- Depression, normally surrounded by a ridge, on the surface of a planet or satellite, and usually caused by meteorite impact. Water action and volcanic activity can also cause craters to form. A long controversy about the origin of the craters of the Moon was settled in the 1960s and early 1970s by evidence that most are meteoritic in origin but some are volcanic. Meteorite craters have been used to date the surfaces of the Moon, Mars and Mercury, because the number of craters per unit area increases with the time the surface has been exposed to impacts, especially if the surface was exposed during the early history of the solar system when there were more impacting objects than now. Large meteorites produce explosion craters with such violence that it can be impossible to find meteorite fragments nearby. In moonrock – which has effectively no atmospheric protection from outer space – there are meteorite craters all the way down to micrometeorite pits visible only with the aid of a microscope.
- (written as Crater)constellation of the Southern hemisphere
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