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Meteors and Meteorites

Meteors and Meteorites

Both meteorites and meteors are classified more broadly as meteoroids, the simplest definition of which is a small body in space. The term meteor is applied to any streak of light in the upper atmo­sphere that is produced by a small body when it enters from space. They are more commonly referred to as shooting stars. Efforts have been made to predict and record the occurrence of the most impressive phenomena, and there is wide interest in observing them among both scientists and laypeople.

Meteorites are natural bodies that have man­aged to travel through the atmosphere to land on the surface of the earth. To accomplish this, they must normally be large and dense enough so that they don’t crumble into fragments or vaporize completely. They are of great scientific interest because they provide an opportunity to examine extraterrestrial material older than any material found on earth. Whereas many of earth’s early rocks have been destroyed by natural geologic pro­cesses, meteorites have remained unchanged since the birth of the solar system. It is now generally accepted that research to determine the age of the materials in meteorites will very likely help us to determine the age of our planet.

Knowledge about and attitudes toward meteor­ites have undergone significant change through the centuries. There is evidence that many ancient people did accept meteorites as heavenly bodies. Both meteors and meteorites inspired a variety of myths and folk beliefs, with the element of super­stition emerging prominently during the medieval period. During the 17th and most of the 18th cen­turies, the possibility that meteorites had origi­nated from beyond the earth was not even considered.

The last decade of the 18th century marked a distinct forward movement in the scientific study of meteorites. Luckily, witnesses were present when several thousand meteorites fell near L’Aigle in northern France. In 1794, Ernst Chladni, a German physicist, published a book asserting that bodies of rock and metal actually do fall from the sky. He is now considered the founder of the study of meteoristics.

From that time on, museums and interested individuals began collecting samples, making fur­ther research possible. At present there are around 1,000 samples available for scientific study. The earliest known meteorite still available for study fell near the village of Ensisheim in the province of Alsace in 1492.

Some specific scientific observations still lagged behind. It wasn’t until the 1930s and 1940s that scientists concluded that some craters had origi­nally been formed by the impact of meteorites.

Chrondites represent the most primitive kind of meteorite, making up about 80% of the total. They appear to have been formed with the same sub­stances as the earth and other planets. Their compo­sition is also very similar to that of the sun, with the exception that the sun also contains large amounts of hydrogen, helium, and other noble gases.

In the 1950s, Clair Patterson, of the California Institute of Technology, began the process of dat­ing chrondites by a radiometric process that made use of a uranium-lead clock. He found their age to be 4.55 billion years, and the range of 4.5 to 4.7 billion years has been corroborated by five differ­ent radiometric dating methods. The basis for all of these methods has been the measurement of the radioactive decay that has occurred. About 70 meteorites have now been accurately dated. Radiometric dating has also been used to deter­mine ages of many other subjects, including the earth, the moon, fossils, early humans, and a vari­ety of geological events.

Comparing chrondites with rocks that appeared to have originated on earth, Patterson found a similar composition of lead isotopes. A logical conclusion is that both may have been formed when silicates condensed from the sun’s nebula. It has also been concluded that the universe may be as old as 14 to 17 billion years.

Scientists now generally believe that most mete­orites are pieces broken off when asteroids—or minor planets—collided. Asteroids themselves are large chunks of debris that circle the sun in the belt between Mars and Jupiter’s orbits. Like the earth, these meteorites are composed mostly of silicates and metals. A few rare meteorite samples may have come from Mars or the moon, probably as the result of a crater being formed by another body impacting those surfaces.

In the 1980s, sediments from around the world were examined and found to contain chemicals of the same age. That suggested that a large body had slammed into the earth approximately 65 million years earlier. It is even possible that our moon might have been formed as the result of a collision between the earth and a body about the size of Mars.

It is estimated that about 30,000 meteorites with a mass larger than 3.5 ounces fall to the earth each year. They are classified by the amount of silicate and nickel-iron that they contain and fall into the three broad categories of irons, stones, and stony irons.

Meteorites are classified as falls or finds, and each is given a name that is usually based on the geographical area in which it fell. To be classified as a fall, the event must be witnessed. Because meteorites usually signal their arrival by a light display and a variety of sounds, it is not surprising that some are observed at the time of impact. Reporting of these events can allow scientific study to begin soon after the event. The largest recorded fall occurred in 1976, in Jilin, Manchuria. The total weight of its pieces was 2 metric tons.

The term finds applies to those that are discov­ered by accident and subsequently identified by the chemical and mineral content or their structure. The largest known meteorite find is the African Hoba meteorite, weighing approximately 66 tons.

The study of known meteorites is expected to reveal even more information about the solar sys­tem. The search for meteorites that are yet to be discovered will be ongoing. Two challenges that have been recognized are the need to search in the comparatively neglected desert areas and the need to identify very small meteorites.

Betty A. Gard

See also Comets; Dinosaurs; Extinction and Evolution; Extinctions, Mass; Geology; Nuclear Winter; Paleontology

Further Readings

Bevan, A., & De Laeter, J. (2002). Meteorites: A journey through time and space. Washington, DC: Smithsonian Institution Press in association with University of New South Wales Press.

Norton, O. R. (2002). The Cambridge encyclopedia of meteorites. New York: Cambridge University Press.

Zandra, B., & Rotaru, M. (2001). Meteorites: Their impact on science and history. Cambridge, UK: Cambridge University Press.

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