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A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids. When it enters the atmosphere, impact pressure causes the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting/falling 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 a natural object that has come from elsewhere in space. Meteorites have been found on the Moon[1][2] and Mars.[3]
Meteorites that are recovered after being observed as they transited the atmosphere or impacted the Earth are called falls. All other meteorites are known as finds. As of February 2010, there are approximately 1,086 witnessed falls having specimens in the world's collections. In contrast, there are over 38,660 well-documented meteorite finds.[4]
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.
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Meteorites are always named for the place where they were found,[5] 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)). Some meteorites have informal nicknames: the Sylacauga meteorite is sometimes called the "Hodges meteorite" after Ann Hodges, the woman who was struck by it; the Canyon Diablo meteorite, which formed Meteor Crater has dozens of these aliases. However, the single, official name designated by the Meteoritical Society is used by scientists, catalogers, and most collectors.
Most meteoroids disintegrate when entering 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 large impact craters. Instead, they typically arrive at the surface at their terminal velocity and, at most, create a small pit. Even so, falling meteorites have reportedly caused damage to property, livestock and people.
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.[6][7] 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 craters.[8] Although such disruption events are uncommon, they can cause a considerable concussion to occur; the famed Tunguska event probably 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. 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 2006.[9])
Several phenomena are well-documented during witnessed meteorite falls too small to produce hypervelocity craters.[10] 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 atmospheric entry, 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 tumbling, it may develop a conical "nose cone" or "heat shield" 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. Reports, however, vary greatly, with some meteorites being reported as "burning hot to the touch" upon landing,[11][12] and others forming a frost upon their surface.[13]
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 most cases, the largest meteorites in a shower are found farthest down-range in the strewn field.
Most meteorites are stony meteorites, classed as chondrites and achondrites. Only 6% of meteorides are iron meteorites or a blend of rock and metal, the stony-iron meteorites. Modern classification of meteorites is complex, the review paper of Krot et al. (2007)[14] summarizes modern meteorite taxonomy.
About 86% of the meteorites that fall on Earth are chondrites,[4][15][16] 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. Certain types of 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 (meaning they do not contain chondrules), some of which are 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 (the HED meteorites) 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. Due to the low abundance of irons in collection areas such as Antarctica, where most of the meteoric material that has fallen can be recovered, it is possible that the actual percentage of iron-meteorite falls is lower than 5%.
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.
Tektites (from Greek tektos, molten) are not themselves meteorites, but are rather natural glass objects up to a few centimeters in size which were formed—according to most scientists—by the impacts of large meteorites on Earth's surface. A few researchers have favored Tektites originating from the Moon as volcanic ejecta, but this theory has lost much of its support over the last few decades.
Most meteorite falls are recovered on the basis of eye-witness accounts of the fireball or the actual impact of the object on the ground, or both. Therefore, despite the fact that meteorites actually fall with virtually equal probability everywhere on Earth, verified meteorite falls tend to be concentrated in areas with high human population densities such as Europe, Japan, and northern India.
A small number of meteorite falls have been observed with automated cameras and recovered following calculation of the impact point. The first of these was the Příbram meteorite, which fell in Czechoslovakia (now the Czech Republic) in 1959.[17] 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.[18] Another program in Canada, the Meteorite Observation and Recovery Project, ran from 1971 to 1985. It too recovered a single meteorite, Innisfree, in 1977.[19] 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.[20]
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 Harvey H. Nininger that meteorites are much more common on the surface of the Earth than was previously thought.
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.[21]
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.[22]
A few meteorites were found in Antarctica between 1912 and 1964. 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.
The Antarctic Scientific Exploration of China has conducted successful meteorite searches since 2000. A Korean program (KOREAMET) was launched in 2007 and has collected a few meteorites.[23] The combined efforts of all of these expeditions have produced more than 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).[24]
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 deserts 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[25], ~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.
In 1986-87, a German team installing a network of seismic stations while prospecting for oil discovered about 65 meteorites on a flat, desert plain about 100 km southeast of Dirj (Daraj), Libya. A few years later, a desert enthusiast saw photographs of meteorites being recovered by scientists in Antarctica, and thought that he had seen similar occurrences in northern Africa. In 1989, he recovered about 100 meteorites from several distinct locations in Libya and Algeria. Over the next several years, he and others who followed found at least 400 more meteorites. 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.[26] Dark-colored meteorites can be easily spotted in these places, where they have also been well-preserved due to the arid climate, and in the case of the Dal al Gani meteorite field, favorable geology consisting of basic rocks (clays, dolomites, and limestones) and lacking erosive quartz sand[27].
