A depression caused by the high-speed collision of a meteoroid, asteroid, or comet with a solid surface. Impact craters are found on the Moon, on all the terrestrial planets, and on planetary satellites except those where resurfacing processes are in operation, such as Io. They range in size from tiny pits a few micrometres across up to vast impact basins thousands of kilometres wide. Young impact craters may be distinguished from volcanic craters by their circular shape, steep inner walls, and shallower outer slopes, as well as their ejecta blankets, secondary craters, and bright rays. Larger impact craters have terracing on their inner walls, flat floors, and central peaks. Old impact craters become progressively eroded and may lose some of these distinguishing features; others may be partially submerged or filled by lava, ice flows, or sediments (see Ghost Crater).