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cratersCircular depressions on the surface of the Moon and several other bodies in the Solar System. They have diameters ranging from less than 1 meter to more than 1000 km, although the larger structures are more usually termed basins. The largest lunar craters are considered to be the 200-km walled plains. Some of the more interesting lunar craters are listed in the table. Craters appear all over the Moon's surface and are most prominent when near the terminator. Small craters are everywhere more numerous than larger ones and crater-size distribution curves provide the basis for the estimation of relative ages by crater counting. Most of the surface of Mercury is heavily cratered and on Mars the craters predominate in the southern hemisphere (see Mars, surface features). Many planetary satellites are now known to be cratered, some very heavily.
The vast majority of craters are now believed to have originated by meteoritic impact, although in some cases their present appearance results from later modification by erosion or volcanic processes. A few craters in the lunar maria, particularly those associated with sinuous rilles, may be volcanic in origin. Some, such as those at the summits of the volcanoes of the Earth or Mars, are solely volcanic. Other small craters result from subsidence or surface collapse.
In profile, a typical crater has a floor slightly below the level of the surrounding surface and a raised rim that slopes gently outward but falls more steeply toward the crater floor. The inner slope of the rims of large craters may be terraced, while the terrain beyond the rim may be hummocky and broken with numerous very small craters. Some large craters also have mountainous central peaks.
The impact events that created most of the surviving craters on Solar System bodies are thought to have occurred between 3000 and 4000 million years ago as the bodies collided with numerous planetesimals. Possibly the planetesimals were themselves the debris from collisions between larger objects that accreted at the same time as the planets, about 4600 million years ago, between the orbits of Mars and Jupiter. Most of the present-day survivors of the planetesimals, the asteroids, orbit between Mars and Jupiter but the Apollo and Amor objects do stray within the orbits of the Earth or Mars and, together with cometary nuclei, still produce occasional impacts, although at a much reduced rate (see also near-Earth asteroid).
During a crater-forming impact, the kinetic energy of the impacting body, which may be several kilometers across and moving at many kilometers per second, goes toward vaporizing and compressing much of itself and the rock on which it impinges. It is the subsequent explosion of these gases that excavates the crater, throwing material (ejecta) away from the crater center to form the rim and the crater's hummocky ejecta blanket. Chunks within the ejecta give rise to secondary impact craters nearby. In a large crater the steep inner rim can then slump to form the observed terraces, while the central peaks may arise from a rebound of the rocks immediately below the site of the explosion. Craters may also initially have ray systems, i.e. bright streaks radiating outward from the crater. In the equatorial and midlatitude regions of Mars, the ejecta deposits surrounding many impact craters consist of several overlapping lobes, so that they appear to be surrounded by ‘moats' and rampart-like ridges. These so-called rampart craters have an ejecta morphology that is believed to result when an impacting object rapidly melts subsurface ice. The presence of liquid water mixed in with the ejecta material allows it to flow along the ground and produce the characteristic moat-and-rampart effect.
Later impacts may destroy an existing crater completely, form a new overlapping crater, or partly fill the crater with ejecta. Lava may well up through cracks in the floor of the largest impact features to produce lava plains, such as the lunar maria. There may be rim obliteration and erosion and the occurrence of isostatic uplift. As a result the depth-to-diameter ratio, which for lunar craters may be as high as 1:5, decreases. On the Earth, erosion by wind, water, ice, and tectonic forces has removed nearly all the evidence that our planet was subject to the same ancient bombardment as the Moon; only a few recent impact craters, such as the Barringer crater in Arizona, remain relatively intact. Over 120 space-impact sites have been identified on the Earth. They occur worldwide, and most are more than 50 million years old.