One of the four large Galilean moons of Jupiter number IV, discovered in 1610. It is the darkest of the Galilean satellites and also the least dense, which suggests that it contains a high proportion of water, though detailed surface images returned by Galileo indicate that the surface at least may contain more rock and dust than previously supposed. by Voyager and Galileo images show that the surface is heavily cratered but has little relief. The most prominent surface feature is a multi-ring structure called Valhalla, which consists of a central bright zone 600 kilometres 375 miles across, surrounded by numerous concentric rings spaced at intervals of 20 to 100 kilometres 12 to 60 miles. At least seven other multi-ring features have been recognized.