Charon

Charon, first discovered in 1978 by James Chirsty, is the largest moon of the dwarf planet Pluto. There has been some debate as to whether or not Charon is a dwarf planet of its own, or a moon, because it is so massive. In 2015, NASA's New Horizons spacecraft will visit both Pluto and Charon to gather more information on the two.

Pluto left, Charon right. Picture taken by Hubble

Charon is over half the size of Pluto, 1,207 km in diameter, and 11.6% the mass. Because of its relatively large size, it orbits Pluto in a peculiar manner. The barycenter (the point at which two bodies orbit each other) between the two is actually outside of pluto. This has been a major factor in the debate on whether Charon is a moon or not. So far, all other bodies that we call moons have barycenters within their parent planets, thus making Charon a bit of an odd ball.

Pluto and Charon actually orbit each other, as shown.

The surface of the moon is fairly interesting. Pluto has many volatile ices, and thus we might expect that Charon would have the same, but that's not quite true. Charon is covered in ice, but it's mostly frozen water or ammonia hydrate crystals. Solar winds would have smoothed away much of this ice, but crystals are present which indicates cryo-volcanic activity, one of the most awesome things to happen on planetary bodies. One interesting feature of the moons surface is its variable albedo. The albedo changes with latitude, the highest albedo being at the equator, with a low albedo at the poles.

A depiction of how Charon might look from the surface of Pluto

Charon's interior is similar to that of Pluto, but with a higher concentration of ice. The planet probably is half ice, but it's difficult to know if this ice is differentiated into a mantle like on Pluto, or if it's mixed in with the rock. Hopefully the New Horizons space craft can shed some light on this.

1990 Hubble Space Telescope picture of Pluto and Charon.

New Horizons space probe