Bowl-like depression ("crater" means "cup" in Latin) on the surface of a planet, moon, or asteroid. Craters range in size from a few centimeters to over 1,000 km across, and are mostly caused by impact or by volcanic activity, though some are due to cryovolcanism. Click on Term to Read More formed by high-speed impact of a Small rocky or metallic object in orbit around the Sun (or another star)., asteroid, or Conglomeration of frozen water and gases (methane, ammonia, CO2) and silicates that that formed in the outer solar system and orbits the Sun. In recent years, the description of comets has shifted from dirty snowballs to snowy dirtballs with more dust than ice. However, the ratio is less than 10-to-1. Click on Term to Read More on 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 km across. There is a general morphological progression from large to small craters: large craters often have several rings and smooth floors; intermediate complex craters tend to have a Exposed core of uplifted rocks in center of a complex impact crater. Central peak material typically shows evidence of intense fracturing, faulting, and shock metamorphism. Click on Term to Read More (formed by melting and rebounding of the Outermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the Click on Term to Read More) and smooth floors; small simple craters have a rough bowl-shaped floor. Craters have been classified into three categories: simple craters, complex craters, and impact basins.
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; ~120 are known, very few older than 1 Ga. Mercury, which lacks an atmosphere and is geologically inert, has a landscape peppered with craters dating to over 4 Ga.
Some or all content above used with permission from J. H. Wittke.