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Classification of Meteorites
Not until the early 19th cent. did scientists fully accept the fact that meteorites came to the earth from outer space. Since then many studies have been made of their composition and crystalline structure; the use of microchemical analysis, X rays, and the mass spectrograph has facilitated such work. The age of meteorites can be determined by measuring their radium and helium contents. Some meteorites might be fragments of comets; others, small asteroids whose orbital paths crossed that of the earth. Geochemical analysis has shown that more than 170 known meteorites are of lunar origin and more than 100 are of Martian origin. One of the Martian meteorites—known as ALH84001—is believed by some scientists to show evidence of there once having been primitive bacterial life on Mars, but most experts disagree with this conclusion. The lunar and Martian meteorites are thought to have been broken away from the moon and Mars by the impact of large asteroids.
Three general categories are used to classify meteorites. The siderites, or irons, are composed entirely of metal (chiefly nickel and iron). The aerolites, or stony meteorites, show a diversity of mineral elements including large percentages of silicon and magnesium oxides; the most abundant type of aerolite is the chondrite, so called because the metal embedded in it is in the form of grainlike lumps, or chondrules. The siderolites, which are rarer than the other types, are of both metal and stone in varying proportions.
As a meteor speeds through the atmosphere, its outer surface becomes liquefied; the friction of the atmosphere finally reduces its velocity (if the meteor is not large), and the surface cools and solidifies into a dark, smooth crust. Lines of flow in the hardened surface can indicate its motions in flight. Cone-shaped meteorites show that one end was directed forward. Others, which are unevenly shaped, probably spun while falling. The smallest meteorites, dust-sized particles known as micrometeorities, may pass through the atmosphere without heating up due to friction because of their very small mass.
Formation of Craters
See K. Mark, Metorite Craters (1995); O. R. Norton and D. S. Norton, Rocks from Space: Meteorites and Meteorite Hunters (2d ed. 1998).
meteorite(mee -tee-ŏ-rÿt) Interplanetary debris that falls to the Earth's surface. The first documented one was the stone that fell near Ensisheim, Alsace in 1492 but it was not until 1803 that meteorites were accepted by the scientific community as being extraterrestrial. One that is seen to hit the ground is known as a fall; one that is discovered accidentally some time after having fallen is termed a find. Roughly 6 falls and 10 finds are added to the list annually. About 3300 hit the Earth each year; the majority go unrecorded, falling in oceans, deserts, and other uninhabited regions.
Meteorites can be crudely divided into three types: stony meteorites, which subdivide into chondrites and achondrites, iron meteorites, and stony-iron meteorites. Iron meteorites have densities of about 7.8 g cm–3 and contain on average 91% iron, 8% nickel, and 0.6% cobalt. Stony meteorites have densities about 3.4 g cm–3 and on average contain 42% oxygen, 20.6% silicon, 15.8% magnesium, and 15.6% iron, no other element exceeding 2%. Stony-irons are intermediate in composition. The relative percentages of each type are shown in the table. The fall percentages give a reasonable approximation to the actual meteorite population of the Solar System. However the fragile carbonaceous chondrites mostly disintegrate on entry.
The largest iron meteorite that has been found is the 60-tonne Hoba meteorite, the largest stony meteorite being 1 tonne, part of the Norton County, Kansas achondrite. The found meteorite might represent only a small percentage of the in-space mass of the body. Meteorite entry is accompanied by a brilliant bolide, the meteorite usually being retarded to free-fall velocity at a height of about 20 km. It hits the ground at about 300 km per hour. The surface of the meteorite can easily be heated up to several thousand kelvin during entry. This usually produces a black glassy fusion crust, which covers the meteorite. Most meteorites fragment on entry, scattering pieces over an elliptical area: the carbonaceous chondrite meteorite Allende, which fell in 1969 in Mexico, scattered 5 tonnes of material over an area 48 km long by 7 km wide.
Meteorites are named after the nearest topographical feature to the fall point. Recognizing meteorite finds relies mainly on analysing chemically for the presence of nickel, which is present in all iron meteorites. Etched and polished irons also show Widmanstätten figures. Stony meteorites contain nickel-iron particles that can be extracted magnetically from a crushed sample.