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 profitability 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.
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 5,000 meteorites as of mid-2009. 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.
The recovery of meteorites from Oman is currently prohibited by national law, but a number of international hunters continue to remove specimens now deemed "national treasures." This new law provoked a small international incident, as its implementation actually preceded any public notification of such a law, resulting in the prolonged imprisonment of a large group of meteorite hunters primarily from Russia, but whose party also consisted of members from the U.S. as well as several other European countries.
The Black Stone in the wall of the Kaaba in Mecca is thought to be a meteorite by some secular historians, but there is little support for this in the scientific literature [28]
Beginning in the mid-1990s, amateur meteorite hunters began scouring the arid areas of the southwestern United States. To date, meteorites numbering possibly into the thousands have been recovered from the Mojave, Sonoran, Great Basin, and Chihuahuan Deserts, with many being recovered on dry lake beds. Significant finds include the Superior Valley 014 Acapulcoite, one of two of its type found within the United States[29][30] as well as the Blue Eagle meteorite, the first Rumuruti-type chondrite yet found in the Americas.[31] Perhaps the most notable find in recent years has been the Los Angeles meteorite, a martian meteorite that was discovered by Robert Verish somewhere in the Mojave desert, only to be recognized years later in a pile of rocks in his back yard.[32] A number of finds from the American Southwest have yet to be formally submitted to the Meteorite Nomenclature Committee, as many finders think it is unwise to publicly state the coordinates of their discoveries for fear of confiscation by the federal government, and of 'poaching' by other hunters at known find sites.[33] Several of the meteorites found recently are currently on display in the Griffith Observatory in Los Angeles.
The German physicist, Ernst Florens Chladni, was the first to publish the then audacious idea that that meteorites were actually rocks from space. He published his booklet, "On the Origin of the Pallas Iron and Others Similar to it, and on Some Associated Natural Phenomena", in 1794. In this he compiled all available data on several meteorite finds and falls concluded that they must have their origins in outer space. The scientific community of the time responded with resistance and mockery.[34] It took nearly 10 years before a general acceptance of the origin of meteorites was achieved through the work of the French scientist Jean-Baptiste Biot and the British chemist, Edward Howard.
One of the leading theories for the cause of the Cretaceous–Tertiary extinction event that included the dinosaurs is a large meteorite impact. The Chicxulub Crater has been identified as the site of this 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.
There are several reported instances of falling meteorites having killed both people and livestock, but a few of these appear more credible than others. The most infamous reported fatality from a meteorite impact is that of an Egyptian dog that was killed in 1911, although this report is highly disputed. This particular meteorite fall was identified in the 1980s as Martian in origin. However, there is substantial evidence that the meteorite known as Valera hit and killed a cow upon impact, nearly dividing the animal in two, and similar unsubstantiated reports of a horse being struck and killed by a stone of the New Concord fall also abound. Throughout history, many first and second-hand reports of meteorites falling on and killing both humans and other animals abound, but none have been well documented.
The first known modern case of a human hit by a space rock occurred on 30 November 1954 in Sylacauga, Alabama.[35] There a 4 kg stone chondrite[36] crashed through a roof and hit Ann Hodges in her living room after it bounced off her radio. She was badly bruised. The Hodges meteorite, or Sylacauga meteorite, is currently on exhibit at the Alabama Museum of Natural History.
Other than the Sylacauga event, the most plausible of these claims was put forth by a young boy who stated that he had been hit by a small (~3 gram) stone of the Mbale meteorite fall from Uganda, and who stood to gain nothing from this assertion. The stone reportedly fell through a number of banana leaves before striking the boy on the head, causing little to no pain, as it was small enough to have been slowed by both friction with the atmosphere as well as that with banana leaves, before striking the boy. Although it is impossible to prove this claim either way, it seems as though he had little reason to lie about such an event occurring.
Several persons have since claimed[37] 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 tips.
Meteorite falls may also be the source of cultish worship. The cult in the Temple of Artemis (Diana) at Ephesus, one of the Seven Wonders of the Ancient World possibly originated with the observation of a meteorite fall which was understood by contemporaries to have fallen to the earth from Zeus, the principal Greek deity.
