four moons of Jupiter are shown to scale. These four largest moons of Jupiter shown in increasing distance from Jupiter are (left to right) Io, Europa, Ganymede, and Callisto. These global views show the side of volcanically active Io which always faces away from Jupiter, icy Europa, the Jupiter-facing side of Ganymede, and heavily cratered Callisto. Io, which is slightly larger than Earths moon, is the most colourful of the Galilean satellites. Its surface is covered by deposits from actively erupting volcanoes, hundreds of lava flows, and volcanic vents which are visible as small dark spots. Calistos dark surface is pocked by numerous bright impact craters.