Meteorites bring to our notice a type of rock existing in the Solar System but different from anything that occurs in the outer shell of the planet Earth. Studies of the chemical elements and compounds and the minerals present in meteorites allow their origin and history to be better understood; isotope ratios of given elements can give absolute ages. Most meteorites are believed to be fragments of asteroids. Two problems arise: firstly, meteorites seem to have had only a short exposure to cosmic rays; secondly, the dynamics by which a meteorite gets from the asteroid belt to the Earth's orbit is nontrivial, mainly relying on the orbital eccentricity being pumped up by gravitational perturbation when the semimajor axis approaches one of the Kirkwood gap resonances. The carbonaceous chondrites are thought to originate as parts of cometary nuclei. The ages of meteorites have been obtained by radiometric-dating methods and have been estimated at up to 4.7 × 109 years. This indicates that meteorites were formed at about the same time as the Solar System.
an iron or stone body that falls to the earth from interplanetary space; meteorites are the remnants of meteoroids that have not been completely destroyed in their passage through the atmosphere.
General information Meteorites are subdivided into three main classes: iron, stony-iron, and stony. However, a continuous gradation between classes is observed. Meteorites are characterized by an angular shape with smoothed out protrusions, a thin molten crust, and distinctive pits called regmaglypts. The interior of stony meteorites have an ashen gray color; less often the interior is black or nearly white. Numerous small inclusions of white nickel-iron and the bronze-yellow mineral troilite usually can be seen; fine dark gray veins are often present. Stony-iron meteorites contain much larger inclusions of nickel-iron. After polishing, the surface of iron meteorites takes on a mirror-like metallic luster. Occasionally there are falls of meteorites that have a more or less regular conical (so-called oriented) shape or a polyhedral shape resembling that of a crystal. These forms occur as a result of the action of the atmosphere (fragmentation and ablation) on the meteoroid during its passage through the atmosphere.
Meteorites are named after the town or geographic object nearest to their point of impact. Many meteorites are found by chance and are designated as finds; those that are observed during their descent are designated as falls.
Meteorites range in size from a few millimeters to a few meters, and the weight from fractions of a gram to tens of tons. The largest unbroken meteorite—the Hoba West iron meteorite, found in southwest Africa in 1920—weighs about 60 tons; the second largest—the Cape York iron meteorite, found in Greenland in 1818—weighs 34 tons. About 35 meteorites whose weight exceeds 1 ton are known.
As a result of the fragmentation of meteoroids, meteorites fall in groups, each consisting of tens, hundreds, or even thousands of individual meteorites. Such falling groups are called meteor showers, and each shower is counted as one meteorite. The Sikhote-Alin iron meteorite shower, with a total weight of about 70 tons, fell in Primor’e Krai, ‘USSR, on Feb. 12, 1947. A phenomenon, possibly caused by the fall and explosion of what is known as the Tunguska meteorite, was observed still earlier, on June 30, 1908, in central Siberia. Each year no fewer than 1,000 meteorites fall to the earth. However, many remain undetected because they fall into the seas and oceans or in sparsely inhabited areas. In the course of a year, only 12-15 meteorites find their way into various museums and scientific institutions throughout the world (see Table 1).
Up to Jan. 1, 1974, 146 meteorites (falls and finds) had been collected in the USSR.
|Table 1. Number of meteorites recorded as of Jan. 1, 19661|
|1 After M. Hey|
Phenomena accompanying falls of meteorites Descents of meteorites to the earth are accompanied by visual, acoustic, and mechanical phenomena. A bright fireball, called a bolide, accompanied by a tail and emitting sparks, races rapidly across the sky. A trail in the form of a streak of dust remains in the sky along the path of the bolide. The trail, initially rectilinear, quickly curves under the action of air currents, which move in different directions at different altitudes, and assumes a zigzag shape. At night, a bolide may illuminate the terrain for hundreds of square kilometers. Explosion-like sounds occur a few tens of seconds after the bolide disappears and are followed by a roaring sound, crackling, and a gradually fading rumble, which are caused by shock (ballistic) waves. Along the projection of the bolide trajectory on the surface of the earth, shock waves sometimes cause the ground and buildings to shake, windows to rattle and even crack, doors to open, and other effects.
