... Credit Copyright: NASA , Voyager Explanation: Callisto is a dirty battered world, showing the most beaten surface of Jupiter 's major moons. Made of a rocky core covered by fractured ice, Callisto 's past collisions with large meteors are evident as large craters surrounded by concentric rings. The four largest moons of Jupiter: Io , Europa , Ganymede , and Callisto were all discovered by Galileo and Marius in 1610 with early telescopes and are now known as the Galilean satellites. ...