Cosmochemistry

What are Meteorites?

Meteorites are rocks that have been deflected into the earth's orbit following collisions in space. Most are widely believed to come from asteroids; about a dozen come from the Moon and about 15 are thought to come from Mars. Meteorites are of great scientific importance. Some have preserved the record of processes occurring before and during the formation of the planets. This is apparent from the great age of most of them: 700 million years older than the oldest known earth rock, and 500 million years older than nearly all lunar rocks. The study of meteorites provides important information about the origin of the solar system and the formation of the planets. Lunar and martian meteorites provide constraints on the evolution of these bodies that cannot be learned except by expensive sample-return missions.

Small rocky objects in interplanetary space are called meteoroids. When meteoroids enter the earth's atmosphere at great speed, they produce fireballs that can be brighter than the full moon. During the first part of their flight through the atmosphere, friction with air molecules melts the outer surface layer of the meteoroid and raises its temperature to incandescence. The light phenomenon produced by the passage of a meteoroid through the atmosphere is known as a meteor. Turbulent air ablates the melt from the meteoroid’s surface, exposing a new surface that then begins to melt. Meteoroids can lose more than 95% of their mass during atmospheric passage. At a height of about 10 kilometers, the meteoroid has slowed significantly. Friction with the surrounding air has been reduced and the meteoroid stops glowing. The outer melted surface, known as the fusion crust, is retained. Throughout atmospheric passage, the interior of the meteoroid remains cold and the internal structure and composition of the rock is unaffected. When a meteoroid reaches the surface, it is called a meteorite. They are never hot enough to ignite fires when they land.

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Types:

Meteorites are divided into three main groups:

1. Irons (Figs. 1, 2) - alloys of iron and nickel
2. Stones (Fig. 3) - consisting largely of silicate minerals (similar to earth minerals) and, in most cases, 5-20% metallic iron-nickel (although some varieties are metal free), and
3. Stony-irons (Fig. 4) - which have roughly equal proportions of iron-nickel and silicate.

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Size & Density

Meteorites may vary in weight from a few grams to many tons, but most weigh a few kilograms and are about 5-50 cm in length. Iron meteorites have a very high density and weigh about three times as much as earth rocks of similar size; most stony meteorites weigh about one and a half times as much as comparably sized earth rocks.

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Characteristic Features

Meteorites are generally irregular in shape with rounded edges, but none are completely round. The fusion crust of most freshly fallen meteorites is black (Fig. 3); on some rare types the crust is light gray. Over the course of decades or centuries, weathering on earth can change the crust of a meteorite to rusty brown (Fig. 1). The smooth surface of the fusion crust is often pitted with thumbprint-shaped depressions (called regmaglypts) which are more pronounced on irons (see the Kinsella iron meteorite; Fig. 1). The interiors of most stony meteorites are distinctly lighter in color than the crusts. Meteorites are never porous or hollow.

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Tests

The best single test is to grind a corner of the suspected object with an emery wheel or emery or silicon-carbide paper. An iron meteorite will show a bright white metallic surface and the vast majority of stony meteorites will show tiny flecks of shiny silver-colored metal. Because the iron-nickel metal is always magnetic it can be attracted with a hand magnet or refrigerator magnet. Irons are strongly magnetic while most stones will attract a magnet weakly. (Weak magnetism can be demonstrated by hanging the magnet on a string, then showing that it moves when the sample is passed near to it.)

If the specimen has a black interior, is highly magnetic and feels less dense than a piece of iron metal of similar size, it is probably the iron oxide magnetite. Magnetite is the most common material confused with meteorites, particularly in the Western United States. The identification of magnetite can be confirmed by seeing if the specimen gives a black streak on an unpolished (e.g., bottom side of) ceramic tile.

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What to do if you think you have a meteorite?

If you think you have a meteorite, remove a small piece, about half the size of a large grape, for examination. Small pieces can be removed from most specimens with a hammer and chisel, with a hacksaw or by sawing at a local rock shop. Please send the sample along with your name, address, phone number, fax number, and e-mail address to:

Professor J. T. Wasson
Institute of Geophysics
University of California
Los Angeles, California 90095-1567
USA

A reward of $500 or more is offered for the first specimen (i.e., the main piece) of any new meteorite. The UCLA Collection of Meteorites is the largest of its kind on the West Coast and contains over 1600 samples from about 650 different meteorites.

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If you see a meteorite fall

In order to assist in the recovery of a recently fallen meteorite, you should note the exact direction in which the fireball disappeared. Select a distant landmark that is in line with the end point of the fireball's path and note carefully your own position. Note also the time and brightness, size and shape of the fireball, and listen for any accompanying sounds.

If you see find a meteorite on the ground following an observed fall, take a photograph of it before picking it up. Look around for other specimens; many meteorites break into several pieces before hitting the ground.

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Additional Information on Meteorites

Most encyclopedias will contain short articles on meteorites and give additional references to more specific articles. Good reports on meteorites can be found in the Encyclopedia Britannica, World Book Encyclopedia and Encarta. Many good meteorite books are available on the market and can be found in the science section of large bookstores and on the Internet. Additional information can be found at the web sites for NASA, JPL and the Meteoritical Society. Each of these sites features links to several others.

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Images

Fig. 1: The Kinsella IIIAB iron meteorite. This meteorite was found in 1946 in Alberta, Canada. Note the depressions on the rusty fusion crust formed during atmospheric passage. (Photo by Gail Natzler)

Fig. 2: Close-up view of the Widmanstätten pattern of the Buenaventura IIIAB iron meteorite. The pattern is an inter-growth of two different iron-nickel minerals (kamacite and taenite) having different amounts of nickel. The meteorite was found in Mexico around 1969. (Photo by G. Natzler)

Fig. 3: The La Criolla L6 chondrite. This meteorite fell on 6 January 1985 in Argentina. Note how the thin black fusion crust contrasts with the light gray interior of the stone. (Photo by G. Natzler)

Fig. 4: The Emery mesosiderite. This stony-iron meteorite was found in South Dakota in 1962. It consists of about 50% iron-nickel metal and 50% silicates including the large triangular clast. (Photo by G. Natzler)