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 (Murchison, Victoria). 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. Seventy four amino acids were found in it, compared to the 20 which are characteristic of life.

The amino acids found in the Murchison meteorite showed an excess of left-handed amino acids near the surface but more nearly equal concentrations of left- and right-handed amino acids near the center. This suggests some contamination, since all life on the Earth contains only left-handed amino acids. The nearly equal concentrations in central samples points to the extra-terrestrial origin of the meteorite. But the latest analyses do show a likely excess of left-handed even after contamination has been accounted for. There was an 18% elevation of left-handed isovaline compared to a 7-10% excess in some other amino acids. Kvenvolden.

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.

One of the interesting aspects of the Allende meteorite is the presence of a significant amount of magnesium in the meteorite in the form of the isotope ²⁶Mg. The normal stable isotope of magnesium is ²⁴Mg. The magnesium was found in the mineral anorthite, which would not normally include Mg because the smaller magnesium ion would not fit well into the crystal lattice. This suggested that this "stowaway magnesium" was produced by the radioactive decay of the aluminum isotope ²⁶Al which has a half-life of 730,000 years. The implication that the short-halflife aluminum was around to be included in the crystallization of the anorthite is significant for detective work about the origin of the Sun. Chondrite meteorites like the Allende are thought to be about the age of the solar system, and the presence of ²⁶Al could have come from a nearby supernova that seeded these short-lived isotopes in the material from which the Sun was formed.

Antarctic Meteorite GRA 95229

Antarctic Meteorite GRA 95229 showed a 12-14% chiral excess of L-type amino acid for isoleucine and alloisoleucine, whereas most meteorites in which amino acids have been found have shown nearly equal mixtures of L- and R-handed amino acids. The researchers believe that this meteorite is exceptionally pristine, which raises the question about the mechanism that led to unequal concentrations.