Pluto's Largest Moon Charon

Charon is the largest moon of Pluto. It is named after the ferryman who took the dead over the river Styx in ancient Greek mythology. Charon is just about half of Pluto's size, and was discovered in 1978 by astronomy James Christy. Observations showed the appearance and disappearance of a large bulge on the limb of Pluto, which turned out to be this orbiting moon. The two objects are gravitationally locked to one another, orbiting a point slightly above the surface of Pluto (called the barycenter) every 6.39 days.

Given its location in the solar system, it was expected that Charon would be heavily cratered from billions of years of impacts on its frozen, dormant surface. In fact, many craters were found during the close up look provided by the New Horizons spacecraft as it flew through the Pluto system in July 2015. However, many unexpected surface features were found as well. Large chasms, longer and deeper than the Grand Canyon, were found. Large smooth areas implied that some sort of surface resurfacing has occurred in recent times. Ejecta from large craters has puzzling patterns and appears to be of a very different material than what is found on the rest of the surface.

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