IMPACT CRATER

any feature on the continents that has been created by the impact of cosmic bodies (meteorites, asteroids, or comets) on the Earth's surface. In fact, the collision of a Mars-sized body with the Earth early in the history of the solar system may have caused the formation of the Moon and the continents themselves, or rather the ocean/continent dichotomy of the Earth's outer layers. Impact craters differ from one another principally in terms of size, progressing from small, simple cup-shaped depressions originally with raised rims, through hollows having a central hill (or peak), to those with more complicated central peaks and basins, and finally to multi-ringed basins up to 1,000 kilometres in diameter. The surfaces of the Moon, Mercury, and certain regions of Mars and Venus, as well as most of the satellites of the solar system, are dominated by impact craters. They are much rarer on the Earth, which has about 100 ranging in size from a few tens of metres to 160 kilometres. These have been identified with varying degrees of certainty. The reason for the relative scarcity of impact craters on the Earth is that the processes that formed its surfacelargely those of weathering, erosion, and mountain buildinghave undoubtedly removed the majority of the older structures, such as are observed on the surfaces of the other planets mentioned above. There is abundant evidence that impact cratering was not only an important surface process in planetary history but that large impact events produced effects that were crustal in scale. The formation of multi-ring basins on the early Moon, for example, is just as important a process in defining the tectonic framework of that body as plate-tectonic phenomena are on the Earth. Evidence from several planets indicates that the effects of very large-scale impacts go beyond the simple formation of an impact structure and serve to localize increased internal geologic activity over an extended period of geologic time. Although no longer occurring with the same frequency and magnitude as during the early solar system, large-scale impact events have continued to affect the local geology of the inner planets. The Moon travels through essentially the same orbital space as the Earth. Since the Moon has not been subjected to the surficial geomorphic processes and tectonic activity that prevail on Earth, the lunar surface serves as an invaluable repository of information about the rate and effects of large body impacts that are likely to have occurred on the Earth. Telescopic observations, orbiting remote-sensing spacecraft, and the Apollo manned explorations of the Moon conducted by the United States during the early 1970s have provided the necessary data to establish surface impact rates. These are quite comparable to rates derived independently from astronomical studies for fluxes of asteroidal and cometary objects in near-Earth space. (For details pertaining to this and related questions, see asteroid, comet, and meteorite.) Some important conclusions for the Earth follow. Asteroidal objects as large as 20 kilometres in diameter probably have struck the planet during the last few billion years, and bodies measuring 10 kilometres across apparently may collide with it every 50,000,000 to 100,000,000 years. Cometary nuclei of similar size may have nearly comparable rates of collision. A 10-kilometre stony object with a density of about three grams per cubic centimetre (0.12 pound per cubic inch) striking the terrestrial surface at a velocity of 25 kilometres per second (15.5 miles per second) would have kinetic energy in excess of 100,000,000 megatonsfar greater than that contained in the world's total nuclear arsenal. On land, the impact of such an object would produce a transient crater 60 to 70 kilometres in diameter; the subsequent collapse of the basin walls would result in a final crater having a diameter of possibly 100 to 125 kilometres. Moreover, an impact of this kind would probably exert a planetwide influence on biological evolution because it could trigger mass extinctions of entire species, as perhaps in the case of the dinosaurs 65,000,000 years ago. Additional reading Two general books on the subject are Ronald Greeley, Planetary Landscapes (1985); and Bruce Murray, Michael C. Malin, and Ronald Greeley, Earthlike Planets: Surfaces of Mercury, Venus, Earth, Moon, Mars (1981). Collected papers from scientific meetings include Bevan N. French and Nicholas M. Short (eds.), Shock Metamorphism of Natural Materials (1968); and Leon T. Silver and Peter H. Schultz (eds.), Geological Implications of Impacts of Large Asteroids and Comets on the Earth (1982), in which see especially Richard A.F. Grieve, The Record of Impact on Earth: Implications for a Major Cretaceous/Tertiary Impact Event, pp. 2537. C. Ronald Seeger