Ganymede, the third of the Galilean satellites, is the largest satellite in the solar system. The surface of Ganymedeis mainly rock, although there are two contrasting surface features on the satellite. The first area is darker, and with many impact craters, while the other is later and has very few craters. It is believed that about 4 million years ago, plate tectonics on the satellite caused the formation of these lighter areas, probably from lava flow. This also caused liquid water to rush to the surface, which is now frozen into huge mountains of ice.