This image shows the scarred surface of Jupiter's moon, Callisto. It was taken by Voyager 2 in 1979. Littered with thousands of impact craters, it is the most heavily cratered of Jupiter's moons and is one of the most heavily cratered objects in the Solar System. Callisto's is considered to be geologically inactive, so the crust is thought to be over 4 billion years old. It also has an extremely thin atmosphere composed of carbon dioxide and oxygen.