Callisto is one of the four Galilean moons of planet Jupiter. It is comprised of equal parts rock and ice and is geologically dead, with it's surface covered with the oldest craters in the solar system. Callisto has a very thin atmosphere carbon dioxide and oxygen. It is almost as large as Mercury.
Callisto, along with Ganymede, Europa, and Io, were discovered by the Italian astronomer Galileo Galilei in 1610.
Pan, zoom, or go full-screen with this megapixel image of Callisto.
A couple of notable surface features on Callisto are Agard crater and Bran crater:
In the night sky, at certain seasons when the Inner Moons are on the other side of our primary and starry skies are clear, I can (I fancy) see the earth.
- Lin Carter, Jandar of Callisto