The ordinary chondrites (sometimes called the O chondrites) are a class of stony chondritic

meteorite A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...

s. They are by far the most numerous group, comprising 87% of all finds. Hence, they have been dubbed "ordinary". The ordinary chondrites are thought to have originated from three parent asteroids, with the fragments making up the H chondrite, L chondrite and

LL chondrite LL may refer to: * Ll or ll, a digraph that occurs in several natural languages Arts and entertainment *LL, the production code for the 1967 ''Doctor Who'' serial ''The Evil of the Daleks'' * ''Labyrinth Lord'', a fantasy role-playing game * ''L ...

groups respectively.

Origin

It is suspected that they are not representative of typical

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

parent bodies, but rather of a select few which are advantageously placed to send impact fragments to Earth-crossing orbits. Such positions are e.g. near

Kirkwood gap A Kirkwood gap is a gap or dip in the distribution of the semi-major axes (or equivalently of the orbital periods) of the orbits of main-belt asteroids. They correspond to the locations of orbital resonances with Jupiter. The gaps were first n ...

s and/or secular resonances in the main asteroid belt. In fact, only the one rather insignificant asteroid 3628 Božněmcová has been identified to have a spectrum close to the ordinary chondrites. A probable parent body of the H chondrites (comprising about 46% of the ordinary chondrites) is 6 Hebe, but its spectrum is dissimilar due to what is likely a metal impact melt component. It is likely that the ordinary chondrites comprise a detailed sample of but a few select asteroids which happen to have been in the right place at the right time to send many fragments toward Earth at the present moment in solar system history. On the other hand, observations of

243 Ida 243 Ida is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at Vienna Observatory and named after Ida (nurse of Zeus), a nymph from Greek mythology. Later telesc ...

by the Galileo spacecraft found weathering of Ida's surface, and the reflection spectra of freshly exposed parts of the surface resembled that of OC meteorites, while the older regions matched the spectra of common S-type asteroids.

Chemical composition

The ordinary chondrites comprise three mineralogically and chemically distinct groupings. They differ in the amount of total iron, of iron metal and iron oxide in the silicates: * The H chondrites have the Highest total iron, high metal, but lower iron oxide (Fa) in the silicates * The L chondrites have Lower total iron, lower metal, but higher iron oxide (Fa) in the silicates * The

s have Low total iron and Low metal, but the highest iron oxide content (Fa) in the silicates

References

External links

The Catalogue of Meteorites

- Meteorites Australia
Gallery of Ordinary Chondrites
by James St. John, a geologist at Ohio State University {{Meteorites it:Condrite#Condriti ordinarie

Origin

Chemical composition

See also

References

External links