METEORITE CRATER

depression that results from the impact of a meteorite with any planetary body. Data collected by Ranger, Orbiter, Surveyor, and Apollo spacecraft indicate that roughly 3,000,000,000,000 craters with diameters greater than one metre (about three feet) exist on the Moon's surface. The space probe observations also allow lunar craters to be classified by size and morphological characteristics and, at least inferentially, by modes of origin. Thus, most lunar scientists now recognize primary and secondary impact craters, volcanic calderas, maars, summit craters, and drainage or collapse craters as probable genetic types on the lunar surface. Impact craters have been discovered on the surface of Mars as well as on various other planets and satellites. It is reasonable to assume that impacts occur frequently on all unprotected planetary surfaces in the solar system and, by extrapolation, on any similar bodies elsewhere in the universe. In general, the number of craters per unit of surface area correlates closely with the nature and density of the planetary atmosphere and its capability for erosion, as well as with the flux of bombarding particles (meteorites) over time in a planet's neighbourhood. Impact craters are much less common on the Earth than on the Moon. This is partly because friction from the Earth's atmosphere effectively burns up most of the smaller bodies entering it from outer space. Thus, any craters formed on the Earth's surface will tend to be larger than the average size of all entering meteorites. Over the last eon (1 109 years) meteorites large enough to survive the plunge through the air and then excavate big craters appear to have been too few to produce the expected population of impact structures on the continents. The combined effects of the hydrosphere and the atmosphere in eroding the land surface evidently have erased most meteorite craters between 10 m (33 feet) and about 1 to 2 km (2/3 to 1 1/4 miles) in diameter. If such craters were more abundant in the first 23 109 years of terrestrial history, owing to a greater meteorite flux in the early solar system, their traces have long since been removed by erosion, deposition, and metamorphism. Since 1955, the list of definite and probable impact structures on the Earth's surface has grown significantly from the 20 to 27 proposed by investigators up until then. (See the Table.) The chief reasons for the sharp increase in the numbers of identified craters have been formulation of the astrobleme concept, more intensive searching, and development of new criteria for recognizing impact structures. The astrobleme concept provided a better understanding of what to look for in relation to meteorite craters. An astrobleme is simply the scar of an impact crateri.e., any part of the structure that has survived erosion. Older impact structures may completely lack discernible crater rims, but they still retain some degree of circularity, have distinctive internal deformation features, or contain remnants of the original rock fragments (breccia) shocked in the impact. Thus, in the broader sense, a meteorite crater can be defined as any structure now at or near the surface containing evidence of a shock-producing impact in which the disturbance by deformation and fragmentation of rock or soil is generally circular in outline (plan) and dies out rapidly upon reaching diameter-to-depth ratios of 3 to 1 or greater.