meteorite

meteorite A natural object from space that hits the surface of the Earth or other planetary body. Impacts by large meteorites are believed to have created most of the craters on the planets and their satellites. An estimated 50–100 tonnes of cosmic debris enters the Earth's atmosphere every day, but only about a tonne reaches the ground. The fate of a body entering the Earth's atmosphere depends mainly on its mass and velocity. The smallest objects (micrometeorites) are decelerated and drift slowly down to the surface. Objects with masses between about 10^-6 g and 1 kg burn up to produce meteors. Bodies with masses between about 1 kg and 1000 tonnes are substantially slowed by atmospheric drag, but penetrate the atmosphere. The atmosphere has no significant slowing effect on bodies of over 1000 tonnes. The average entry velocity of incoming bodies which fall as meteorites is about 20 km/s. Incoming bodies with high velocities are more likely to disintegrate in the atmosphere than those with lower velocities; those with an entry velocity in excess of 30 km/s suffer more than 99% ablation. However, the composition of an incoming body also affects whether it survives to reach the ground.

Meteorites were observed and collected for thousands of years, but their extraterrestrial origin was not accepted until J. -B.Biot investigated the l'Aigle meteorite shower in 1803. A meteorite fall is preceded by a brilliant fireball, often accompanied by hissing noises and detonations like thunder. As the meteorite is slowed down it may break up, showering fragments in a scatter ellipse. Meteorites range in mass from a few grams to 60 tonnes or more. A meteorite seen to hit the ground is known as a fall, whereas one discovered by chance at some later date is known as a find. In both cases, the meteorite is named after the place where it is picked up. The great majority (nearly 80%) of known meteorites come from Antarctica where they have been preserved in deep-freeze since they fell.

There are three main classes of meteorite, divided according to their composition: iron meteorites, stony meteorites, and stony-iron meteorites. From observed falls, stony meteorites appear to be about twenty times more abundant in our part of the Solar System than are iron and stony-iron meteorites combined. However, the true ratio is probably even higher because stones tend to be more friable (crumbly) than irons, and so disintegrate more readily in the atmosphere. However, stony meteorites are under-represented in collections because once on the ground they are more susceptible to weathering and disintegration. Stones also tend to resemble terrestrial rocks and so are more difficult to recognize than are irons and stony-irons.

About 1000 meteorites have been observed to fall, but this is only a small fraction of the total number of incoming objects, most of which fall unseen in the oceans or unpopulated areas. By contrast, over 20 000 meteorites have been found, many of them since 1969 when it was discovered that meteorites are preserved on the surface of the Antarctic ice sheet. The large numbers and new types found among Antarctic meteorites have greatly stimulated research.

Meteorites are the most ancient rocks known, about 4.5 billion years old, about the same as the age of the Solar System. Hence, they may carry clues to the formation of the Solar System and the bodies within it. Although most meteorites are believed to be fragments of asteroids or asteroid-sized bodies, recent studies have identified a small number which appear to have come from the Moon. Another group, the SNC meteorites, probably originated on Mars. These meteorites were presumably ejected from their parent bodies by massive impacts.