The appearance of a bolide is caused by the entry into the earth’s atmosphere of a meteoroid whose velocity reaches 15 or more km/sec. Air resistance causes the meteoroid to decelerate, and its kinetic energy is transformed into heat and light. Consequently, the surface of the meteoroid and the air shell around it are heated to several thousand degrees. The matter of the meteoroid is brought to a boil, evaporates, and in the molten state is partially carried away by air currents and is dispersed into tiny droplets, which quickly solidify to form tiny balls of meteoric dust. The dust trail of the bolide is formed from the products resulting from this process, called ablation. A meteoroid becomes luminous at an altitude of about 130–80 km, and at 20–10 km its speed is usually greatly reduced (see Figure 1). In
this part of the path, called the arrest region, heating and vaporization of the meteoroid (or its fragments) cease, the bolide disappears, and the thin molten layer on the surface of the fragments rapidly solidifies, forming a fusion crust. Microscopic examination reveals the crust’s complex structure, reflecting the effects of exposure to the atmosphere; grooves, scattered drops, and a porous or slag structure of the crust are frequently observed. After passing through the arrest region, the dark fragments of the meteoroid, which are covered with a hardened crust, plummet down to the earth under the influence of gravity. While falling, they cool and, on reaching the ground, are either warm or hot, but not incandescent. Upon impact with the earth, the meteorite forms a depression, whose size and shape depend largely on the rate of fall. There are about 40 recorded cases of meteoritic impacts with buildings, but no significant damage occurred.
Chemical composition Meteorites contain no chemical elements that are unknown on the earth; nearly all the known elements have been found in them. The most abundant chemical elements in meteorites are Al, Fe, Ca, O, Si, Mg, Ni, and S. The chemical composition of certain meteorites may deviate significantly from the norm. For example, the Ni content in iron meteorites ranges from 5 to more than 30 percent. The average content of precious metals and rare elements (per 1 ton of meteoritic matter) is 10 g Ru, 5 g Rh, 10 g Pd, 5 g Ag, 3 gm Os, 5 g Ir, 20 g Pt, and 5 g Au. It has been established that the content of certain chemical elements is closely related to the content of other elements. Thus, for example, it has been shown that the higher the Ni content in a meteorite, the lower the Ga content. The isotopic composition of the many chemical elements found in meteorites has been shown to be identical with the isotopic composition of the same elements of terrestrial origin. The presence of radioactive chemical elements and their decay products in meteorites has made it possible to determine that the age of the matter that makes up meteorites is 4.5 billion years. In interplanetary space, meteorites are exposed to the action of cosmic rays, and stable and unstable cosmogonic isotopes are formed in them. The cosmic-ray exposure age of meteorites, that is, the duration of their independent existence, which varies from a few million years to hundreds of millions of years, has been determined from the content of cosmogonic isotopes. Measurement of these isotopes also makes it possible to determine the terrestrial age of an old fall, that is, the time interval that has elapsed since a particular meteorite impacted with the earth, which may be tens or hundreds of thousands of years.
The content of cosmogonic isotopes in meteorites and the presence of the tracks formed by high-energy particles make it possible to study variations in the intensity of cosmic rays in space and with time and to determine the original mass of meteorites (prior to striking the earth).
Mineral composition In contrast to the chemical composition, the mineral composition of meteorites is distinctive: a number of minerals that are unknown or that occur very rarely on the earth have been found in meteorites. These include schreiber-site, daubreelite, oldhamite, lawrencite, and merrillite, which are present in meteorites in small quantities. In recent years, several dozen new, previously unknown, minerals have been discovered in meteorites. Many of these have been named after meteoriticists, for example, farringtonite, ureyite, niningerite, and krinovite. The presence of these minerals indicates the unique conditions of formation of meteorites, conditions that differ from those under which terrestrial rocks were formed. The most common minerals found in meteorites are nickel-iron, olivine, pyroxenes—anhydrous silicates (enstatite, bronzite, hypersthene, diopside, augite), and, occasionally, plagioclase.
