How to recognize meteorites?

Here is a flyer to help you recognize meteorites (PDF).

What are meteorites?

Meteorites are rocks that have been injected as meter-sized objects into the Earth's orbit following collisions on their parent bodies. Most come from asteroids; More than 200 come from the Moon and about 150 are thought to come from Mars.

Meteorites are of great scientific importance. Some have preserved the record of processes that occurred before and during the formation of the planets. This is apparent from the great age of most meteorites: 600 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.

Martian meteorites provide clues about the evolution of this planet that cannot otherwise be obtained until very expensive sample-return missions are completed in future decades. Most lunar meteorites sample regions on the Moon that were not visited during the Apollo 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, air friction melts the 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. The brighter meteors made by meteoroids with masses >100 grams are called fireballs. If a meteoroid is still moving above the speed of sound after it falls below a height of about 55 kilometers it will create sonic booms.

Most stony meteoroids undergo fragmentation and produce showers of glowing objects. Air friction removes the melt from their surface as it forms, exposing new surfaces that also melt. Meteoroids can lose more than 95% of their mass during atmospheric passage.

By the time meteoroids have fallen to a height of about 20 kilometers, their velocities have been reduced to about the speed of sound and the meteoroids stop glowing. The final surface melt on these objects solidifies to form a fusion crust.

Throughout atmospheric passage, the interiors of the meteoroids retain their original temperature (near the freezing point of ice); the internal structure and composition of these rocks are unaffected by the melting of the rocks’ surface. When meteoroids reach the Earth, they are called meteorites. The surface of freshly fallen meteorites might be warm to the touch, but they are never hot enough to ignite fires when they land.

Meteorites are divided into two main classes: primitive and differentiated

Primitive meteorites

The most abundant meteorites are the chondrites; they comprise about 90% of the meteorites that fall. Most of these rocks have been altered by thermal or aqueous processes. Those that have largely avoided alteration consist of minerals and mineral assemblages (such as chondrules) produced in the Solar Nebula, the cloud of gas and dust that formed in the planetary region at the same time as the Sun. The processes by which these particles were collected and compacted to form the chondrites are still poorly understood, but it is clear that these primitive objects experienced little chemical change in their early history.

Eventually, much larger objects were produced from these primitive chondritic planetesimals ("infinitesimal planets"). Thus it is reasonable to call the chondrites the building blocks of the planets.

Chondrites show a wide range in properties indicating formation at different places and times (for example, some have high (~20 weight %) metal contents; others are metal free). Metal-free chondrites contain similar amounts of iron and nickel as metal-rich chondrites, but these elements are present solely as oxides or sulfides. The most primitive chondrites consist of minor phases with a wide variety of chemical properties; the millimeter-size grains of these rocks formed one at a time and were never in equilibrium with each other.

[Figure A]. These millimeter-sized objects formed during brief melting events in the Solar Nebula. The melting events were very short, with durations of seconds. The physical process that created them is not known; shock waves and lightning are commonly discussed possibilities.

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A characteristic of chondrites is their content of chondrules
Figure A

Three complete chondrules about 1 mm in diameter are visible in this transmitted-light photo of the Tieschitz chondrite. The colored minerals are mainly olivine and vary appreciably in shape and size from chondrule to chondrule. Width of image about 4 mm.

Differentiated meteorites

The differentiated meteorites were formed by melting and the separation of phases ("differentiation"). Heating metal-rich chondrites to sufficiently high temperatures causes them to melt; immiscible metal and silicate liquids are produced. The denser metallic melt then moves downward under buoyancy forces and the silicate melt moves upwards. This is the main way that iron meteorites and differentiated stones (basalts such as the eucrites and howardites) formed. The more refractory silicates did not melt but remained as the "mantle" of the parent asteroid; the main mantle mineral was olivine (the same phase as the semi-precious gem peridot).

The iron meteorites consist of iron-nickel metal together with some other phases. Many iron meteorites show a Widmanstätten or octahedral pattern such as exhibited by polished and etched samples of the Buenaventura iron [Figure B].

The light phase that forms bands oriented in four directions is a low-nickel (about 6% nickel) alloy of iron. This section of Buenaventura also includes some irregular patches of schreibersite, an iron-nickel phosphide mineral.

The most common differentiated stones are the basalts; they are similar in composition to lavas extruded by volcanoes on Earth. The texture of the Pasamonte eucrite is typical of those produced in rapidly cooled lavas [Figure C]. This meteorite created a long dusty tail in the atmosphere when it fell in New Mexico in 1933.