Some Native Americans treated meteorites as ceremonial objects. In 1915, a 135-pound iron meteorite was found in a Sinagua (c.1100-1200 AD) burial cyst near Camp Verde, Arizona, respectfully wrapped in a feather cloth.[38] A small pallasite was found in a pottery jar in an old burial found at Pojoaque Pueblo, New Mexico. Nininger reports several other such instances, in the Southwest US and elsewhere, such as the discovery of Native American beads of meteoric iron found in Hopewell burial mounds, and the discovery of the Winona meteorite in a Native American stone-walled crypt.[38]
In the 1970s a stone meteorite was uncovered during an archaeological dig at Danebury Iron Age hillfort, Danebury England. It was found deposited part way down in an Iron Age pit. Since it must have been deliberately placed there, this could indicate one of the first (known) human finds of a meteorite in Europe.
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.[41]
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METEORITE, a mass of mineral matter which has reached the earth's surface from outer space. Observation teaches that the fall of a meteorite is often preceded by the flight of a fireball (see Meteor) through the sky, and by one or more loud detonations. It was inferred by Chladni (1794) that the fireball and the detonations result from the quick passage of the meteorite through the earth's atmosphere.
The fall of stones from the sky, though not credited by scientific men till the end of the 18th century, had been again and again placed on record. One of the most famous of meteorites fell in Phrygia and was worshipped there for many generations under the name of Cybele, the mother of the gods. After an oracle had declared that possession of the stone would secure to the Romans a continual increase of prosperity, it was demanded by them from King Attalus about the year 204 B.C., and taken with great ceremony to Rome. It is described by the historian as "a black stone, in the figure of a cone, circular below and ending in an apex above." Plutarch relates the fall of a stone in Thrace about 470 B.C., during the time of Pindar, and according to Pliny the stone was still preserved in his day, 500 years afterwards. Both Diana of the Ephesians "which fell down from Jupiter," and the image of Venus at Cyprus, appear to have been conical or pyramidal stones. One of the holiest relics of the Moslems is preserved at Mecca, built into a corner of the Kaaba; its history goes back far beyond the 7th century; the description of it given to Dr Partsch suggests that the stone had fallen from the sky. The oldest existing meteorite of which the fall is known to have been observed is that which fell at Ensisheim in Elsass on the 10th of November 1492. It was seen to strike the ground and was immediately dug out; it had penetrated to a depth of 5 ft. and was found to weigh 260 lb. It was long suspended by a chain from the roof of the parish church, and is now kept in the Rathhaus of the town.
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Place. |
Date. |
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In England. |
Wold Cottage, Thwing, York- shire . |
Dec. 13, 1795. |
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Launton, Oxfordshire |
Feb. 15, 1830. |
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Aldsworth, Gloucestershire |
Aug. 4, 1835. |
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Rowton, Shropshire |
April 20, 1876. |
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Middlesbrough, Yorkshire . |
March 4, 1881. |
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In Scotland. |
High Possil, Glasgow. . |
April 5, 1804. |
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Perth. . |
May 17, 1830. |
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In Ireland. |
Mooresfort, Tipperary. . |
Aug. 1810. |
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Adare, Limerick. . |
Sept. io, 1813. |
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Killeter, Tyrone. . |
April 29, 1844. |
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Dundrum, Tipperary . |
Aug. 12, 1865. |
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Crumlin, Antrim . |
Sept. 13, 1902. |
It was not till scientific men gave credence to the reports of the fall of heavy bodies from the sky that steps were taken for the formation of meteorite collections. The British Museum (Natural History) at South Kensington now contains specimens belonging to 566 distinct falls; of these falls 325 have been actually observed; the remaining specimens are inferred to have come from outer space, because their characters are similar to those of the masses which have been seen to fall. Of these meteorites the following twelve have fallen within the British Isles: - Meteoritic falls are independent of thunderstorms and all other terrestrial circumstances; they occur at all hours of the day and night, and at all seasons of the year; they favour no particular latitudes. The number of stones which reach the ground from one fireball is very variable. In each of the two Yorkshire falls only one stone was found; the Guernsey County meteor yielded 30; at Toulouse, as many as 350 are estimated to have fallen; at Hessle, over Soo; at Knyahinya, more than z000; at L'Aigle, from z000 to 2000; at both Pultusk and Mocs no fewer than ioo,000 are estimated to have reached the earth's surface. The largest single mass seen to fall is one of those which came down at Knyahinya, Hungary, in 1866, and weighed 547 lb; but far larger masses, inferred from their characters to be meteorites, have been met with. The larger of the Cranbourne masses, now in the British Museum (Natural History), before rusting weighed 32 tons; the largest of the masses brought by Lieut. Peary from western Greenland weighs 362 tons. A mass found at Bacubirito in Mexico is 13 ft. long, 6 ft. wide and 5 ft. thick, and is estimated to weigh so tons.
