One of the four large Galilean moons of Jupiter (number II). The images returned by the Voyager 1 spacecraft showed a bright reflective surface, criss-crossed by a complex network of dark lines. Only five impact craters have been identified. Some of the dark lines, if not all, may be cracks or fractures created when the crust expanded. The absence of craters shows that the crust has been greatly modified since the satellite first formed. The structure is thought to consist of a rocky core overlaid by an icy crust.
