understanding of the composition of meteorites and asteroids has evolved with time. Early studies relied on meteorites, the bodies that had fallen to Earth, since no space travel or observations in space were possible during the previous centuries. Recently space missions to asteroids and remote sensing have enabled observations of asteroids in space to be integrated with the data from samples taken from meteorites collected on Earth.
The ice content in asteroids generally increases as the distance of their orbits from the sun increases. Geologists classify meteorites collected on Earth according to the composition (reflected in the types of minerals present) and how much they have been metamorphosed, or changed by events that have subjected the meteorites to higher temperatures and pressures. Many meteorites were part of planetesi-mals (small planets) that had formed iron-rich cores, and then were probably destroyed in a catastrophic collision with another large asteroid early in the history of the solar system, spreading the asteroid debris of both planetesimals across the solar system. other asteroids, comets, and meteorites appear not to have ever been part of larger planets, and may represent some of the primordial matter from the solar nebula from which the solar system formed.
Meteorites are classified on the basis of their composition and structure. The aim of such a classification scheme is to group together all the known bodies that may share a common parent body, whether it was a large asteroid, comet, planet, or moon. This is achieved by placing known asteroids into groups and subgroups based on their important mineralogical, physical, chemical, and isotopic properties.
Meteorites are made from material similar to that which makes up the Earth, including common silicate minerals, plus iron and nickel metals. some meteorites also contain small lumps of material called chondrules, which represent melt droplets that formed before the meteorite fragments were accreted to asteroids, and thus represent some of the oldest material in the solar system. some meteorites also contain presolar grains, often comprising tiny carbon crystals in the form of diamonds.
Traditional classification schemes for meteorites broke them into three main groups based on composition. These groups include stony meteorites composed mosty of rocky material, iron meteorites composed mostly of metallic material, and mixtures called stony-iron meteorites. These groups were then divided into subgroups; for instance, the stony meteorites were divided into chondrites and achron-drites according to whether or not they contained chondrules. The iron meteorites were divided into textural groups including structures such as octa-hedrites, hexahedrites, and ataxites. more recently these textures have not been used for classification but only for descriptive purposes, and the iron meteorites are further divided according to their chemistry. stony-iron meteorites were divided into pallasites and mesosiderites.
more modern schemes use a simpler classification, in which meteorites are classified as either chondrites or nonchondrites. The nonchrondrites are divided into primitive and differentiated types. The differentiated nonchondrites have three groups, including the achondrites, stony-irons, and irons.
Recognizing meteorites on Earth is difficult since they have similar minerals to many Earth rocks. many meteorites develop a fused crust on their surface from the heat during entry through Earth's atmosphere. most of the meteorites from Earth used for classifications have been collected from Antarctica, where the only place for rocks on the ice fields to come from is space. Mos^ about 85 percent of all meteorites falling on Earth, are chondrites, thought to represent largely primitive solar material. For the nonchrondrites, many classifications and descriptions are aimed at determining whether they come from larger parent bodies that broke up during early impacts, and if so, what the characteristics of this parent body may have been.
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