From observations of the path and time of flight of the luminous meteor it is calculated that meteorites enter the earth's atmosphere with absolute velocities ranging from io to 45 m. a second; but the speed of a meteorite after the whole of the resisting atmosphere has been traversed is extremely small and comparable with that of an ordinary falling body. According to Professor A. S. Herschel's experiments, the meteorite which fell at Middlesbrough must have struck the ground with a velocity of only 412 ft. a second. In the case of the Hessle fall, several stones fell on the ice, which was only a few inches thick, and rebounded without breaking the ice or being broken themselves. The depth to which a meteorite penetrates depends on the speed, form, weight and density of the meteorite and on the nature of the ground. At Stannern a meteoric stone weighing 2 lb entered to a depth of only 4 in.; the large Knyahinya stone already mentioned made a hole 11 ft. deep.
The area of the earth's surface occupied by towns and villages being comparatively small, the probability of a shower of stones falling within a town is extremely minute; the likelihood of a living creature being struck is still more remote. The first Yorkshire stone, that of Wold Cottage, struck the ground only z o yds. from a labourer; the second, that of Middlesbrough, fell on the railroad only 40 yds. away from some platelayers at work; a stone completely buried itself in the highway at Kaba; one fell between two carters on the road at Charsonville, throwing the ground up to a height of 6 ft.; the Tourinnes-la-Grosse meteorite broke the pavement and was broken itself; the Krahenberg stone fell within a few paces of a little girl; the Angers stone fell close to a lady standing in her garden; the Braunau mass went through the roof of a cottage; at Macao, in Brazil, where there was a shower of stones, some oxen are said to have been killed; at Nedagolla, in India, a man was so near that he was stunned by the shock; while at Mhow, also in India, a man was killed in 1827 by a stone which is a true meteorite, and is represented by fragments in museum collections.
Though the surface of a meteoric stone becomes very hot during the early part of the flight through the air, it is cooled again during the later and slower part of the flight. Meteorites are generally found to be warm to the touch if immediately dug out; at the moment of their impact they are not hot enough to char woody fibre on which they chance to fall, nor is the surface then soft, for terrestrial matter with which the surface comes into contact makes no impression upon the meteorite. Where many stones fall at the same time they are generally distributed over a large area elongated in the direction of the flight of the luminous meteor, and the largest stones generally travel farthest. At Hessle, for instance, the stones were distributed over an area of 10 m. long and 3 m. broad.
Meteorites are almost invariably found to be completely covered with a thin crust such as would be caused by intense heating of the material for a short time; its thinness shows the slight depth to which the heat has had time to penetrate. They are presumably cold and invisible when they enter the earth's atmosphere, and become heated and visible during their passage through the air; doubtless the greater part of the superficial material flicks off as the result of the sudden heating and is left behind floating in the air as the trail of the meteor. The crust varies in aspect with the mineral composition of the meteorite; it is generally black; it is in most cases dull but is sometimes lustrous; more rarely it is dark-grey in colour. Each stone of a shower is in general completely covered with crust; but occasionally, as in the case of the Butsura fall, stones found some miles apart fit each other closely and the fitting surfaces are uncrusted, showing that a meteorite may break up during a late and cool stage of the flight through the atmosphere. A meteorite is generally covered with pittings which have been compared in size and form to thumbmarks; the pittings are probably caused by the unequal conductivity, fusibility and frangibility of the superficial material. As picked up, complete and covered with crust, meteorites are always irregularly-shaped fragments, such as would be obtained on breaking up a rock presenting no regularity of structure.
About one-third, and those the most common, of the chemical elements at present recognized as constituents of the earth's crust have been met with in meteorites; no new chemical element has been discovered. The most frequent or plentiful in their occurrence are: aluminium, calcium, carbon, iron, magnesium, nickel, oxygen, phosphorus, silicon and sulphur; while less frequently or in smaller quantities are found antimony, arsenic, chlorine, chromium, cobalt, copper, hydrogen, lithium, manganese, nitrogen, potassium, sodium, strontium, tin, titanium, vanadium. The existence of minute traces of several other elements has been announced; of these special mention may be made of gallium, gold, iridium, lead, platinum and silver. Iron occurs chiefly in combination with nickel, and phosphorus almost always in combination with both nickel and iron (schreibersite); carbon occurs both as indistinctly crystallized diamond and as graphitic carbon, the latter generally being amorphous, but occasionally having the forms of cubic crystals (cliftonite); free phosphorus has been found in one meteorite; free sulphur has also been observed, but may have resulted from the decomposition of a sulphide since the fall of the stone.
