Meteorites

Most people have looked up into the night sky and seen the fleeting flashes of light that are known as meteors. These flashes are caused by small sand-sized particles that are debris from comets, which melt in the atmosphere and never reach the surface of Earth. Sometimes these flashes come in showers, such as the famous Perseid meteor shower, which occurs from July 23 to August 22 when Earth crosses the debris-strewn orbit of comet Swift-Tuttle.

Meteorites, on the other hand, are extraterrestrial material that have made it to Earth's surface and can weigh many tons. This material is not related to comets but rather to other astronomical bodies. Deceleration of meteorites begins high in the atmosphere where the surface of the incoming body heats up to incandescence causing melting and ablation and forming a (usually) black fusion crust on the exterior. Whether a meteoroid makes it to Earth's surface (and becomes a meteorite) or not depends on many factors including the mass, initial velocity, angle of entry, composition, and shape of the body. Like the Moon, Earth has been subjected in the past to periods of intense meteorite bombardment, but fortunately many incoming meteoroids disintegrate well up in the atmosphere.

The largest meteorite on Earth, weighing some 60 tons, is called Hoba and lies where it fell in Namibia. There are various other meteorite giants, including Chaco (Argentina), weighing 37 tons; Ahnighito (Greenland), 31 tons; and Bacubirito (Mexico), 22 tons.

The orbits of five recovered meteorites are shown in the figure below. Their orbits suggest that their origin lies in the asteroid belt between Mars and Jupiter. These orbits were calculated from photographs taken by networks of cameras in Europe, Canada, and the United States.

In 1982 a 31-gram (1.1-ounce) meteorite discovered in the Allan Hills region of Antarctica was determined to be from the Moon. Since then, more than twenty fragments of the Moon and more than twenty fragments of the planet Mars have been found on Earth.

One interesting thing that can be done with meteorites, and one of the reasons why they are so important to science, is to date them by radioisotope methods. It turns out that most meteorites have a formation age around 4.56 billion years, when the solar nebula (the hot swirling cloud of dust and gas from which the Sun formed) began to cool enough for solid material, and hence planets, to form. Thus, meteorites represent a fossil record of the early conditions of the solar nebula.

Meteorite Craters

A consequence of large meteorites striking Earth is the formation of craters. This occurs when a large body weighing in excess of 100 tons strikes Earth's surface at sufficiently high velocity. The kinetic energy of the meteorite is converted to heat, which vaporizes the surrounding rock as well as much of the meteorite, producing an explosion equivalent to a large nuclear device. One of the most famous craters is Meteor Crater (also called Barringer Crater) near Flagstaff, Arizona, which is about 50,000 years old and 1.2 kilometers (0.7 miles) in diameter. The impacting iron mass was approximately 50 meters (164 feet) across, and the consequence of it striking Earth at 60,000 kilometers per hour (37,200 miles per hour) can be seen in the accompanying image.

Meteor Crater is not particularly large among the 150 or so impact craters that exist on Earth, and even larger ones can produce global climatic and environmental changes. The asteroid that is thought to have wiped out the dinosaurs was about 10 kilometers (6.2 miles) in diameter and struck off the coast of Yucatan, Mexico, 65 million years ago, producing a crater 300 kilometers (186 miles) in diameter. This event is theorized to have created enormous amounts of dust, which blocked out the Sun, possibly for years, and led to the extinction of 75 percent of all living species.

Can such an event happen in modern times? Since Earth is actually orbiting the Sun through a swarm of solar system debris, the answer has to be yes. In fact, in 1908 there was an enormous atmospheric explosion above Tunguska, Siberia. The resulting blast leveled 2,000 square kilometers (770 square miles) of forest, and the shock wave circled the globe. Such an event is predicted to happen once every few hundred years or so. As recently as 1947, the Sikhote-Alin meteorite crashing north of Vladivostok, Russia, made an array of craters, some of which were one-fourth the size of a football field.

Asteroids Turned Meteorites

Tens of thousands of small bodies called asteroids are found in the asteroid belt, with the largest one, Ceres, discovered in 1801, being about 930 kilometers (575 miles) in diameter. Most of these are in stable orbits around the Sun and are really just small planets, or planetoids. From time to time, these asteroids crash into one another, sending their fragments in all directions. But there are some empty zones in the asteroid belt, known as Kirkwood gaps (after American astronomer Daniel Kirkwood), which are caused by a special gravitational relationship with Jupiter. If some asteroid fragments from a collision are thrown into one of these gaps, Jupiter's enormous gravity has the effect of sending them into a more elliptically shaped orbit (as seen in the figure on page 104) that can intersect Earth's orbit. That is how fragments of the asteroid belt can end up crashing into Earth as meteorites.

