METEORITE

METEORITE

METEORITE, Meteor that reaches the surface of the earth or of another planet before it is entirely consumed. Meteorites found on earth are classified into types, depending on their composition: irons, those composed chiefly of iron, a small percentage of nickel, and traces of other metals such as cobalt; stones, stony meteors consisting of silicates; and stony irons, containing varying proportions of both iron and stone. Although most meteorites are now believed to be fragments of either asteroids or comets, geochemical studies have shown that a few Antarctic stones came from the moon and Mars, from which they presumably were ejected by the explosive impact of asteroids. Asteroids themselves are fragments of large bodies called planetesimals, formed some 4.6 billion years ago, while the earth was forming. Irons are thought to represent the cores of planetesimals, and stones (other than the aforementioned Antarctic ones) the crust. Meteorites generally have a pitted surface and fused charred crust. The larger ones strike the earth with tremendous impact, creating huge craters.

The largest known meteorite, estimated to weigh about 55 metric tons, is situated at Hoba West near Grootfontein, Namibia. The next largest, weighing more than 31 metric tons, is the Ahnighito (the Tent); it was discovered, along with two smaller meteorites, in 1894 near Cape York, Greenland, by the American explorer Robert Edwin Peary. Composed chiefly of iron, the three masses had long been used by the Inuit (Eskimo) as a source of metal for the manufacture of knives and other weapons. The Ahnighito was brought to the U.S. by Peary and is on display in the Hall of Meteorites at the American Museum of Natural History in New York City. The largest known crater believed to have been produced by a meteorite was discovered in 1950 in northwestern Québec, Canada. It consists of a circular pit 4 km (2.5 mi) in diameter, containing a lake and surrounded by concentric piles of shattered granite.

The serious study of meteorites as non-earth objects began in the 20th century. Scientists now use sophisticated chemical analysis, X rays, and mass spectrography in determining their origin and composition. In 1996, the results of a study of a Mars rock recovered 12 years earlier from the Allan Hills region of Antarctica suggested that life may have existed on that planet. Microscopic quwantities of liquid water were found trapped in salt crystals in a 4.5-billion-year-old meteorite that fell to earth at Monahans, Tex., in 1998.