Callisto is Jupiter’s second-largest moon, the third-largest moon in the Solar System, and the outermost Galilean moon. This processed image of Callisto is from unfiltered images taken by Voyager 2 on July 8 1979. It has a dark, icy surface covered in craters. But its induced magnetic field indicates the possibility of a salty ocean beneath all that ice.

(from New Thinking Allowed)