The most obvious features on the surface of the Moon are the circular walled areas called craters. These have been known since telescopic observations began. The Earth has very few easily recognizable craters of any great size, although in recent years careful observations have revealed many more which are somewhat obscured by water, vegetation, erosion and so forth. Astronomers expected that Mercury, Venus and Mars would be like the Earth, rather than the Moon, and it was a great surprise when the Mariner 4 spacecraft flew past Mars in 1965 and revealed a crater-strewn surface similar to the Moon. These observations covered only a small part of the surface and it was not until the planet-wide photo¬graphic coverage by Mariner 9 in 1972 that large areas relatively free of craters were discovered. Mariner 10 photography in 1974 covered nearly half the surface of Mercury and showed this planet to have a surface very similar to that of the Moon. Radar mapping of Venus in the 1970s has shown that it too is covered with craters although the number of small craters is less than on Mars, Mercury or the Moon.

For many years the origin of the lunar craters was a matter of great controversy between supporters of volcanic origin and those of meteoritic impact origin. It was only settled to most people’s satisfaction when spacecraft data became available. Most astronomers now believe that the majority of lunar craters were formed by meteoritic impact, although some are of volcanic origin.

If a large meteorite, or small asteroid, collides with the surface of the Moon the relative velocity is some tens of kilometers per second, which is greater than the speed of sound in the lunar material. This material cannot move out of the way of the meteorite faster than sound and so it is therefore compressed. This slows down the meteorite and the energy of its motion is converted into heat which can be sufficient to vaporize a large mass of material. Eventually the meteorite is stopped some kilometers below the surface having formed a pocket of hot, highly compressed and vaporized rock. This then explodes and it is the explosion that forms the crater. If a graph is drawn of crater diameters against depths for lunar craters, terrestrial meteorite craters, and craters caused by man-made explosions, all the points lie on the same smooth curve, thereby con¬firming the explosive origin of the lunar craters. An objection sometimes raised to this impact theory is that meteorites generally strike the Moon obliquely and that the craters should be elliptical and not circular. This would be true if the meteorite simply gouged out a hole, but in fact the explosion after the meteorite has stopped is not affected by the direction of impact and so a circular crater is formed which completely obliterates any elliptical feature produced when the meteorite initially penetrated the surface The theory of impact origin is now well understood and is supported by laboratory experiments. As a result most workers in this subject agree that impact-produced craters have the following features.

A nearly circular rim crest with the slope of the rim steeper on the inside than on the outside. Interior walls that arc terraced in large craters and steep in in smaller ones.

A surrounding hummocky and undulating blanket of material ejected in the explosion that caused the crater. It has radial And dune-like features, is usually brighter than its surroundings and extends at least one crater diameter outward from the rim crest. A system of radial rays extending beyond the ejecta blanket due to projectiles from the impact site that form secondary craters.

A depression inside the rim that is generally deep. When it is large, there is a central peak that was probably formed by the elastic rebound of rock from beneath the surface, during the impact event.

Although the crater itself is not appreciably affected by the angle of impact, the ejecta blanket and the rays form preferentially on the side opposite the direction of impact. In contrast to these features, volcanically-produced craters display the following characteristics:
A usually polygonal form.

A rim with the interior and exterior slopes nearly the same and with few, if any, wall terraces.

A relatively smooth ejecta blanket which is often darker than the surroundings.

No extensive ray systems.

Craters that are usually shallow with collapsed depressions on the rim and in the floor.

The distinction between the two types of crater !H most easily made for the fresh, young features such as Maunder and Kopff . Older craters, however, have generally had their features subdued by later events and it is not always easy to make a distinct ion. Being depressions, craters are the hosts of vary¬ing amounts of fill. The very large craters, such as the basins of Serenitatis. Nectaris, Crisium, Imbrium, Moscoviense, Ingenii and Orientale are partly or nearly filled with dark mare material. For example, in the case of Orientale The dark mare material appears to overlie the lighter material in the interior of the basin. Some small craters occur in CRATER CHAINS which can be as much as 300 km long: they occur over most parts of the lunar surface. The individual craters are usually less than 15km in diameter. It would be too much of a coincidence for them to have been formed by the random process of impact and so they must be of volcanic origin. They presumably mark out lines of crustal weakness.

The Earth is the only planet known to have volcanoes active today. We have referred above to the way in which they are almost always found along plate boundaries. Of particular interest to the study of the large craters on other planets are those volcanic craters on the Earth which are larger than two kilometers across and are known as CALDERAS. It is popularly supposed that these volcanoes have blown their tops off but this is incorrect; the material now present is newly cooled magma. The largest calderas develop late in the history of a volcano during a violent eruption. It seems that the underground reservoirs of magma are emptied by the eruption and that the summit of the volcano, having lost its support, collapses to leave a crater. The Earth’s largest caldera, at Aso-san in Japan, is 23 km by 16 km – much less than the 65 km diameter of the caldera of Olympus Mons on Mars.

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