Callisto, composed of rock and ice, is covered with numerous impact craters. The largest of these is Valhalla Basin, a 2748-km (1650-mi) multi-ring impact basin near the top of the image. Craters on Callisto appear as bright and dark spots whose morphology changes. Newer craters appear as sharply defined bright and dark spots. Over time older craters appear as more diffuse splotches.