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 meteorites have come 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.

Population

79 percent 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 five percent 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 eight percent of the incoming material and are thought to represent crustal material of some of the larger asteroids and occasionally Moon or Mars. About six percent 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 two percent. 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.

Classification

Meteorites are classified according to their structure and mineral composition. The three main classes of meteorite are stones, stony irons, and irons. Each chondrite is described using the following two-character group name and a petrologic type. Starting from the most primitive type 3, decreasing numbers (2~1) indicate higher degrees of aqueous alteration, and increasing numbers (4~7) indicate degrees of thermally metamorphism.

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

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.

Another theory is that the island of Manhattan, part of New York City, was formed by a huge meteorite that fell prior to the supercontinent Pangea splitting apart to eventually form the present age's continents.

The only reported fatality from meteorite impacts is an Egyptian dog that 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 kilogram 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 be struck by meteorites but no verifiable meteorites have been produced as a result of these claims.

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.

Legend has it that Alamo defender Jim Bowie used a knife popularly called a Bowie knife forged from a meteorite that fell in Nigeria, Africa.

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 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.

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