lunar crater

Lunar craters are impact craters on Earth's Moon. The Moon's surface has many craters, all of which were formed by impacts. The International Astronomical Union currently recognizes 9,137 craters, of which 1,675 have been dated.

History

The word ''crater'' was adopted from the Greek word for "vessel" (, 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. These were named craters by Johann Hieronymus Schröter (1791), extending its previous use with volcanoes.

Robert Hooke in ''Micrographia'' (1665) proposed two hypotheses for lunar crater formation: one, that the craters were caused by projectile bombardment from space, the other, that they were
the products of subterranean lunar volcanism.

Scientific opinion as to the origin of craters swung back and forth over the ensuing centuries. The competing theories were:
# volcanic eruptions blasting holes in the Moon
# meteoric impact
# a theory known as the Welteislehre developed in Germany between the two world wars which suggested glacial motion creating the craters.
Grove Karl Gilbert suggested in 1893 that the Moon's craters were formed by large asteroid impacts. Ralph Baldwin in 1949 wrote that the Moon's craters were mostly of impact origin. Around 1960, Gene Shoemaker revived the idea. According to David H. Levy, Shoemaker "saw the craters on the Moon as logical impact sites that were formed not gradually, in eons, but explosively, in seconds."

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 meteoroid striking the surface at a speed of .

In March 2018, the discovery of around 7,000 formerly unidentified lunar craters via convolutional neural network developed at the University of Toronto Scarborough, Canada was announced. A similar study in December 2020 identified around 109,000 new craters using a deep neural network.

Characteristics

Because of the Moon's lack of water, atmosphere, and 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 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.

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 ; this is generally a splash effect caused by the kinetic energy of the impacting object being turned to heat and melting some lunar material.

Statistics

There are at least 1.3 million craters larger than in diameter; of these, 83,000 are greater than in diameter, and 6,972 are greater than in diameter.

Lunar crater categorization

In 1978, Chuck Wood and Leif Andersson of the Lunar & Planetary Lab devised a system of categorization of lunar impact craters.
They used a sampling of craters that were relatively unmodified by subsequent impacts, then grouped the results into five broad categories. These successfully accounted for about 99% of all lunar impact craters.

The LPC Crater Types were as follows:

* ''ALC'' — small, cup-shaped craters with a diameter of about or less, and no central floor. The archetype