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Figure B

Close-up view of the Widmanstätten pattern of the Buenaventura IIIAB iron meteorite. The pattern is an intergrowth of two different iron-nickel minerals (kamacite and taenite) that contain different amounts of nickel. The dominant pattern is formed by the low-nickel phase kamacite (the light-colored bands). Note that there are thin bands oriented at about 45º and 135º relative to the base of the picture; two other sets of bands are wider and somewhat irregular in shape. Some regions with grainy textures and irregular shapes are iron-nickel phosphides. The meteorite was found in Mexico around 1969. (Photo by G. Natzler)

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Figure C

This thin-section view of the eucrite Bluewing 001 reveals a basaltic (more specifically “subophitic”) texture, indicative of rapid crystallization of a fast-cooling magma at the surface of an asteroid or planet. The major minerals (pyroxene, various bright colors; plagioclase, shades of grey) form elongated intergrown grains up to a few millimeters in length. This 6.1-g meteorite was found in Nevada in 2000. (Photo by P. Warren)

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Figure D

The Emery mesosiderite. This stony-iron meteorite was found in South Dakota around 1962. It consists of about 50% iron-nickel metal and 50% silicates including the large triangular clast. It is speculated that these meteorites formed during the collision of an asteroid with a molten core with a second asteroid with basaltic materials on the surface. (Photo by G. Natzler)

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Figure E

The Seymchan pallasite. In Seymchan the yellow silicate olivine grains are 4 to 7 mm across and are embedded in a matrix of nickel-iron metal. These meteorites formed by the mixing liquid metal with solid olivine at the core-mantle interface of an asteroid. The irregular shapes of the olivine indicate formation by crushing, probably during an impact. Length of section about 14 cm. (photo by J. Wasson)

A curious kind of differentiated meteorite is the group of mesosiderites; these rocks consist of a mixture of basalt (which is commonly formed near the surface of a differentiated asteroid or planet), and metal (which commonly forms cores in planetary bodies). Figure D shows a sample of the Emery mesosiderite from the UCLA collection

The members of one class of differentiated meteorite, the pallasites, consist of roughly equal amounts of metal and olivine; they formed at the boundary between the core and the mantle of their differentiated parent asteroid. In Figure E the olivine of the Seymchan pallasite is yellow and the metal is light gray.

Finding meteorites in the field

Properties important for distinguishing meteorites from normal rocks

Many of us like to check out the rocks we see when we take nature walks; we dream that we may someday find a meteorite. Here in southern California a large fraction of public lands is desert where, because vegetation is scarce, rocks are easy to see. Here are some properties that can be used to help identify meteorites.

Size and Density

Meteorites may vary in weight from a less than a gram to many tons, but typical masses of recovered meteorites are in the mass range 100 to 10000 grams; typical lengths are about 3 to 30 cm. Iron meteorites have very high densities (7.4 to 7.9 times that of water) and weigh about three times as much as Earth rocks of similar size; most chondrites weigh about one and a half times as much as comparably-sized earth rocks.

Characteristic Features
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Figure 1

The La Criolla L6 chondrite. This meteorite fell on 6 January 1985 in Argentina. Note how the thin matte black fusion crust contrasts with the light gray interior of the stone. Left-to right length about 18 cm. (Photo by G. Natzler)

Meteorites are generally irregular in shape with rounded edges, but none are completely spherical. The fusion crust of most freshly fallen meteorites is generally black [Figure 1]; on some rare types the crust is light gray. Interiors of most stones, as with the La Criolla chondrite, are generally gray in color.

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Figure 2

The Ness County L6 chondrite. This meteorite was recovered from a field in Kansas; it has weathered to a mix of brown (iron oxide) and black (original fusion crust). The rounded dimples are typical of chondrite falls. Left-to-right length about 18 cm. (Photo by J. Wasson)

Over the course of decades or centuries, weathering on Earth can change the fusion crust of a meteorite to rusty brown [Figure 2].

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Figure 3

The Kinsella IIIAB iron meteorite weighs about 3.5 kg and is about 20 cm long . This meteorite was found in 1946 in Alberta, Canada. Note the that the depressions on the rusty fusion crust formed during atmospheric passage are deeper than those in the chondrites. (Photo by Gail Natzler)

The smooth surface of the fresh meteorite is often pitted with thumbprint-shaped depressions (called regmaglypts) that are more pronounced on irons than on stones (see the Kinsella iron meteorite [Figure 3].)

No meteorites are porous or hollow.

Tests
a) Shape, color and texture. The first clue that a rock is a possible meteorite is its general shape and color. Freshly fallen stones are black; weathered stones or irons are generally dark brown.

b) Magnetism. Because meteoritic metal is always magnetic, it can be attracted with a 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 swings when the stony sample is passed near to it. But note that not all meteorites are magnetic: lunar meteorites, martian meteorites and some eucrites are essentially metal free.

c) Exposure of the interior. An important test is to grind a corner of the suspected object with an emery wheel, emery stick 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.

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

What to do if you think you have a meteorite?
We are willing to examine samples that pass the above tests but (because we are not paid for this), please limit the number of samples to two or three. If you think you have a meteorite, remove a small piece, about half the size of a large grape, for examination. Although pieces can be removed with a hammer and chisel, this may create small fragments not well suited for examination. It is better to saw off small pieces. For iron meteorites this can be done with a hacksaw. Stones can be sawed with a ceramics saw at home or at a local rock shop.

Please send the sample along with your name, address, phone number, fax number, and e-mail address to:

Dr. A. E. Rubin
Dept. of Earth and Space Sciences
University of California
Los Angeles, California 90095-1567
USA

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. Listen for any accompanying sounds. It is also important to record the time and the brightness, size and shape of the fireball.

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

Additional Information on meteorite properties
Wikipedia has good short articles on meteorites and gives additional references to more specific articles. More information can be found at the web sites for NASA, JPL and the Meteoritical Society. Each of these sites features links to several others.