have been classified as a planet, because the draft explicitly defined a planetary satellite as one in which the barycenter lies within the major body.