SUMMARY: Primitive meteorites are remnants of the solar nebula, containing intermixed rock and metal flakes. Processed meteorites are fragments of larger asteroids and therefore can be metallic like a planet's core or rocky like its mantle or crust.
METEORITE: the actual particle from space that passes through the atmosphere and lands on Earth
The Ahnighito Meteorite, located at
the American Museum of Natural
History in New York.
Most meteorites are difficult to distinguish from terrestrial rocks without detailed scientific analysis. Meteorites are usually covered with a dark, pitted crust resulting from their fiery passage through the atmosphere. Some can be distinguished from terrestrial rocks by their metal content, which is high enough to attract a magnet.
Primitive meteorites appear to be remnants of the birth of the solar system. Radioactive dating shows them to be about 4.6 billion years old, meaning that they have remained essentially unchanged since they first accreted in the solar nebula.
Most meteorites are primitive. They are pieces of rock that accreted in the solar nebula and orbited the Sun for billions of years before finally falling to Earth. The individual flakes may represent the tiny particles that first condensed from the gas of the solar nebula.
Two Different Kinds of Primitive Meteorites
Stony primitive meteorites (above, left)
- Composed of rocky minerals with a small fraction of pure metallic flakes mixed in
- Matches the composition of asteroids in the inner part of the asteroid belt
Carbon-rich primitive meteorites (above, right)
- Contain substantial amounts of carbon compounds, with some even including a small amount of water
- Matches the composition of dark, carbon-rich asteroids in the outer part of the asteroid belt
Processed meteorites apparently once were part of a large object that “processed” the original material of the solar nebula into another form. Radiometric dating shows that these meteorites are generally younger than the primitive meteorites.
The compositions of processed meteorites are similar to the crusts, mantles or cores of the terrestrial planets. Thus, they must be fragments of larger asteroids whose interiors melted so that the heavier metals sank to the center and the lighter rocks rose to the surface. This process is called differentiation .
Two Different Kinds of Processed Meteorites
Metal-rich processed meteorites (above, left)
- Made mostly of high-density iron and nickel mixed with smaller amounts of other metals
- Resemble the terrestrial planet cores in composition
Rocky processed meteorites (above, right)
- Have lower densities and are made of rock
- Resemble terrestrial planet mantles and crusts in composition