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Lunar craters are craters on Earth's Moon. The Moon's surface is saturated with craters, almost all of which were formed by impacts.
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History
The word crater was adopted by Galileo from the Latin word for cup. Galileo built his first telescope in late 1609, and turned it to the Moon for the first time on November 30, 1609. He discovered that, contrary to general opinion at that time, the Moon was not a perfect sphere, but had both mountains and cup-like depressions, the latter of which he gave the name craters.
Scientific opinion as to the origin of craters swung back and forth over the ensuing centuries. The competing theories were (a) volcanic eruptions blasting holes in the Moon, (b) meteoric impact, (c) a strange theory known as the Welteislehre developed in Germany between the two World Wars which suggested glacial action creating the craters.
Evidence collected during the Apollo Project and from unmanned spacecraft of the same period proved conclusively that meteoric impact, or impact by asteroids for larger craters, was the origin of almost all lunar craters, and by implication, most craters on other bodies as well.
Characteristics
Because of the Moon's lack of liquid water, an atmosphere, or tectonic plates, there is little erosion, and craters are found that exceed two billion years in age. The age of large craters is determined by the number of smaller craters contained within it, older craters generally accumulating more small, contained craters.
The smallest craters found have been microscopic in size, found in rocks returned to Earth from the Moon. The largest crater called such is about 360 kilometers (200 miles) in diameter, located near the lunar South Pole. However, it is believed that many of the lunar maria were formed by giant impacts, with the resulting depression filled by upwelling lava.
Craters typically will have some or all of the following features:
- a surrounding area with materials splashed out of the ground when the crater was formed; this is typically lighter in shade than older materials due to exposure to solar radiation for a lesser time
- raised rim, consisting of materials ejected but landing very close by
- crater wall, the downward-sloping portion of the crater
- crater floor, a more or less smooth, flat area, which as it ages accumulates small craters of its own
- central peak, found only in some craters with a diameter exceeding 16 miles (26 km); this is generally a splash effect caused by the kinetic energy of the impacting object being turned to heat and melting some lunar material.
Locations of major craters
The red marker on these images illustrates the location of the named crater feature on the near side of the Moon.
See also
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