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The term meteorite is derived from the Greek. It means that which originates in the atmosphere. Today, we define a meteoroid as a small object travelling through space; a meteor as its luminous trail in the sky; and meteorites are the fragments of the meteoroid that collide with the Earth and survive to reach the surface.
Meteoritic materials are constantly reaching the Earth. Several falls are observed and recovered every year. Most of these are stones or metallic masses of only a few kilograms, small enough to be held in your hand. The rarer, larger falls are produced when a mass of hundreds or thousands of kilograms strikes the atmosphere, often breaking up to scatter fragments over a large area. Even larger projectiles can also strike at intervals of thousands of years, producing impact craters when they crash into the surface.
A surprising variety of rocks are in the Earth's meteorite collections. The traditional description of these meteorites are based on their appearance and bulk composition. There are three classes of meteorite: iron, stony and the rarer stony-iron types. Iron meteorites are nearly pure metallic nickel-iron. Their extraterrestrial origin is obvious when we bear in mind that iron and most other metals naturally occur on the earth in the form of oxides rather than in the pure metallic state. The stony meteorites more closely resemble terrestrial rocks, and they are not generally recognized as extraterrestrial in origin unless their fall was witnessed. The stony-irons contain a mixture of stone and metallic iron, as suggested by their name.
A more useful categorization, rather than one dependent on appearance and bulk composition of the meteorites, is based on their history or that of their parent bodies (the asteroids and/or comets of which the meteorites are fragments). Some meteorites have a chemistry indicative of little alteration since their formation (and probably since the formation of the solar system). These are known as primitive meteorites. All primitive meteorites are stony, although not all stony meteorites are primitive. Primitive meteoritic material remains largely unaltered by its violent arrival at the Earth's surface. Those meteorites that have experienced major chemical or physical changes since their formation are known as differentiated meteorites. Like igneous rocks on Earth, these rocks solidified out of a molten state. Differentiated meteorites appear to be fragments of differentiated parent bodies that had experienced major episodes of heating, along with the loss of volatile materials. All of the iron and stony-iron meteorites (and many of the stony meteorites) are instances of differentiated meteorites.
Primitive meteorites have chemical compositions that are relatively unchanged since they formed in the cooling solar nebula ~4.5billion years ago. Except for a shortage of gaseous and other volatile constituents such as hydrogen, helium, argon, carbon and oxygen, the composition of the primitive meteorites is thought to be the same as that of the sun.
The most primitive meteorites are a special group, called the carbonaceous meteorites. These are relatively rich in carbon and also in volatile compounds such as water. From their composition we conclude that they were formed in a cooler region of the solar nebula from the other primitive meteorites.
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