Meteorite

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Meteorites

A meteorite is an extraterrestrial body that survives its impact with the Earth's surface without being destroyed. While in space it is called a meteoroid. When it enters the atmosphere, air resistance causes the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting star. The term bolide refers to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface.

More generally, a meteorite on the surface of any celestial body is an object that has come from elsewhere in space. Meteorites have been found on the Moon and Mars.

Meteorites that are recovered after being observed as they transitted the atmosphere or impacted the Earth are called falls. All other meteorites are known as finds. As of mid-2006, there are approximately 1050 witnessed falls having specimens in the world's collections. In contrast, there are over 31,000 well-documented meteorite findsMeteoritical Bulletin Database.

Meteorites are always named for the place where they were found Meteoritical Society Guidelines for Meteorite Nomenclature, usually a nearby town or geographic feature. In cases where many meteorites were found in one place, the name may be followed by a number or letter (e.g., Allan Hills 84001 or Dimmitt (b)).

Meteorites have traditionally been divided into three broad categories: stony meteorites are rocks, mainly composed of silicate minerals; iron meteorites are largely composed of metallic iron-nickel; and, stony-iron meteorites contain large amounts of both metallic and rocky material. Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy. See Meteorites classification.

Fall phenomena

Most meteoroids disintegrate when entering the Earth's atmosphere, however an estimated 500 meteorites ranging in size from marbles to basketballs or larger do reach the surface each year; only 5 or 6 of these are typically recovered and made known to scientists. Few meteorites are large enough to create impact craters. Instead, they typically arrive at the surface at their terminal velocity (free-fall) and, at most, create a small pit. Even so, falling meteorites have caused damage to property, livestock, and even people in historic times.

Very large meteoroids may strike the ground with a significant fraction of their cosmic velocity, leaving behind a hypervelocity impact crater. The kind of crater will depend on the size, composition, degree of fragmentation, and incoming angle of the impactor. The force of such collisions has the potential to cause widespread destruction Chapman et al. (2001)Make your own impact at the University of Arizona. The most frequent hypervelocity cratering events on the Earth are caused by iron meteoroids, which are most easily able to transit the atmosphere intact. Examples of craters caused by iron meteoroids include Barringer (Meteor Crater), Odessa Meteor Crater, Wabar craters, and Wolfe Creek crater; iron meteorites are found in association with all of these craters. In contrast, even relatively large stony or icy bodies like small comets or asteroids, up to millions of tons, are disrupted in the atmosphere, and do not make impact cratersBland P.A. and Artemieva, N A. (2006) The rate of small impacts on Earth. Meteoritics and Planetary Science 41, 607-631.. Although such disruption events are uncommon, they can cause a considerable concussion to occur; the famed Tunguska event likely resulted from such an incident. Very large stony objects, hundreds of meters in diameter or more, weighing tens-of-millions of tons or more, can reach the surface and cause large craters, but are very rare. However, such events are generally so energetic that the impactor is completely destroyed, leaving no meteorites. (The very first example of a stony meteorite found in association with a large impact crater, the Morokweng Crater in South Africa, was reported in May, 2006Maier, W.D. et al. (2006) Discovery of a 25-cm asteroid clast in the giant Morokweng impact crater, South Africa. Nature 441, 203-206.)

Several phenomena are well-documented during witnessed meteorite falls too small to produce hypervelocity craters Sears, D. W. (1978) The Nature and Origin of Meteorites, Oxford Univ. Press, New York. The fireball that occurs as the meteoroid passes through the atmosphere can appear to be very bright, rivaling the sun in intensity, although most are far dimmer and may not even be noticed during daytime. Various colors have been reported, including yellow, green and red. Flashes and bursts of light can occur as the object breaks up. Explosions, detonations, and rumblings are often heard during meteorite falls, which can be caused by sonic booms as well as shock waves resulting from major fragmentation events. These sounds can be heard over wide areas, up to many thousands of square km. Whistling and hissing sounds are also sometimes heard, but are poorly understood. Following passage of the fireball, it is not unusual for a dust trail to linger in the atmosphere for some time.

As meteoroids are heated during passage through the atmosphere, their surfaces melt and experience ablation. They can be sculpted into various shapes during this process, sometimes resulting in deep "thumb-print" like indentations on their surfaces called regmaglypts. If the meteoroid maintains a fixed orientation for some time, without rotating, it may develop a conical "nose cone" shape. As it decelerates, eventually the molten surface layer solidifies into a thin fusion crust, which on most meteorites is black (on some achondrites, the fusion crust may be very light colored). On stony meteorites, the heat-affected zone is at most a few mm deep; in iron meteorites, which are more thermally conductive, the structure of the metal may be affected by heat up to 1 cm below the surface. Meteorites are sometimes reported to be warm to the touch when they land, but they are never hot.

