One of the four Galilean moons of Jupiter (number III) and the largest natural satellite in the solar system.
Voyager 1 and Voyager 2 returned high-resolution images of Ganymede, revealing detail as fine as 3 kilometres. There are several different types of terrain, notably dark areas that are heavily cratered and a lighter grooved terrain that constitutes around 60 per cent of the surface photographed. The dark area is thought to represent the remains of the most ancient crust. The variety indicates that evolution has taken place. The density is about twice that of water, and the composition is thought to be about half ice, half silicate rock. It is not known whether the two materials are mixed throughout or whether there may be an ice mantle overlying a rocky core.