Some meteoritic minerals, for example, lawrencite, are highly unstable under terrestrial conditions and quickly combine with atmospheric oxygen. As a result, abundant oxidation products appear on meteorites in the form of rust spots; this leads to the disintegration of the meteorites. Crystalline cosmic water is present in some rare types of meteorites, and small grains of diamond are found in others, which are just as rare. The latter are a result of an impact metamorphism to which the meteorite has been subjected. Various gases that occur in different quantitative ratios have been identified in meteorites. The mineral composition of meteorites convincingly attests to a common origin of various types of meteorites.
Structure The surfaces of most iron meteorites, when polished and etched with a solution of nitric or some other acid, display complex patterns, called Widmanstatten figures. They consist of intersecting bands with bright narrow bordering strips. Polygonal fields are observed in some intermediate areas. Widmanstatten patterns appear as a result of the varying action of the etching solution on the surface of the meteorite. Nickel-iron consists of two mineral phases: kamacite, with a low nickel content, and taenite, with a high nickel content. Therefore, the bands, which consist of kamacite, are etched more strongly than the fields, which are filled with a fine mechanical mixture of kamacite and taenite grains. The narrow strips bordering the bands consist of taenite and do not submit at all to etching. The band-lamellae of kamacite in meteorites lie along the planes of an octahedron. Therefore, meteorites in which Widmanstatten patterns are observed are called octahedrites. Encountered less often are iron meteorites that consist entirely of kamacite and, on etching, reveal thin parallel lines, called Neumann lines. The internal microstructure of such meteorites shows a crystalline structure resembling that of a cube, or hexahedron. Therefore, these meteorites are called hexahedrites. Iron meteorites (ataxites) that do not display any pattern are encountered just as rarely; they contain the greatest amount of nickel.
Stony-iron meteorites (pallasites) resemble an iron sponge, whose cavities are filled with the transparent yellow-green mineral olivine. Another type of stony-iron meteorite, called mesosiderite, when broken, shows abundant inclusions of nickel-iron in the primary stony mass.
Stony meteorites are subdivided into two main groups: chondrites and achondrites. Chondrites are meteorites with distinctive globules, called chondrules, ranging in size from microscopic grains to the size of a pea. The chondrules are apparently rapidly solidified drops. Chondrites constitute about 85 percent of all falls of stony meteorites. Achondrites, a much rarer group, are meteorites that contain no chondrules.
Origin The most widespread opinion is that meteorites are fragments of asteroids. It has been established that meteoroids move in elliptical orbits similar to those of asteroids. A large number of small asteroids, with diameters much less than 1 km, constitute a group intermediate between asteroids and meteoroids. As a result of collisions among the moving small asteroids, there occurs a continual process of the fragmentation of small asteroids into smaller parts, which supplement the meteoroid content of interplanetary space. Meteorites constitute samples of solid matter of extraterrestrial origin that are accessible to direct study and that provide diverse information on the early stage of the formation of the solar system and on the system’s subsequent evolution. Thus, the study of meteorites, which reveals more and more new facts, has great cosmogonic significance. It is also important in the study of the deep regions of the earth.
Some researchers also classify tektites as meteorites; tektites are distinctive glassy bodies that are found in different localities on the earth’s surface. However, the conditions of the formation of and the nature of tektites distinguish tektites from meteorites.
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Mason, B. Meteority. Moscow, 1965. (Translated from English.) Wood, J. Meteority i proiskhozhdenie solnechnoi sistemy. Moscow, 1971. (Translated from English.)
Zavaritskii, A. N., and L. G. Kvasha. Meteority SSSR. Moscow, 1952.
Meteoritika, fascs. 1–30. Moscow, 1941–70. (Collection of articles.)
Heide, F. Kleine Meteoritenkunde. Berlin, 1957.
The Solar System, vol. 4. Edited by B. M. Middlehurst and G. P. Kuiper. New York, 1963.
Hey, M. N. Catalogue of Meteorites, 3rd ed. London, 1966.
E. L. KRINOV