Meteoroids that experience disruption in the atmosphere may fall as meteorite showers, which can range from only a few up to thousands of separate individuals. The area over which a meteorite shower falls is known as its strewn field. Strewn fields are commonly elliptical in shape, with the major axis parallel to the direction of flight. In many cases, the largest meteorites in a shower are found farthest down-range in the strewn field.

Meteorite types

About 86% of the meteorites that fall on Earth are chondrites, which are named for the small, round particles they contain. These particles, or chondrules, are composed mostly of silicate minerals that appear to have been melted while they were free-floating objects in space. Chondrites also contain small amounts of organic matter, including amino acids, and presolar grains. Chondrites are typically about 4.55 billion years old and are thought to represent material from the asteroid belt that never formed into large bodies. Like comets, chondritic asteroids are some of the oldest and most primitive materials in the solar system. Chondrites are often considered to be "the building blocks of the planets."

About 8% of the meteorites that fall on Earth are achondrites, some of which appear to be similar to terrestrial mafic igneous rocks. Most achondrites are also ancient rocks, and are thought to represent crustal material of asteroids. One large family of achondrites may have originated on the asteroid 4 Vesta. Others derive from different asteroids. Two small groups of achondrites are special, as they are younger and do not appear to come from the asteroid belt. One of these groups comes from the Moon, and includes rocks similar to those brought back to Earth by Apollo and Luna programs. The other group is almost certainly from Mars and are the only materials from other planets ever recovered by man.

About 5% of meteorites that fall are iron meteorites with intergrowths of iron-nickel alloys, such as kamacite and taenite. Most iron meteorites are thought to come from the core of a number of asteroids that were once molten. As on Earth, the denser metal separated from silicate material and sank toward the center of the asteroid, forming a core. After the asteroid solidified, it broke up in a collision with another asteroid.

Stony-iron meteorites constitute the remaining 1%. They are a mixture of iron-nickel metal and silicate minerals. One type, called pallasites, is thought to have originated in the boundary zone above the core regions where iron meteorites originated. The other major type of stony-iron meteorites is the mesosiderites.

Note: Tektites (from Greek tektos, molten), natural glass objects up to a few centimeters in size, were formed--according to most scientists--by the impact of large meteorites on Earth's surface, although a few researchers have favored an origin from the Moon as volcanic ejecta. Tektites are NOT meteorites.

Meteorite recovery

Falls

Most meteorite falls are recovered on the basis of eye-witness accounts of the fireball and/or the actual impact of the object on the ground. However, a small number have been observed with automated cameras and recovered following calculation of the impact point. The first of these was the Pribram meteorite, which fell in Czechoslovakia (now the Czech Republic) in 1959Ceplecha, Z. (1961) Multiple fall of Pribram meteorites photographed. Bull. Astron. Inst. Czechoslovakia, 12, 21-46 NASA ADS. In this case, two cameras used to photograph meteors captured images of the fireball. The images were used both to determine the location of the stones on the ground and, more significantly, to calculate for the first time an accurate orbit for a recovered meteorite.

Following the Pribram fall, other nations established automated observing programs aimed at studying infalling meteorites. One of these was the Prairie Network, operated by the Smithsonian Astrophysical Observatory from 1963 to 1975 in the midwestern US. This program also observed a meteorite fall, the Lost City chondrite, allowing its recovery and a calculation of its orbitMcCrosky, R.E. et al. (1971) J. Geophys. Res. 76, 4090-4108. Another program in Canada, the Meteorite Observation and Recovery Project, ran from 1971 to 1985. It too recovered a single meteorite, Innisfree, in 1977Campbell-Brown, M. D. and Hildebrand, A. (2005) A new analysis of fireball data from the Meteorite Observation and Recovery Project (MORP). Earth, Moon, and Planets 95, 489 - 499. Finally, observations by the European Fireball Network, a descendant of the original Czech program that recovered Pribram, led to the discovery and orbit calculations for the Neuschwanstein meteorite in 2002Oberst, J. et al. (2004) The multiple meteorite fall of Neuschwanstein: Circumstances of the event and meteorite search campaigns. Meteoritics & Planetary Science 39, 1627-1641 NASA ADS.

