Meteorite

The Willamette Meteorite, the largest ever to be found in the United States

A meteorite is a small extraterrestrial body that impacts the Earth's surface. While in space they are called meteoroids, and while falling through Earth's atmosphere they are called meteors. Most of them were small asteroids, approximately boulder-sized or less. Some came from the Moon and Mars. When it enters the atmosphere, air resistance causes the body to heat up and emit light, thus forming a fireball or shooting star. More generally, meteorites fall on every solid planetary body, creating craters such as those on the Moon. Meteorites are so far the most important materials for studying planetary science.

Classification

Meteorites are classified according to their structure and mineral composition. The three main classes of meteorite are stones, stony irons, and irons. They can be also divided into primitive and differentiated meteorites.

• Stones
  • Chondrites
    • Carbonaceous chondrites: CI (Ivuna), CM (Mighei), CV (Vigarano), CO (Omans), CK (Karoonda), CR (Renazzo), CH (High iron), CB (Bencubbinites)
    • Ordinary chondrites: H (High iron), L (Low iron), LL (Low, low iron)
    • E (Enstatite) chondrites: EL (Low iron), EH (High iron)
    • R (Rumuruti) chondrites
    • K (Kakangari) chondrites
  • Achondrites
    • HEDs: Howardites, Eucrites, Diogenites
    • Lunar
    • Martian
    • Primitive achondrites: Acapulcoites, Lodranites, Brachinites, Winonaites
    • Others: Angrites, Aubrites, Ureilites
• Stony irons: Pallasites, Mesosiderites
• Irons: IAB, IC, IIAB, IIC, IID, IIE, IIF, IIG, IIIAB, IIICD, IIIE, IIIF, IVA, IVB

Population

79% of meteorites are chondrites - balls of mafic minerals with small grain size indicative of rapid cooling. In most chondrites small spherules, called chondrules, can be found. Chondrites are typically about 4.6 billion years old and are thought to represent material from the asteroid belt. There are different models to explain how chondrules formed. Carbonaceous chondrites, some of which are thought to be unaltered solar nebula material, constitute about 5% of meteorites and contain small amounts of organic materials, including amino acids. Also, presolar grains are identified in carbonaceous chondrites. The isotope ratios of carbonaceous chondrites are similar to those of the Sun.

Achondrites are similar to terrestrial mafic igneous rocks and sometimes are brecciated. Achondrites constitute about 8% of the incoming material and are thought to represent crustal material of some of the larger asteroids and occasionally Moon orMars. About 6% of meteorites are iron meteorites with intergrowths of iron-nickel alloys, such as kamacite. Unlike chondrites, the crystals are large and appear to represent slow crystallization. Iron meteorites are thought to be the core material of one or more planets that subsequently broke up. Stony iron meteorites constitute the remaining 2%. They are a mixture of iron-nickel and silicate minerals. They are thought to have originated in the boundary zone above the core regions where iron meteorites originated.

Meteorites in history

One theory suggests that a large meteorite impact caused the mass extinction of the dinosaurs. It is also theorized that meteorites caused other mass extinction events throughout the history of the Earth.

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 [1] occurred on November 30 1954 in Sylacauga, Alabama. There a 4 kg stone chondrite meteorite [2] 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 [3] 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.

Notable meteorites

Possible fossils of bacteria inside a Martian meteorite ALH84001

• Heat Shield Rock - Found on Mars.
• Orgueil meteorite - Object of a 1965 hoax that involved embedding a seed within part of the meteorite.
• Murchison meteorite and Allende meteorite - These CM2 and CV3 chondrites fell in the same year of 1969 and greatly contributed to meteoritics and planetary science.
• Canyon Diablo meteorite - Used by pre-historic Native Americans.
• Sikhote-Alin Meteorite - Massive impact event that occurred on February 12, 1947.
• ALH84001 - Mars meteorite that was claimed to prove the existence of life on Mars.
• Kaidun meteorite - Possibly from the martian moon Phobos.
• Tagish Lake meteorite - Possibly from a D/T-type asteroid or comet.

See also

• Meteor
• Solar System
• Planetary science

External links

• Astromaterials Curaiton at NASA Johnson Space Center
• The Meteoritical Society
• The Natural History Museum (London) Meteorite Catalogue Database
• Antarctic Meteorite Research Center, National Institute of Polar Research (Tokyo)

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