One way to study asteroids is to measure the intensity of sunlight at different wavelengths reflecting off their surfaces. This is then compared to the light reflected off pulverized meteorites in the laboratory. Reflectance spectra from various asteroids can be matched with different types of meteorites, further strengthening the connection between asteroids and meteorites.

There are three basic types of meteorites: stones, stony-irons, and irons. Stones are divided into two main subcategories: chondrites and achondrites. Chondrites are the main type of stony meteorite, constituting 84 percent of all witnessed meteorite falls. Most chondrites are characterized by small spherical globules of silicate, known as chondrules. Interestingly, carbonaceous chondrites also contain organic compounds such as amino acids, which may have contributed to the origin of life on Earth. Chondrites are the most primitive of the meteorites, suffering little change since their origin. Achondrites, on the other hand, come from chondritic parent bodies that have been heated to the melting point, destroying their chondrules and separating heavy and light minerals into a core and mantle. These are known as differentiated meteorites. Early volcanism occurred on the surface of their parent bodies forming a thin crust. A subcategory of achondrites called SNC achondrites are believed to have come from Mars.

Stony-irons are a metal-silicate mixture. Meteorites from one subcategory, the pallasites, contain large crystals of the mineral olivine imbedded in a matrix of metal. These are thought to form at the boundary of the molten metal core of an asteroid and its olivine-bearing silicate mantle.

Irons are actually alloys of mostly iron with a small percentage of nickel. As the liquid metal core of an asteroid slowly cools over a period of millions of years, the different alloys of nickel-iron (kamacite and taenite) form an intertwining growth pattern known as a Widmanstätten pattern, which is indicative of extraterrestrial iron meteorites.

Meteorites are true extraterrestrials, valuable not only to science but also to the discoverer. If you happen to find a piece of the Moon lying on the ground (as some people have), you can plan your retirement from that day onward. Today, a thriving market exists as an increasing number of new meteorites are being discovered yearly, many finding their way to the marketplace. A growing number of aficionados eagerly await these new discoveries.

see also Asteroids (volume 2); Comets (volume 2); Close Encounters (volume 2); Impacts (volume 4); Mars (volume 2); Moon (volume 2).

Joel L. Schiff

Bibliography

Bagnall, Philip M. The Meteorite and Tektite Collector's Handbook. Richmond, VA: Willmann-Bell, Inc., 1991.

Hutchison, Robert, and Andrew Graham. Meteorites. New York: Sterling Publishing Co., Inc., 1993.

McSween, Harry Y., Jr. Meteorites and Their Parent Planets, 2nd ed. Cambridge, UK:Cambridge University Press, 1999.

Norton, O. Richard. Rocks from Space, 2nd ed. Missoula, MT: Mountain Press, 1998.

meteorites Small, extraterrestrial bodies, most of which probably originate in the asteroid belt, that enter the Earth's atmosphere and land on the surface. Most are only a few centimetres in size. Meteorites are classified into four main groups according to their composition and structure as: chondrites; achondrites; stony-irons; and irons. See also ANTARCTIC METEORITE; AUBRITE; BASALTIC METEORITES; CARBONACEOUS CHONDRITE; EUCRITE; SHERGOTTYITE/NAKHLITE/CHASSIGNITE METEORITES; STONY METEORITE; and TEKTITE.

me·te·or·ic / ˌmētēˈôrik/ • adj. 1. of or relating to meteors or meteorites: meteoric iron. ∎ fig. (of the development of something, esp. a person's career) very rapid: her meteoric rise to the top of her profession.2. chiefly Geol. relating to or denoting water derived from the atmosphere by precipitation or condensation.DERIVATIVES: me·te·or·i·cal·ly / -ik(ə)lē/ adv.

me·te·or·ite / ˈmētēəˌrīt/ • n. a meteor that survives its passage through the earth's atmosphere such that part of it strikes the ground. More than 90 percent of meteorites are of rock, while the remainder consist wholly or partly of iron and nickel.DERIVATIVES: me·te·or·it·ic / ˈˌmētēəˈritik/ adj.

meteorite Part of a large meteoroid (a small particle or body following an Earth-crossing orbit) that survives passage through the Earth's atmosphere and reaches the ground. Most of a meteoroid burns up in the atmosphere to produce meteors, but c.10% reaches the surface as meteorites and micrometeorites. Meteorites generally have a pitted surface and a fused charred crust. There are three main types: iron meteorites (siderites), stony meteorites (aerolites), and mixed iron and stone meteorites. Some are tiny particles, but others weigh up to 200 tonnes.

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