Finds

Until the 20th century, only a few hundred meteorite finds had ever been discovered. Over 80% of these were iron and stony-iron meteorites, which are easily distinguished from local rocks. To this day, few stony meteorites are reported each year that can be considered to be "accidental" finds. The reason there are now over 30,000 meteorite finds in the world's collections started with the discovery by a man named Harvey H. Nininger that many meteorites may be found if you know how and where to look.

The Great Plains of the US

Nininger's strategy was to search for meteorites in the Great Plains of the United States, where the land was largely cultivated and the soil contained few rocks. Between the late 1920s and the 1950s, he traveled across the region, educating local people about what meteorites looked like and what to do if they thought they had found one, for example, in the course of clearing a field. The result was the discovery of over 200 new meteorites, mostly stony types Website by A. Mitterling.

In the late 1960s, one area of the Great Plains was found to be a particularly good place to find meteorites. This was Roosevelt County, in eastern New Mexico. After the discovery of a few meteorites in 1967, a public awareness campaign resulted in the finding of nearly 100 new specimens in the next few years, with many being found by a single person, Mr. Ivan Wilson. In total, nearly 140 meteorites were found in the region since 1967. In the area of the finds, the ground was originally covered by a shallow, loose soil sitting atop a hardpan layer. During the dustbowl era, the loose soil was blown off, leaving any rocks and meteorites that were present stranded on the exposed surfaceHuss, G.I. and Wilson, I.E. (1973) A census of the meteorites of Roosevely County, New Mexico. Meteoritics 8, 287-290 NASA ADS.

Antarctica

A few meteorites had been found by field parties in Antarctica between 1912 and 1964. Then in 1969, the 10th Japanese Antarctic Research Expedition found nine meteorites on a blue ice field near the Yamato Mountains. With this discovery, came the realization that movement of ice sheets might act to concentrate meteorites in certain areas. After a dozen other specimens were found in the same place in 1973, a Japanese expedition was launched in 1974 dedicated to the search for meteorites. This team recovered nearly 700 meteorites. Shortly thereafter, the United States began its own program to search for Antarctic meteorites, operating along the Transantarctic Mountains on the other side of the continent: the ANtarctic Search for METeorites (ANSMET) program. European teams, starting with a consortium called "EUROMET" in the late 1980s, and continuing with a program by the Italian Programma Nazionale di Ricerche in Antartide have also conducted systematic searches for Antarctic meteorites. Most recently, a Chinese program, the Antarctic Scientific Exploration of China, has conducted highly successful meteorite searches since the year 2000. The combined efforts of all of these expeditions have produced over 23,000 classified meteorite specimens since 1974, with thousands more that have not yet been classified. For more information see the article by Harvey (2003)Harvey, Ralph (2003) The origin and significance of Antarctic meteorites Chemie der Erde 63, 93-147.

Australia

At about the same time as meteorite concentrations were being discovered in the cold desert of Antarctica, collectors discovered that many meteorites could also be found in the hot desert of Australia. Several dozen meteorites had already been found in the Nullarbor region of Western and South Australia. Systematic searches between about 1971 and the present recovered over 500 moreBevan, A.W.R. and Binns, R.A. (1989) Meteorites from the Nullarbor region, Western Australia: I. A review of past recoveries and a procedure for naming new finds. Meteorites 24, 127-133 NASA ADS, ~300 of which are currently well characterized. The meteorites can be found in this region because the land presents a flat, featureless, plain covered by limestone. In the extremely arid climate, there has been relatively little weathering or sedimentation on the surface for tens of thousands of years, allowing meteorites to accumulate without being buried or destroyed. The dark colored meteorites can then be recognized among the very different looking limestone pebbles and rocks.

The Sahara and rising commercialization

In 1986-1987, a German team installing a network of seismic stations while prospecting for oil discovered ~65 meteorites on a flat, desert plain about 100 km SE of Dirj (Daraj), Libya. These were the first hint that vast numbers of meteorites could be found in certain parts of the Sahara. A few years later, an anonymous engineer who was a desert enthusiast saw photographs of meteorites being recovered in Antarctica, and thought he had seen similar occurrences on Jeep adventure tours he had organized in north Africa. In 1989, he returned to Algeria and recovered about 100 meteorites from at least 5 locations. Over the next four years, he and others who followed found at least 400 more meteorites at the same locations, plus new areas in Algeria and Libya. The find locations were generally in regions known as regs or hamadas, flat, featureless areas covered only by small pebbles and minor amounts of sandBischoff A. and Geiger T. (1995) Meteorites from the Sahara: find locations, shock classification, degree of weathering and pairing. Meteoritics 30, 113-122. ADS. Dark-colored meteorites can be easily spotted in these places, where they have been well-preserved due to the arid climate.

