Meteorites

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The quick flashes of light in the sky most people call ``shooting stars'' are meteors---pieces of rock glowing from friction with the atmosphere as they plunge toward the surface at speeds around 20 to 40 kilometers/second. Before the rock hits the Earth's atmosphere it called a meteoroid. Most of the meteors you see are about the size of a grain of sand and burn up at altitudes above 50 kilometers (in the mesosphere). If the chunk of rock is a meter or larger in size, it is called an asteroid. Large meteors and asteroids can make the much brighter "fireballs" that may even briefly rival the brightness of the Sun. More meteors are seen after midnight because your local part of the Earth is facing the direction of its orbital motion around the Sun. Meteoroids moving at any speed can hit the atmosphere. Before midnight your local part of the Earth is facing away from the direction of orbital motion, so only the fastest moving meteoroids can catch up to the Earth and hit the atmosphere. The same sort of effect explains why an automobile's front windshield will get plastered with insects while the rear windshield stays clean.

If the little piece of rock makes it to the surface without burning up, it is called a meteorite. As with the asteroids, there are many ways of grouping meteorites. Just as was done for the asteroids, you can group the meteorites into three basic types based on their composition.

1. Stones: they are made of silicate material with a density around 3 (relative to the density of water) and look like ordinary Earth rocks. This makes them hard to distinguish from Earth rocks so they do not stand out. About 95% to 97% of the meteorites are of these type. About 85% of the stones are primitive, unchanged since they first solidified about 4.6 billion years ago. Most of the primitive stones have chondrules---round glassy structures 0.5 to 5 millimeters across embedded in the meteorites. They are solidified droplets of matter from the early solar nebula and are the oldest part of a primitive meteorite.
   
   A meteorite containing chondrules (courtesy of the Planetary Data Center).

   The oldest of the stone meteorites are the carbonaceous meteorites. They contain silicates, carbon compounds (giving them their dark color), and a surprisingly large amount of water (about 22% of their mass). They are probably chips of C-type asteroids. Some of the carbonaceous meteorites have organic molecules called amino acids. Amino acids can be connected together to form proteins that are used in the biological processes of life. There is the possibility that meteorites like these may have been the seeds of life on the Earth. In addition, these type of meteorites may have provided the inner planets with a lot of water. The terrestrial planets may have been so hot when they formed that most of the water in them at formation evaporated away to space. The impact of millions to billions of carbonaceous meteorites (and comets) in the early solar system may have replenished the water supply on the terrestrial planets.

   About 10% to 12% of the stones are from the crust of differentiated parent objects. Therefore, they are younger (only 4.4 billion years old). The lighter-colored stones are chips from the S-type of asteroids.

2. Stoney-Irons: only 1% of the meteorites are of this type. They have a variable mixture of metal (iron and nickel) and rock (silicates) and have densities ranging from 4 to 6 times that of water. They come from a differentiated object at the boundary between the metal core and the rock crust or from objects that were only partially differentiated. They are 4.4 billion years old.

3. Irons: although they make up about 40% of the meteorites found worldwide, only 2 to 3% of the meteorites are these type. They make up so many of the ones found because they are easily distinguished from Earth rocks. They are noticeably denser than Earth rocks, they have a density around 7 times that of water. They come from the core of a differentiated body and are made of iron and nickel. They are 4.4 billion years old. Irons sometimes have large, coarse-grained crystalline patterns (``Widmanstatten patterns'') that is evidence that they cooled slowly.
   
   An iron meteorite (courtesy of the Planetary Data Center).

The primitive meteorites are probably the most important ones because they hold clues to the composition and temperature in various parts of the early solar nebula. Because of this, some astronomers put the meteorites into two groups: primitive and processed (not primitive).

See Arizona State University's Center for Meteorite Studies excellent website for more details about meteorites and distinguishing them from terrestrial rocks.

Meteorites at the Los Angeles Museum of Natural History

Here are some images of the meteorites on display at the Los Angeles Museum of Natural History plus a display of "meteor-wrongs"---terrestrial rocks that look like meteorites.

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Two iron meteorites. On the left is a Canyon Diablo meteorite found at Meteor Crater in Arizona in 1891. It is a coarse octahedrite iron meteorite. On the right was found in 1945 at Laguna Manatiales, Santa Cruz in Argentina. This one clearly shows the Widmanstatten pattern of two nickel-iron alloys (kamacite with high iron and taenite with high nickel) intergrown in in a crisscross pattern. Iron meteorites are analagous to the iron cores of planets.

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A stony-iron (on left) and a stony meteorite (on right). The stony-iron meteorite is a Pallasite found in Imilac in the Atacama Desert of Chile. Stony-iron meteorites contain both silicate and nickel-iron and Pallasites (named after the asteroid Pallas) contain approximately equal proportions of silicate and nickel-iron. Stony-irons are analogous to a planet's core-mantle boundary which is why there are so few of them found. The stony meteorite is a ordinary chondrite (has a lot of tiny spherical silicate chondrules embedded in it) found on a ranch near Neenach in the western Antelope Valley area of California. The rancher broke it into four pieces as he unearthed it with his plow.

