Meteorite fall

Observations from meteorite falls suggest that organic compounds that may be prebiotic are available in a concentrated form within meteorite samples. Hot-water washes and other extraction processes show the production of many of the compounds required for life. Most importantly from the ALH84001 find is that material can be moved between planetary bodies and organic molecules can be transported... 

Iron meteorites offer the unique opportunity to examine metallic cores from deep within differentiated bodies. Most of these samples were exposed and dislodged when asteroids collided and fragmented. Although irons constitute only about 6% of meteorite falls, they are well represented in museum collections. Most iron meteorites show wide variations in siderophile-element abundances, which can be explained by processes like fractional crystallization in cores that mimic those in achondrites. However, some show perplexing chemical trends that may be inconsistent with their formation as asteroid cores. 

When a meteorite falls onto the Earth s surface, it becomes for the most part shielded from the effects of cosmic rays. The cosmic-ray-produced nuclides cease to form so the exposure clock stops. However, because many of the cosmic-ray produced nuclides are radioactive, several new clocks start that can be used to estimate the time that a meteorite has been on the Earth s surface. 

In the eyes of a distant observer using a fixed coordinate system, a meteorite falling in the gravitational field of the earth describes a parabolic path. An observer standing on earth uses the rotating frame of reference of the earth. For him, the complicated path of the falling meteorite simplifies to a straight vertical line. 

Meteorites fall into different classes, and the distinct classes have distinct oxygen isotopic compositions. The slope 1/2 mass-dependentchemical fractionation line... 

Cosmogenic radioactivity 22Na is created as a collision fragment in meteorites when cosmic rays strike the meteorite during its journey to the Earth. Atoms of Mg and Si are split into fragments, some of 22Na. When meteorites fall this radioactivity is counted in the lab, alive at the time of fall, and gives information on the meteorite s history in space. 

Solid bodies of extraterrestrial material that penetrate the atmosphere and reach the Earth s surface are called meteorites. Other extraterrestrial materials include micrometer-sized interplanetary dust particles (IDEs) collected in the lower stratosphere and polar ices. Most meteorites and IDEs are fragments of asteroids, but some IDEs may represent cometary material and some meteorites are fragments of the planets Mars and Earth s moon. Meteorites recovered following observed falls are called/a// those which cannot definitely be associated with observed falls are called finds. Meteorites are given names based on the location where they were recovered (e.g., the Allende meteorite fell in Allende, Mexico). Meteorites recovered in Antarctica and the deserts of Australia and northern Africa are given names and numbers, because numerous samples are found in the same locations. Fragments thought to be of the same meteorite fall, which, in Antarctica or hot deserts, may have different numbers or even names because they were found in different locations, are called... 

Since ordinary chondrites account for —80% of all meteorite falls (Table 1), it was once believed that their parent bodies were common in the main asteroid belt. However, spectral studies show that most asteroids are dark and featureless (C and related types), and most of the brighter S type differ from ordinary chondrites. H group chondrites probably come from one or more S-type asteroids, possibly 6 Hebe (Burbine et al, 2002). Ordinary chondrites are probably rare in the main part of the asteroid belt (Meibom and Clark, 1999), though they may account for —20% of the near-Earth objects (Binzel et al, 2002). A few ordinary chondrites including Tieschitz (Figure 1(a)) do not fit comfortably into the H, L, and LL groups and may be derived from separate bodies. 

Iron and stony-iron meteorites constitute —6% of meteorite falls (Grady, 2000). Despite their scarcity among falls, iron meteorites are our only samples of 75 of the 135 asteroids from which meteorites originate (Keil et ai, 1994 Scott, 1979 Meibom and Clark, 1999 see also Chapter 1.05), suggesting that both differentiated asteroids and the geologic processes that produced them were common. 

Inspection of Equation (2) shows that in the limit of CRE ages long compared to 1/X, the concentration of a cosmogenic radionuclide approaches the constant value P/X. Hence, in this case, the activity at the time of a meteorite fall (or the collection of a lunar sample) is equal to the production rate. Once the production rate is known for one meteorite, one may analyze for the same radionuclide in a second meteorite with, perhaps, a shorter CRE age. If P was the same in the second meteorite (an important assumption), then we can solve Equation (2) for the CRE age. In a kind of bootstrapping process, measurements of other cosmogenic nuclides in the second meteorite then can serve as the basis for calculating their production rates, and so on. 

Rubin A. E., Kallemeyn G. W., Wasson J. T., Clayton R. N., Mayeda T. K., Grady M., Verchovsky A. B., Eugster O., and Lorenzetti S. (2003) Formation of metal and silicate globules in Gujba a new Bencubbin-like meteorite fall from Nigeria. Geochim. Cosmochim. Acta (in press). 

Meteorites are superficially described as being either falls or finds. A meteorite fall is scientifically more useful than a find because the exact time that it hit Earth s surface is known. Einds, on the other hand, are simply that—meteorites that have been found by chance. The largest meteorite find fo date is fhaf of the 66-ton (60-metric ton) Hoba meteorite in South Airica. Meteorites are either named after the recific geographic location in which they fall, or after the nearest postal station to the site of the fall. 

Even though many thousands of meteorites fall to Earth each year it is rare to hear of one hitting a human beir. The chances of a human fatality resulting from the fall of a meteorite have been calculated as one death. 

Meteorites are generally classified into two categories falls and finds. A meteoritic fall occurs when an observer actually sees a meteorite fall to Earth and is able to track and recover the meteorite. The term meteoritic find is used to describe a meteorite that has been found on Earth s surface, although there is no evidence as to when it fell to Earth. In one summary of meteorites, reported in The Handbook of Iron Meteorites, 55 percent of all discoveries were falls and 45 percent finds. 

Another special class of particles is meteoritic dust of cosmic origin. Smaller meteoritic particles (r < 1 m) can reach the lower layers of the atmosphere without significant modifications. However, larger meteorites falling through the atmosphere partly or totally evaporate due to frictional heating. In the troposphere, spherical droplets from the condensation of the resulting vapour can be identified (e.g. Wirth and Prodi, 1972). Pettersson estimates (see Cadle, 1973) that 14 x 10f t of meteoritic materials are collected annually by the atmosphere of our planet. 

Table I. Numbers of Classifled Non-Antarctic Meteorite Falls and Finds, Including Those from Hot and Cold Deserts...

Besides long gas retention ages, three lines of evidence link all undifferentiated and most differentiated meteorites to asteroids mineralogy spectral reflectance and the orbits of nine meteorite falls. 

Stony meteorites are most abundant and make up 94.2% by number of the so-called falls (i.e., meteorites that were collected soon after they fell). Irons are rare at only 4.5% and stony irons are even less common at 1.2%. In the modem classification in Table 18.1 all meteorites (falls only) are subdivided into ... 

The evidence that stony meteorites collected in Antarctica are weathered implies that certain chemical elements are mobilized within the affected meteorite specimens. In addition, glacial meltwater and atmospheric carbon dioxide invade the affected meteorite specimens together with halogens, sulfur-bearing componnds, and organic molecules. Therefore, meteorites that fell on the East Antarctic ice sheet are altered mineralogically as well as chemically and, for that reason, their trace-element concentrations may differ from those of non-Antarctic meteorite falls. 

Just when you think all meteorites are the same, one comes along with purple salt in it. A group of seven boys playing basketball saw a meteorite fall near Monahans, Texas, in 1998. The rock was brought to Johnson Space Center and was broken open... 

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