Although meteorites had been sold commercially and collected by hobbyists for many decades, up to the time of the Saharan finds of the late 1980s and early 1990s, most meteorites were deposited in or purchased by museums and similar institutions where they were exhibited and made available for scientific research. The sudden availability of large numbers of meteorites that could be found with relative ease in places that were readily accessible (especially compared to Antarctica), led to a rapid rise in commercial collection of meteorites. This process was accelerated when, in 1997, meteorites coming from both the Moon and Mars were found in Libya. By the late 1990s, private meteorite-collecting expeditions had been launched throughout the Sahara. Specimens of the meteorites recovered in this way are still deposited in research collections, but most of the material is sold to private collectors. These expeditions have now brought the total number of well-described meteorites found in Algeria and Libya to over 2000.

As word spread in Saharan countries about the growing profitibility of the meteorite trade, meteorite markets came into existence, especially in Morocco, fed by nomads and local people who combed the deserts looking for specimens to sell. Many thousands of meteorites have been distributed in this way, most of which lack any information about how, when, or where they were discovered. These are the so-called "Northwest Africa" meteorites.

Oman

In 1999, meteorite hunters discovered that the desert in southern and central Oman were also favorable for the collection of many specimens. The gravel plains in the Dhofar and Al Wusta regions of Oman, south of the sandy deserts of the Rub' al Khali, had yielded about 2000 meteorites as of mid-2006. Included among these are a large number of lunar and martian meteorites, making Oman a particularly important area both for scientists and collectors. Early expeditions to Oman were mainly done by commercial meteorite dealers, however international teams of Omani and European scientists have also now collected specimens.

Meteorites in history

One of the leading theories for the cause of the Cretaceous-teritary mass extinction that included the dinosaurs is a large meteorite impact. There has been a lively scientific debate as to whether other major extinctions, including the ones at the end of the Permian and Triassic periods might also have been the result of large impact events, but the evidence is much less compelling than for the end Cretaceous extinction.

The only reported fatality from meteorite impacts is an Egyptian dog who was killed in 1911, although this report is disputed. The meteorites that struck this area were identified in the 1980s as Martian in origin.

The first known modern case of a human hit by a space rock occurred on November 30 1954 in Sylacauga, Alabama. There a 4 kg stone chondrite Ann Hodges in her living room after it bounced off her radio. She was badly bruised. Several persons have since claimed [http://home.earthlink.net/~magellon/news1.html" target="_blank" >^* to have been struck by 'meteorites' but no verifiable meteorites have resulted. Indigenous peoples often prized iron-nickel meteorites as an easy, if limited, source of iron metal. For example, the Inuit used chips of the Cape York meteorite to form cutting edges for tools and spear tops.

Notable meteorites

Allan Hills 84001 - Mars meteorite that was claimed to prove the existence of life on Mars.
Canyon Diablo - Iron meteorite used by pre-historic Native Americans.
Cape York - One of the largest meteorites in the world.
Ensisheim - The oldest meteorite whose fall can be dated precisely (to November 7, 1492).
Heat Shield Rock - Found on Mars.
Hoba - The largest known meteorite.
Kaidun - Possibly from the martian moon Phobos.
Orgueil - Object of a 1965 hoax that involved embedding a seed within part of the meteorite.
Sayh al Uhaymir 169 - Originated from the moon; it fell to earth as a result of meteoroid strikes on the moon.
Sikhote-Alin - Massive iron meteorite impact event that occurred on February 12, 1947.
Willamette - The largest meteorite ever found in the United States.

Apart from meteorites fallen onto the Earth, "Heat Shield Rock" is a meteorite which was found on Mars, and two tiny fragments of asteroids were found among the samples collected on the Moon by Apollo 12 (1969) and Apollo 15 (1971) astronauts ^*.

Meteorite-related news

Soon after 2 am local time (00:00 GMT) on 7 June 2006: on a mountainside in Reisadalen in North Troms in Norway, a bolide was observed by several residents, possibly followed by an impact. There is a question as to how large it was, but an associated explosion was heard throughout the region. ^*
On 12 June 2006, NASA reported that two rocks dubbed "Allan Hills," and "Zhong Shan," found by the Spirit rover on Mars, might be iron meteorites. Unlike in the case of "Heat Shield Rock," there are not yet any supporting compositional data for these objects, so their identities as meteorites are less certain^*.

References

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