Classification of Meteorites

Meteorites are traditionally classified as irons, stones, and stony-irons. The "irons" are composed of nearly pure metallic nickel-iron. This makes it easy to classify them as extraterrestrial in origin because pure metallic iron almost never occurs naturally on the Earth - it is in the form of some oxide. Fraknoi, et al. comment " if you ever come across a chunk of metallic iron, it is sure to be either man-made or a meteorite." The stony meteorites are more common but harder to identify, often requiring isotopic analysis to be sure. The stony-irons, mixtures of metallic iron and stone, are much rarer.

Another important characterization of the meteorites is as differentiated or undifferentiated meteorites. The differentiated meteorites, including the irons and stony-irons, appear to be fragments of larger bodies for which separation according to density took place while they were in the molten state after formation. As a larger body cools, the more dense materials sink toward the center. This gives an important role to the undifferentiated stony meteorites, since they can be presumed to be from smaller bodies which cooled and solidified too quickly for the differentiation to take place. These primitive meteorites are then our best picture of the early history of the solar system since they have had fewer influences for change over the age of the solar system.

The stony meteorites are by far the most numerous of the meteorites. They are commonly described as gray silicates with some metallic grains mixed in.

Of particular importance are the stony meteorites collected in Antarctica from the ice and they have been used for radioactive dating, giving us perhaps our best indication of the age of the solar system.

A different group of meteorites is the carbonaceous meteorites, dark rocks containing a significant amount of carbon.

The vast majority of meteorites are thought to come from the asteroid belt, but a number have been identified as coming from either the Moon or Mars.

The meteorites are rich sources of information about the solar system. Two of the most famous meteorites, the Allende meteorite and the Murchison meteorite, have been studied intensively for clues about solar system history.

Carbonaceous Meteorites

Of the classes of meteorites, stony meteorites are by far the most numerous. But an important group of meteorites are dark stones with a significant content of carbon. They are thought to originate from dark asteroids called C-type asteroids in the outer part of the asteroid belt. The small meteorite at left, called a carbonaceous chondrite, is on display in the Smithsonian Museum of Natural History. It is about 1.5 cm across.

The carbon in these meteorites is a complex, tar-like substance. Along with this material are a number of complex organic molecules. Sixteen amino acids were found in the Murchison meteorite. Eleven of them are rare on the Earth.

A remarkable aspect of these meteorites is the fact that the abundance of water in them is up to 20% compared to 0.1% of the Earth and the abundance of carbon is up to 4% compared to 0.05% of the Earth (Ward & Brownlee).

Murchison Meteorite

The Murchison meteorite is named for the town in Australia close to its impact point. It fell in 1969, as did another famous meteorite, the Allende meteorite. It has been a treasure of organic compounds, and has impacted our understanding about chemicals in the solar system. It is a carbonaceous meteorite containing a complex, tar-like substance. Along with this material are a number of complex organic molecules. Sixteen amino acids were found in it, eleven of which are rare on the Earth.

The amino acids found in the Murchison meteorite were found to be equal mixtures of left and right-handed molecular symmetry, whereas all life on the Earth contains only left-handed amino acids. That fact alone points to the extra-terrestrial origin of the meteorite.

The ratio of ¹³C to ¹²C in the Murchison meteorite is about twice that usually found on the Earth.

Allende Meteorite

The Allende meteorite is named for the town in Mexico where it fell in 1969, the same year as another famous meteorite, the Murchison meteorite. It is most famous for the variety of chemicals found in it. It is thought that as much as 10% of Allende is of previous origin and is therefore older than our solar system.