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Meteorites, what are they?

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They streak through the Earth's upper atmosphere at 8 to 12 thousand mph. Fragments of solid, rocky, and iron materials crash into the Earth's surface on a regular basis. Known as "meteorites", they are fragments of shattered asteroids and other planetary bodies. But, once on Earth, unless seen to fall and land, they blend in well with the rocks on most of the Earth's continental surfaces.

Finding them is not easy and the effects of terrestrial weathering makes it even harder.

The majority of meteorites originate from asteroids shattered by impacts with other asteroids. In a few cases they are pieces of the Moon, the planet Mars, and presumably, comets. Before striking the ground, a meteorite is called a meteoroid. When it enters the Earth's atmosphere, it is slowed by friction with atmospheric molecules - air - causing the meteoroid to heat up. It often glows and leaves a trail of vaporized material and ionized air behind it.

A glowing meteoroid moving through the atmosphere is called a meteor. This is the light phenomena we see and call a "shooting star". It is usually 80 to 100 miles in altitude when we see them. Most of these are dust to sand sized particles, that burn up, or vaporize, high in the atmosphere. If a meteoroid is large enough it can survive its passage through the atmosphere and strike the Earth's surface. We then refer to it as a "meteorite".

Some meteoroids break apart or burst in the atmosphere, their remnants falling to the surface as smaller fragments. Others may break up on impact, creating a crater of varying size. Meteorites that are found after a meteoric event has been witnessed are called an observed "Fall," while those found by chance are called a "Find." Once studied and classified they are named by the Nomenclature Committee of the Meteoritical Society (the Meteoritical Society is an international organization of mostly scientists affiliated with NASA and many universities).

They are usually named after a town or a large geographic landmark closest to the Fall or Find, and collectively termed "localities".

Meteorites are some of the scarcest material on Earth, more rare then gold. They are pieces and fragments of the original material from which the Earth, the terrestrial planets, asteroids, and comets, formed in the early solar system. Because of their scarcity and primitive origins, meteorites are sought after by collectors and researchers alike.

Meteorites have three general compositions:

a. Iron meteorites are composed mainly of metallic iron and nickel often mixed together as an alloy and are believed to resemble some of the mineralogy in the Earth's core.

b. Stony meteorites, called chondrites, mostly contain silicate minerals, such as pyroxene, plagioclase feldspar, and olivine. They also contain minor amounts of metals, particularly nickel and iron alloys. Stony meteorites account for 95% of all meteorites found. Some of the more rare stone meteorites within this group, called achondrites, are similar in a few ways to igneous volcanic Earth rocks like basalts.

c. Stony-iron meteorites contain a mixture of silicate minerals and a nickel-iron alloy. One theory of origin is that they form at the core/mantle boundary of differentiated asteroids. Some of this group are called pallasites and are the most visually striking of all meteorites.

While the previous list of (a), (b), and (c) may seem rather straight forward, meteorites can also contain some exotic compounds and mineralogy. For example, the Murchison meteorite that fell in Australia in 1969 contains over 90 amino acids, most of which are not found on Earth. Another one, Allende, observed to fall in Mexico, has an abundance of white, calcium aluminum inclusions containing interstellar grains that predate the formation of our Sun and solar system. Millbillille, a fall in Australia, is a sample of Asteroid Vesta, and Zagami (Nigeria) is a basaltic meteorite that is a fragment of the planet Mars!

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