Of the mineral constituents of meteorites, the following are by many mineralogists regarded as still unrepresented among native terrestrial products: cliftonite, a cubic form of graphitic carbon; phosphorus; various alloys of nickel and iron; moissanite, silicide of carbon; cohenite, carbide of iron and nickel (corresponding to cementite, carbide of iron, found in artificial iron); schreibersite, phosphide of iron and nickel; troilite, protosulphide of iron; oldhamite, sulphide of calcium: osbornite, oxysulphide of calcium and titanium or zirconium; daubreelite, sulphide of iron and chromium; lawrencite, protochloride of iron; asmanite, a species of silica; maskelynite, a singly refractive mineral with the chemical composition of labradorite; weinbergerite, a silicate intermediate in chemical composition to pyroxene and nepheline.
Of these troilite is perhaps identical with some varieties of terrestrial pyrrhotite; asmanite has characters which approach very closely to those of terrestrial tridymite; maskelynite, according to one view, is the result of fusion of labradorite, according to another view, is an independent species chemically related to leucite. Other compounds are present corresponding to the following terrestrial minerals: olivine and forsterite; enstatite and bronzite; diopside and augite; anorthite, labradorite and oligoclase; magnetite and chromite; pyrites; pyrrhotite; breunnerite. Quartz (silica), the most common of terrestrial minerals, is absent from the stony meteorites; but from the Toluca meteoric iron microscopic crystals have been obtained of which some have certain resemblances to quartz, and others to zircon. Free silica is present in the Breitenbach meteorite but as asmanite. In addition to the above there are several compounds or mixtures of which the nature has not yet been satisfactorily ascertained.
Meteorites are conveniently distributed into three classes,. which pass more or less gradually into each other: the first (siderites or meteoric irons) includes all those which consist mainly of metallic iron alloyed with nickel; only nine of them.. have been actually seen to fall; the second (siderolites) includes those in which metallic iron (alloyed with nickel) and stony matter are present in large proportion; few of them have been seen to fall; those of the third class (aerolites or meteoric stones) consist almost entirely of stony matter; nearly all have been seen to fall.
In the meteoric irons the iron generally varies from 80 to 95% and the nickel from 6 to 10%; the latter is generally alloyed with the iron, and several alloys or mixtures have been distinguished by special names (kamacite, taenite, plessite). Troilite is frequently present as plates, veins or large nodules, sometimes surrounded by graphite; schreibersite is almost always present, and occasionally also daubreelite. The compositeness and the structure of meteoric iron are well shown by the figures generally called into existence when a polished surface is etched. by means of acids or bromine-water; they are due to the inequality of the etching action on thick and thin plates of various constituents, the plates being composed chiefly of two nickel-iron materials (kamacite and taenite). A third nickel-iron material (plessite) fills up the spaces formed by the intersection of the joint plates of kamacite and taenite; it is probably not an independent substance but an intimate intergrowth of kamacite and taenite. The figures were first observed in 1808 and are generally termed "Widmanstatten figures" in honour of their discoverer; the plates which give rise to them are parallel to the faces of the regular octahedron, and such masses have therefore an octahedral structure. A small number of the remaining masses have cubic cleavage; instead of Widmanstatten. figures they yield fine linear furrows when etched; the furrows were found by Neumann in 1848 to have directions such as would result from twinning of the cube about an octahedral. face; they are known as "Neumann lines." For meteoric irons of cubic structure the percentage of nickel is lower than 6 or 7; for those of octahedral structure it is higher than 6 or 7; the plates of kamacite are thinner, and the structure therefore finer the higher the percentage of that metal. A considerable number of meteoric irons, however, show no crystalline structure at all, and have percentages of nickel both below and above 7; it has been suggested that each of these masses may once have had crystalline structure and that it has disappeared as a result of prolonged heating throughout the mass while the meteorite has been passing near a star.
An investigation of the changes of the magnetic permeability of the Sacramento meteoric iron with changing temperature led Dr S. W. J. Smith to infer that the magnetic behaviour can only be explained by imagining the meteorite to consist largely of plates of nickel-iron containing about 7% of nickel (kamacite), separated from each other by thin plates of a nickel-iron constituent (taenite), containing about 27% of nickel and having different thermomagnetic characters from those of kamacite; he suggests, however, that taenite is not a definite chemical compound but a eutectic mixture of kamacite and a nickel-iron compound containing not less than 37% of nickel.
