Lecture
Lecture |
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| Meteorites: Stones from the Sky
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Introduction
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
From the point of view of origin, there are three basic types of meteorites. These are commonly called 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.
The images shown here are from this extensive gallery of meteorite images. It is highly recommended that you take the time to visit this site.
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 figure at the left which shows an iron found
in Henbury, Australia). This pattern is the result of the meteorite having
cooled very slowly under very high pressure. The idea is that the 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.
The iron meteorites we will handle in class are samples of the cores of worlds formed out beyond the orbit of Mars.
The most common meteorites that fall to the Earth are called stony
meteorites. Many are from the outer parts of an asteroid that suffered destruction
by collison, but some are pieces of material that apparently never existed
in a much larger body. Meteorites that come from such a small, undifferentiated
body are called primitive meteorites. The stony meteorites show a wide variety
of appearances: some 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
more rare than irons in collections on Earth. The stony shown in the figure
above is from Silverton, Texas.
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.
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| This is a section of a meteorite found in Kansas, an example of an "iron-stony" |
 | This is a section of a meteorite, from Dalgaranga, Western Australia, an example of a stony-iron |
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. It has been recently learned that some chondrules were formed outside of our solar system and thus were around long before our solar system was even formed. So, not only are carbonaceous chondrite meteorites an important probe into the early history of our solar system, but they may supply us with materials from beyond our solar system. 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 thus do not survive long on the surface of the Earth.
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| This is a section of the most famous carbonaceous chondrite, the Orgueil, found in France a long time ago. It is also one of the most studied meteorites; samples of amino acids are found in this meteorite. Notice the extremely black color. |
 | This is a section of a carbonaceous chondrite found in Allende,
Mexico. The small white specks are chondrules. Notice the large number that
are roughly spherical in shape. |
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