Clavius and other impact craters on the Moon.
An impact crater is a crater formed by the high-speed impact of a meteoroid, asteroid, or comet with a solid surface. Craters are a common feature on most moons (an exception is Io), asteroids, and rocky planets, and range in size from a few cm to over 1,000 kilometers across, in the case a large impact basin. There is a general morphological progression from large to small craters: large craters might have several rings and smooth floors; intermediate craters tend to have a central peak (formed by melting and rebounding of the crust) and smooth floors; small craters have a simple bowl-shaped floor that is rough. Because impact craters degrade at different rates depending on their environment, they are valuable indicators of the age of a surface and the extent to which resurfacing has taken place. On Earth, for example, craters are rapidly degraded and destroyed by weathering processes; about 120 are known, with diameters ranging from 150 meters to 180 kilometers. One of the best preserved and most impressive is the Barringer Crater, near Winslow in northern Arizona. On Mercury, by contrast, which lacks an atmosphere and is geologically inert, the landscape is peppered with craters dating back over 4 billion years.