About eleven out of every twelve of the known meteoric stones belong to a division to which Rose gave the name "chondritic" (xovdpos, a grain); they present a very fine-grained but crystalline matrix or paste, consisting of olivine and enstatite or bronzite, with more or less nickel-iron, troilite, chromite, augite and triclinic feldspar; through this paste are disseminated round chondrules of various sizes and generally with the same mineral composition as the matrix; in some cases the chondrules consist wholly or in great part of glass. Some meteorites consist almost solely of chondrules; others contain only few; in some cases the chondrules are easily separable from the surrounding material. In mineral composition chondritic meteorites approximate more or less to terrestrial lherzolites.
A few meteorites belonging to the chondritic division are remarkable as containing carbon in combination with hydrogen and oxygen; those of Alais and Cold Bokkeveld are good examples.
The remaining meteoric stones are without chondrules and contain little or no nickel-iron; of these the following may be mentioned as illustrative of the varieties of mineral composition: Juvinas, consisting essentially of anorthite and augite; Petersburg, of anorthite, augite and olivine, with a little chromite and nickel-iron (both Juvinas and Petersburg may be compared to terrestrial basalt); Sherghotty, chiefly of augite and maskelynite; Angra dos Reis, almost wholly of augite, but olivine is present in small proportion; Bustee, of diopside, enstatite and a little triclinic feldspar, with some nickeliron, oldhamite and osbornite; Bishopville, of enstatite and triclinic feldspar, with occasional augite, nickel-iron, troilite and chromite; Roda, of olivine and bronzite; and Chassigny, consisting of olivine with enclosed chromite, and thus mineralogically identical with terrestrial dunite.
Almost all meteoric stones appear to be made up of irregular angular fragments, and some of them bear a close resemblance to volcanic tuffs. In the large group of chondritic stones, chondrules or spherules, some of which can only be seen under the microscope while others reach the size of a walnut, are embedded in a matrix apparently made up of minute splinters such as might result from the fracture of the chondrules themselves. In fact, until recently it was thought by some mineralogists that the chondrules owe their form, not to crystallization, but to friction, and that the matrix was actually produced by the wearing down of the chondrules through frequent collision with each other as oscillating components of a comet or during repeated ejection from a volcanic vent of some small celestial. body. Chondrules have been observed, however, presenting forms and crystalline surfaces incompatible with such a mode of formation, and others have been described which exhibit features resulting from mutual interference during their growth. The chondritic structure is different from anything which has yet been observed in terrestrial rocks, and the chondrules are distinct in character from those observed in perlite and obsidian. It is now generally believed that the structural features of meteoric stones are the result of hurried crystallization.
No organized matter has been found in meteorites and they have brought us, therefore, no evidence of the existence of living beings outside our own world.
Authorities. - The literature consists chiefly of memoirs dispersed through the journals of scientific societies. The following separate works may be consulted: A. Brezina, Die MeteoritenSammlung d. k-k. min. Hofkabinetes in Wien (Vienna, 1896); A. Brezina u. E. Cohen, Die Structur and die Zusammensetzung der Meteoriten (Stuttgart, 1886 - 1887); P. S. Bigot de Morogues, Memoire historique et physique sur les chutes des pierres (Orleans, 1812); Chladni, Ueber den Ursprung der von Pallas gefundenen and anderer ihr dhnlicher Eisenmassen (Riga, 1794), and Ueber Feuer-Meteore, and fiber die mit denselben herabgefallenen Massen (Vienna, 1819); E. Cohen, Meteoritenkunde (Stuttgart, 1894-1905); L. Fletcher, An Introduction to the Study of Meteorites, Toth ed. (London, 1908); E. King, Remarks concerning Stones said to have fallen from the Clouds both in these Days and in Ancient Times (London, 1796); S. Meunier, Meteorites (Paris, 1884); C. Rammelsberg, Die chemische Natur der Meteoriten (Berlin, 1870-1879); G. Rose, Beschreibung and Eintheilung der Meteoriten (Berlin, 1864); G. Tschermak, Die mikroskopische Beschaffenheit der Meteoriten (Stuttgart, 1883 - 1885); E. A. Wolfing, Die Meteoriten in Sammlungen and ihre Literatur (Tubingen, 1 8 97) (L. F.)
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