Europa, the brightest of Jupiter’s Galilean satellites, may have a
surface of thin ice crust overlying water or softer ice, with
large-scale fracture and ridge systems appearing in the crust. Europa
has a density about three times that of water, suggesting it is a
mixture of silicate rock and some water. Very few impact craters are
visible on the surface, implying a continual resurfacing process,
perhaps by the production of fresh ice or snow along cracks and cold
glacier-like flows.