This image of Jupiter's moon Callisto was taken by NASA's Galileo spacecraft. It shows the heavily cratered surface of a moon that has remained unchanged for millions of years. In fact, Callisto is the most heavily cratered object in the Solar System. It is believed that the moon is geologically inactive and extremely stable. The darker areas are believed to be formed of a mixture of rock and ice. The brighter material is probaby water ice that was ejected during the numerous meteor impacts. Of Jupiter's four largest moons, Callisto orbits the farthest from the planet.