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. The explosion which was visible with the naked eye, is believed to be from an approximately 40 kg meteoroid hitting the moon with a speed of 90000 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 181 miles 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 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.
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 10 km or less, and no central floor. The archetype for this category is Albategnius C.
BIO — similar to an ALC, but with small, flat floors. Typical diameter is about 15 km. The lunar crater archetype is Biot.
SOS — the interior floor is wide and flat, with no central peak. The inner walls are not terraced. The diameter is normally in the range of 15–25 km. The archetype is Sosigenes.
TRI — these complex craters are large enough so that their inner walls have slumped to the floor. They can range in size from 15–50 km in diameter. The archetype crater is Triesnecker.
TYC — these are larger than 50 km, with terraced inner walls and relatively flat floors. They frequently have large central peak formations. Tycho is the archetype for this class.
Beyond a couple of hundred kilometers diameter, the central peak of the TYC class disappear and they are classed as basins. Large craters, similar in size to maria, but without (or with small amount of) dark lava filling, are sometimes called thalassoids.[A]
Beginning in 2009 Dr. Nadine Barlow of Northern Arizona University began to convert the Wood and Andersson lunar impact-crater database into digital format. Dr. Barlow is also creating a new lunar impact crater database similar to Wood and Andersson's, except hers will include all impact craters greater than or equal to five kilometers in diameter and is based on Clementine (spacecraft) images of the lunar surface.
Small craters of special interest (for example, visited by lunar missions) receive human first names (Robert, José, Louise etc.). One of the biggest lunar craters, Apollo, is named after Apollo missions. Many smaller craters inside and near it bear the names of deceased American astronauts, and many craters inside and near Mare Moscoviense bear the names of deceased Soviet cosmonauts. Besides this, in 1970 twelve craters were named after twelve living astronauts (6 Soviet and 6 American).
Majority of named lunar craters are satellite craters: their names consist of the name of nearby named crater and a capital letter (for example, Copernicus A, Copernicus B, Copernicus C and so on).
^This term was coined by Soviet explorers of the Moon after beginning of exploration of lunar farside. Later, in 1967, on XIII General Assembly of the International Astronomical Union this word was proposed to be included into the list of generic terms of nomenclature of lunar surface features, but this proposal was declined. So, this term remains only a characterization of the features, but not a part of their names.
^"Descriptor Terms (Feature Types)". Gazetteer of Planetary Nomenclature. International Astronomical Union (IAU) Working Group for Planetary System Nomenclature (WGPSN). Archived from the original on 2013-12-10. Retrieved 2014-08-24.