The four Galilean satellites of Jupiter probably experienced early, intense bombardment. Thus, the very ancient surface of Callisto remains scarred by impact craters. The younger, more varied surface of Ganymede reveals distinct light and dark areas, the light areas featuring networks of intersecting grooves and ridges, probably resulting from later ice flows. The impact sites of Europa have been almost completely erased, apparently by water outflowing from the interior and instantly forming vast, low, frozen seas. Satellite photographs of Io, the closest of the four to Jupiter, were revelatory. They showed a landscape dominated by volcanoes, many erupting, making Io the most tectonically active object in the solar system. Since a body as small as Io cannot supply the energy for such activity, the accepted explanation has been that, forced into a highly eccentric orbit, Io is engulfed by tides stemming from a titanic contest between the other three Galilean moons and Jupiter.