A young surface is a the surface of a planet, moon, or other object on which the visible features are of relatively recent origin. On such a surface, older features have been destroyed by erosion, lava flows, or other processes. Young surfaces exhibit few impact craters and are typically varied and complex. In contrast, an "old" surface is one that has changed relatively little over geologic time. The surfaces of Earth, Io, and Europa, for example, are young, whereas the surfaces of Mercury and Callisto are old.