Charon

Charon is the largest of Pluto’s five moons, and the second-largest after Triton among those orbiting a dwarf planet. Charon is massive relative to Pluto: its diameter is almost half that of Pluto, and its mass is about 9% of Pluto’s. This large mass ensures that Charon dominates Pluto’s orbit: it has an orbital period of just 6.4 days around Pluto, compared to 24.8 days for Neptune’s moon Triton (the next mostmassive satellite relative to its primary).Charonthese two objects together form what is known as a binary system—one of only three such systems in the Solar System (the others are Earth–Moon and Saturn’s satellites Titan and Enceladus).

The surface of Charonis heavily cratered, with many impact craters up to 60 kilometers across. There are also several large fault valleys or chasmata up to 650 kilometers long. Like on Mercury, these features indicate widespread geological activity in Charon’s past. The most prominent feature on Charonis Plateau, informally known as Mordor Macula. This dark patch covers an area about the size of New Mexico and appears to be rich in carbonaceous materials similar to those found in primitive meteorites; it may represent a significant reservoirof water ice mixed with more volatile ices such as methane or nitrogen.

Charon has a tenuous atmosphere consisting mostly of nitrogen (); it also contains traces of methane () and carbon monoxide (). These gases escape fromCharonto space at a rate comparable to that at which water vapor escapes from Earth’s atmosphere—a few molecules per second—and they are probably replenished by outgassing from the interior ofCharonsimilar to what occurs onTitanand other icy satellites with atmospheres.”