The largest moon of Pluto, Charon was first spotted telescopically in June 1978. Its diameter of 1,208 km (751 miles) is a little over half that of Pluto and it has more than one-tenth as much mass. It is a very close companion of Pluto, and their orbits are nearly in synchrony (their rotational periods are the same). Because they share a common center of gravity, astronomers sometimes refer to them as a double planet. The discoverer of the moon, James Christy, named it after the ferryman of Greek mythology who plies the River Styx, a key feature in Hades' realm in the Underworld. The choice also coincided with the initials of his wife, Charlene, whose nickname was "Char".
The New Horizons mission discovered that Charon has a large dark area on its north pole, which scientists have now identified as being caused by ice geysers and ice volcanoes. These erupt from cracks in the surface and release a mix of frozen nitrogen, carbon dioxide and methane that carries away sunlight to leave the dark areas behind.
The New Horizons team also discovered a giant canyon on the moon, named Serenity Chasm after the ship from Joss Whedon's cult television show Firefly. It is 37 miles wide and was likely formed by the same process that carved out the enormous craters on Pluto itself. Like the rest of Charon, it is mostly composed of frozen water. Astronomers also now know that the ice on Charon is colored by chemicals in its interior that form reddish compounds known as tholins.