Composite view of Amalthea and Io at the same scale. The visible part of Amalthea is about 150 kilometers across. The colors are approximate. Amalthea is actually much darker than Io, but is displayed at a similar brightness for ease of viewing. The shape of Amalthea is controlled largely by impact cratering and fragmentation. In contrast, Io, like Earth, has gravity sufficient to form it into a slightly ellipsoidal sphere. Amalthea is covered by craters because there are no processes which erode or cover them efficiently. On extremely volcanically active Io, impact craters are covered quickly by lavas and other volcanic materials. Some of the volcanic materials escape from Io and probably contribute to the reddish colors of Amalthea and the other small inner satellites.