DIFFERENT KINDS

A meteoroid which reaches the ground is called a meteorite. A small number of meteorites have been found which scientists believe originated on the Moon or Mars. The composition of meteorites gives us valuable information about the universe. Meteorites typically fall into one of five categories.

Irons
These meteorites are made of a crystalline iron-nickel alloy that resembles the outer core of the earth. Similar in structure to some asteroids (type M), 5.7% of meteorites are irons.

Stony-Irons
These meteorites are mixtures of iron-nickel alloy and non-metallic mineral matter. Scientists believe they are like the material which would be found where the Earth's core meets the mantle. 1.5% of meteorite falls are stony irons.

Stony Meteorites
There are three subclasses of stony meteorites:

Chondrites: These meteorites are the most numerous, comprising 85.7% of all meteorites found. They are characterized by chondrules: small (average diameter of 1 millimeter) spheres of formerly melted minerals that have melded with other minerals to form a solid rock. Chondrites are believed to be among the oldest rocks in the solar system and are similar in composition to the mantles and crusts of earth and the other terrestrial planets.

Carbonaceous Chondrites: These meteorites are very rare and contain elemental carbon, the basic building block for life on earth.

Achondrites: Stony meteorites without chondrules, representing about 7.1% of meteorites. Scientists believe that some of these meteorites originate on the surface of the Moon or Mars.