Stony meteorites: ordinary chondrite on left observed to fall on March 5, 1960 in Gao, Sissili, Burkina; carbonaceous chondrite (has a high carbon content) in the middle observed to fall on February 8, 1969 at Allenda, Chihuahua, Mexico; and an achondrite (without chondrules) that fell sometime in 1960 at Millbillillie in Western Australia. Ordinary chondrites and carbonaceous chondrites are the oldest of the meteorites that formed 4.6 billion years ago in the birth of the solar system. The achondrite meteorite is made of igneous rocks composed of silicate minerals such as plagioclase, olivine, and pyroxene and is analogous to a planet's crust.

Named for where it was found in Littlerock, California, this stony ordinary chondrite has a mass of 19 kilograms. It probably fell about 100 years ago, judging by the freshness of the fusion crust---the outer crusty layer that forms as the rock heats up when passing through the atmosphere at supersonic speeds.

Finally, a set of "meteor-wrongs"---Earth rocks that look like extra-terrestrial meteorites. Here are the questions to ask when looking at a rock to determine if it might be a meteorite:

• Is it magnetic? Except for those from Mars and the moon, almost all meteorites contain some iron-nickel alloy. Iron meteorites will be very strongly attracted to a magnet, while stony meteorites will be only weakly attracted. However, some common terrestrial rocks, notably those containing magnetite, and some industrial slags will also be attracted to a magnet.
• Is there a fusion crust? The 15 seconds of heat generated during its trip through the atmosphere flash-fried the outside of a meteorite, resulting in a thin, often glass, ash-black fusion crust. Over time this crust may weather to a rusty brown and eventually disappear. However, a rock with desert varnish can resemble fusion crust. Breaking it open or maybe just turning it over is enough to show that it is an Earth rock.
• What's its shape? As a meteorite moves through the Earth's atmosphere, it may develop the shaped of a rounded cone as parts of its surface are ablated (melted) away. The surface of a meteorite is usually smooth and featureless but due to ablation, it often has hallow depressions and cavities resembling thumbprints in wet clay. However, it's possible for erosion by wind-blown sand to scrulpt terrestrial rocks into rounded cones as well and then desert varnish completes the disguise.
• Does it have chondrules? Chondrules are microscopic to marble-sized silicate spheres found in most stony meteorites but not found in Earth rocks. However, Earth rocks can have other spherical structures in them, so it takes a trained eye to distinguish them from the chondrules of stony meteorites.
• Does it show a Widmanstatten pattern? When cut, polished, and brushed with a dilute solution of acid, most iron meteorites show a crisscross pattern of the intergrowing of two nickel-iron alloys. Human-made iron objects do not have the Widmanstatten pattern.

I sometimes get asked to identify a strange looking rock that's posing as a meteorite. I send inquirers to the William M Thomas Planetarium's "Is it a Meteorite or a Meteorwrong?" page. A definite positive identification requires chemical analysis at a meteorite laboratory.

Finding Meteorites

To get an accurate number for the proportion of meteorites that fall to the Earth (an unbiased sample), meteorite searchers go to a place where all types of rocks will stand out. The best place to go is Antarctica where the stable, white ice pack makes darker meteorites easy to find. Meteorites that fell thousands of years ago can still be found in Antarctica without significant weathering. Since the 1980s, thousands of meteorites have come from here. For further exploration, check out the Antarctica Meteorite web site at the Planetary Materials Curation office of NASA and the ANSMET site at Case Western Reserve University.

Meteorites stand out against the snow and ice background of Antarctica.

Most (about 99.8%) meteorites are pieces of asteroids, but 143 (at the time of writing) are from the Moon. A select few (125 at the time of writing), the Shergotty-Nakhla-Chassigny (SNC) meteorites, may be from Mars. The relative abundances of magnesium and heavy nitrogen (N-15) gases trapped inside the SNC meteorites is similar to the martian atmosphere as measured by the Viking landers and unlike any meteorites from the asteroids or Moon. Also, the isotope ratios of argon and xenon gas trapped in the meteorites most closely resemble the martian atmosphere and are different than the typical meteorite. The analysis of the soil and rocks by the Mars Pathfinder confirm this.

A meteorite blasted from Mars.

Most SNC meteorites are younger than 1.4 Gyr old, but the one with suggestions of extinct Martian life is about 4.5 Gyr old. The discovery was published in the August 16, 1996 issue of the journal Science. Non-subscribers can find a copy of the article here. Recent studies of the meteorite have cast considerable doubt on the initial claims for fossil microbes in the rock. There is strong evidence of contamination by organic molecules from Earth, so this meteorite does not provide the conclusive proof hoped for. A detailed description of SNC meteorites is given at JPL's Mars Meteorites website.

Below are some small slices of martian meteorites I got to handle at a Chautauqua short course in 2005, courtesy of Tony Irving. One of them, the Zagami meteorite slice, had been analyzed enough (and handled by several intermediary parties at least) that we were allowed to touch it directly, so I can say that I have touched a piece of Mars! Note that the martian rocks are not red but instead, grayish in color like the rocks of Earth. The red color of Mars is from a thin layer of dust on the surface. When the dust is swept away by a dust devil or a rover from Earth, the darker material below is exposed. In all of the images, a U.S. dime coin is used to show the scale of the image.

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last updated: June 8, 2022