Voyager 2 photograph showing surface details of Europa, one of the moons of Jupiter. It is thought that the crust of the moon is water ice 75-100km thick & surrounding a rocky interior. The surface of Europa is featureless except for the striking curvilinear markings which are clearly seen in this image. The individual dark bands are 20-40km in width & stretch for thousands of kilometres across the face of the moon. One explanation for the markings is that expansion caused the crust to fracture & that the fractures later refilled with water, which froze.