Meteorites: Stones from the Sky
Meteorites are fragments of other worlds that have survived the entry into the Earth's atmosphere. Most meteorites originate in the asteroid belt from bodies that formed very early in the history of the solar system. Almost all of the information we have learned about the solar system, such as its age, history, and chemical composition, is due to the detailed study of meteorites.
Types of Meteorites
There are three basic origins of meteorites. This leads to a classification of meteorites into three types: stony, stony-iron, and iron. Meteoriticists recognize many more types of meteorites, and have reconstructed a marvelously detailed history of the solar system from their subtle differences.
Iron Meteorites
[The Union County Meteorite is an iron meteorite.]
The most easily recognized meteorites are the iron meteorites. Since even a casual examination shows that they are not ordinary rocks, they tend to be very common in collections. However, they are rare in space. They are heavy and, except for a thin crust (made by the melting of the surface by the passage through the atmosphere), they look and feel like metal. Chemically, they are mostly iron with a few percent nickel and a little cobalt. When sawed in half and polished, they display a geometrical pattern called a Widmanstatten pattern (see below right). This pattern is the result of the meteorite cooling very slowly under very high pressure. Iron meteorites were once the cores of larger, differentiated bodies, most likely asteroids. Because of differentiation, these large asteroids developed an iron core and a stony outer mantle. Between the core and the mantle was a stony-iron region, more iron toward the core, more stony toward the mantle. Collisions in the asteroid belt break up asteroids, sending particles into the inner solar system. Occasionally one of the bits runs into the Earth and falls as a meteorite.
Stony Meteorites
The most common meteorites that fall to the Earth are called stony meteorites. Many are from the outer parts of an asteroid that was destroyed by collision, but some are pieces of material that was never part of a much larger body. Meteorites that come from such a small, undifferentiated body are called primitive meteorites. The stony meteorites vary in appearance: some are light, some dark, some are coarse grained, some fine grained. Chemically they are also diverse; however, they all have a telltale composition that tells us that they are definitely not from Earth. Their diversity and the fact that they tend to look like ordinary rocks to the untrained eye means that stony meteorites are difficult to recognize in the field. Unless someone sees them fall, they usually go unnoticed. Therefore, although stony meteorites are the most common type out in space, they are rarer than irons in collections on Earth.
Stony-Iron Meteorites
Pieces of material from the boundary zone between the cores and mantles of the now-destroyed asteroids are also found. These very rare meteorites are called stony-iron meteorites. They tend to look like irons with pieces of stone in them, or stones with pieces of iron.
Carbonaceous Chondrite Meteorites
An especially important type of meteorite is the carbonaceous chondrite. These are stony meteorites of a very special kind, usually black or dark gray in color. They are rich in the element carbon (thus their black color) and they contain small spherical droplet-like inclusions called chondules. They are among the most unchanged (primitive) objects in the solar system, having survived literally untouched for 4.6 billion years. Although carbonaceous chondrites are fairly abundant among meteorites that fall to the Earth, they look enough like Earth rocks that they are rare in collections. They also weather very easily and do not survive long on the surface of the Earth.
