Lunar crater

Lunar craters are impact craters on Earth's Moon. The Moon's surface has many craters, almost all of which were formed by impacts.

The word crater was adopted by Galileo from the Greek word for vessel - (Κρατήρ a Greek vessel used to mix wine and water). 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 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.

The formation of new craters is studied in the lunar impact monitoring program at NASA. The biggest recorded creation was caused by an impact recorded on March 17, 2013. Visible to the naked eye, the impact is believed to be from an approximately 40 kg meteoroid striking the surface at a speed of 90,000 km/h.

Because of the Moon's lack of water, and 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 290 kilometres (181 mi) across 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 .

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