|
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 finds. Meteorites are always named for the place where they were found , 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
Meteorite types
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 1959. 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 orbit. Another program in Canada, the Meteorite Observation and Recovery Project, ran from 1971 to 1985. It too recovered a single meteorite, Innisfree, in 1977. 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 2002. Finds
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 . In the late 1960s, Roosevelt County, New Mexico in the Great Plains was found to be a particularly good place to find meteorites. 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 surface. 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). 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 more, ~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 sand. 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. A famous case is the alleged Chinguetti meteorite, a find reputed to come from a large unconfirmed 'iron mountain' in Africa. 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 crashed through a roof and hit Ann Hodges in her living room after it bounced off her radio. She was badly bruised. Several persons have since claimed * 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 Meteorite-related news See also | |||||||||||||
|
| ||||||||||||||
 |
|
| |