METEORITE

any interplanetary particle or chunk of stony or metallic matter known as a meteoroid that survives its passage through the Earth's atmosphere and strikes the ground. The term is often applied to similar objects that reach the surfaces of other planets or satellites. Any source that can eject such material into interplanetary space should, at least in principle, be thought of as a candidate source of meteorites. There is no fundamental reason all meteorites must come from similar sources. It turns out, however, that there are some regions of the solar system that are much more effective than others in introducing material of substantial strength into Earth-crossing orbits. Laboratory and theoretical studies fully confirm the older belief that most meteorites are fragments of asteroids. These same studies show that a small fraction, less than 1 percent of meteorites, come from nonasteroidal sources. The lunar origin of several meteorites is well established, and it is probable that at least eight others came from Mars. There is evidence from fireball data that a small part of the material in cometary orbits (i.e., with aphelia beyond Jupiter) may possess sufficient strength to successfully penetrate the atmosphere. It is not known if any of this material is present in existing meteorite collections. If it is, the best candidate material would be carbonaceous stony meteorites, probably those of type CI (see below), of which five separate falls have been recovered. With these few exceptions, it is safe to regard all meteorites as samples broken from outcrops of rock or metal, which until fairly recently in solar-system history were part of asteroidal bodies, mostly in the inner region of the asteroid belt (between about 2.2 and 2.6 AU). Like rocks from the Moon, the Earth, or any other similar planetary body, their present state is determined by the total effect of events that occurred on the body throughout the entire history of the solar system. There is no a priori reason why such samples must be pristine samples of a primordial solar nebula from which the present solar system evolved. On the other hand, the principal driving force behind asteroid studies has been the plausible belief that small primitive bodies such as asteroids and comets are those most likely to preserve evidence of events that took place in the early solar system. Insofar as this belief is correct, meteorites, samples of these bodies, share this property. Evidence derived from the study of meteorites themselves supports this conclusion. any interplanetary particle or chunk of stony or metallic matter known as a meteoroid (q.v.) that survives its passage through the Earth's atmosphere and strikes the ground. The term is often applied to similar objects that reach the surfaces of other planets or satellites. Meteoroids enter the Earth's atmosphere at a velocity of at least 11 km (7 miles) per second. Interaction with the atmosphere heats them to high temperatures, causing them to become incandescent, vaporize, and heat the surrounding air. This process produces a streak of light, which is referred to as a meteor (q.v.). As meteoroids penetrate into the dense lower atmosphere, they usually fragment and disintegrate. Although several thousand enter the atmosphere each year, only a few hundred actually reach the ground. Occasionally, the remnant of a very large meteoroid weighing several tons or more may collide with the surface, exploding upon impact to create a so-called meteorite crater (q.v.). Meteorites may be classified into two broad groups: undifferentiated and differentiated. Undifferentiated meteorites are those that do not appear to have experienced chemical differentiation associated with igneous melting. They consist of the chondrites, which constitute the most abundant type of meteorite. The chondrites are so called because they contain chondrules, spherules of millimetre size composed primarily of the silicate minerals olivine and pyroxene. The chondrites are commonly divided into three major subclassesordinary, enstatite, and carbonaceouson the basis of chemical differences. The enstatite chondrites are, for example, more chemically reduced than the ordinary variety, so that nearly all their constituent iron is in metallic form. The carbonaceous chondrites, for their part, contain more carbon than either the ordinary or enstatite chondrites. Differentiated meteorites evince the kind of chemical fractionation that is thought to occur on a planetary body that underwent core formation and magmatic differentiation comparable to that observed in volcanic rocks of the Earth and the Moon. Meteorites of this group are generally classified into three categories: iron, stony, and stony iron. The iron meteorites consist chiefly of nickeliron metal and iron sulfides. The stony type are rich in silicates (e.g., olivine, orthopyroxene, and plagioclase) but are poor in metal and sulfides. The principal subclass of the stony meteorites are the achondrites, which characteristically have no chondrules. The stony-iron meteorites contain mixtures of large masses of nickeliron metal and differentiated silicate rock. Meteorites are of considerable scientific interest because their chemical characteristics and their texture and internal structure carry clues to the early history of the solar system. It is widely held that nearly all meteorites originate from meteoroids produced by collisions between asteroids, which aggregated from fragmentary material about the same time as the Earth, some 4.5 billion years ago. There also are indications that a few meteorites, the CI carbonaceous chondrites, are derived from the nuclei of comets, which are considered by many researchers as the most pristine remnants of the dust and gas that formed the solar system. Additional reading Introductory information can be found in Harry Y. McSween, Jr., Meteorites and Their Parent Planets (1987); Robert T. Dodd, Thunderstones and Shooting Stars: The Meaning of Meteorites (1986); John G. Burke, Cosmic Debris: Meteorites in History (1986); Robert Hutchison, The Search for Our Beginning: An Enquiry, Based on Meteorite Research, into the Origin of Our Planet and of Life (1983); and John A. Wood, Meteorites and the Origin of Planets (1968). More advanced treatments are John T. Wasson, Meteorites: Their Record of Early Solar-System History (1985), and Meteorites: Classification and Properties (1974); V.A. Bronshten, Physics of Meteoric Phenomena (1983; originally published in Russian, 1981); and Robert T. Dodd, Meteorites: A Petrologic-Chemical Synthesis (1981). A descriptive and historical treatment of iron meteorites, including beautiful photographs, is Vagn F. Buchwald, Handbook of Iron Meteorites, Their Distribution, Composition, and Structure, 3 vol. (1975). H.H. Nininger, Out of the Sky: An Introduction to Meteorites (1952, reprinted 1959), provides firsthand experiences of fall phenomena on a nontechnical level. See also D.E. Brownlee, Cosmic Dust: Collection and Research, Annual Reviews of Earth and Planetary Sciences, 13:147173 (1985). A catalog of known meteorites, including data regarding their fall, is A.L. Graham, A.W.R. Bevan, and R. Hutchison (eds.), Catalogue of Meteorites, 4th ed. rev. and enlarged (1985). There are two journals devoted to papers on meteorites and related bodies: Meteoritika (annual), published in Russia; and Meteoritics (quarterly). Many papers on meteorites are published in Geochimica et Cosmochimica Acta (monthly). George W. Wetherill