Meteorites

1. Recovery of meteorites

Meteorites traditionally are given the name of a geographic feature associated with the location where they are found. Until quite recently, there were no systematic efforts to recover them. This was largely because meteorites fall more or less uniformly over Earth’s surface and because there was no obvious way to predict where they would fall or could be found. When a meteorite was seen to fall or when a person chanced upon an unusual-looking rock, the specimen was simply taken to a museum or a private collector. In the 1930s and ’40s, enterprising meteorite collectors began crisscrossing the prairie regions of North America, asking farmers to bring them unusual rocks that they had found while plowing their fields. Prairie soil is largely derived from fine glacial loess and contains few large rocks. The collectors realized that there was a reasonable chance that any rocks the farmers unearthed would include meteorites. A better approach to finding meteorites than searching places with few rocks, however, is to search places where they can accumulate over time—i.e., where the surface is quite old and rates of weathering are low. Because meteorites contain minerals, such as iron metal, that are easily weathered, they do not normally last long on Earth’s surface. Liquid water is one of the principal agents of weathering. In desert environments, where there is little water, meteorites survive much longer. Indeed, they tend to accumulate on the surface in arid regions if weathering rates are slower than the rates at which meteorites fall to Earth, provided that little windblown sand accumulates to bury them. Areas of the Sahara in North Africa and the Nullarbor Plain region in Australia have proved to be good places to look for meteorites. The most-successful collection efforts, however, have been in Antarctica. The Antarctic can be viewed as a cold desert. Annual snowfall is quite low over most of the interior, and the intense cold slows weathering rates considerably. Most meteorites that fall on the ice sheet become buried and are stored for 20,000–30,000 years, although some appear to have been in Antarctica for a million years or more. The ice of the Antarctic sheet gradually flows radially from the South Pole northward toward the coast. In places, the ice encounters an obstruction, such as a buried hill, that forces it to flow upward. Strong katabatic winds, which sweep down the gently sloping ice sheets from the centre of the continent, sandblast the upwelling ice with snow and ice particles, eroding it at rates as high as 5–10 cm (2–4 inches) per year and leaving the meteorites stranded on the surface. Areas of upwelling ice, called blue ice for its colour, can be recognized from aerial or satellite photographs, and on foot the dark meteorites are relatively easy to spot against the ice and snow. The drawback of collecting in Antarctica is the harsh conditions that the collection teams must endure for weeks to months while camping out on the ice. Since the 1970s several countries, notably the United States and Japan, have operated scientific collection programs. Some tens of thousands of meteorites have been retrieved from Antarctica by the two countries’ programs, increasing the number of meteorites available to researchers manyfold. These include one-third of all known Martian meteorites, one-third of known lunar meteorites, and numerous other rare or unique samples. Because large numbers of Antarctic meteorites are found within small areas, the traditional geographic naming system is not used for them; rather, an identifier is made up of an abbreviated name of some local landmark plus a number that identifies the year of recovery and the specific sample.

2. Classification

Most meteorites are stony meteorites, classed as chondrites and achondrites. Only about 6% of meteorites 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) summarizes modern meteorite taxonomy. About 86% of the meteorites 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. 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 coalesced 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 are achondrites (meaning they do not contain chondrules), some of which are similar to terrestrial igneous rocks. Most achondrites are also ancient rocks, and are thought to represent crustal material of differentiated planetesimals. One large family of achondrites (the HED meteorites) may have originated on the parent body of the Vesta Family, although this claim is disputed. Others derive from unidentified 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 constitutes the only materials from other planets ever recovered by humans. About 5% of meteorites that have been seen to fall are iron meteorites composed of iron-nickel alloys, such as kamacite and/or taenite. Most iron meteorites are thought to come from the cores of planetesimals that were once molten. As with the Earth, the denser metal separated from silicate material and sank toward the center of the planetesimal, forming its core. After the planetesimal solidified, it broke up in a collision with another planetesimal. Due to the low abundance of iron meteorites in collection areas such as Antarctica, where most of the meteoric material that has fallen can be recovered, it is possible that the percentage of iron-meteorite falls is lower than 5%. This would be explained by a recovery bias; laypeople are more likely to notice and recover solid masses of metal than most other meteorite types. The abundance of iron meteorites relative to total Antarctic finds is 0.4%. 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 that 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.

3. Falls

Most meteorite falls are collected on the basis of eyewitness accounts of the fireball or the impact of the object on the ground, or both. Therefore, despite the fact that meteorites fall with virtually equal probability everywhere on Earth, verified meteorite falls tend to be concentrated in areas with higher 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 Přibram 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.[44] 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.[45] NASA has an automated system that detects meteors and calculates the orbit, magnitude, ground track, and other parameters over the southeast USA, which often detects a number of events each night.