HED meteorite
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The HED meteorites are a grouping of achondrite meteorite types, the:
- Howardites
- Eucrites
- Diogenites
- Dunite
- Olivine Diogenites[1]
These are all thought to have originated from the crust of the asteroid 4 Vesta, their differences being due to different geologic histories of the parent rock. Their crystallization ages have been determined to be between 4.43 and 4.55 billion years from radioisotope ratios.
They are a relatively common type except for the dunite which is represented by only one meteorite, NWA 2968. The HED meteorites account for about 5% of all finds, which is about 60% of all achondrites.[2]
It is thought that the method of transport from 4 Vesta to Earth is as follows[3]:
- An impact on 4 Vesta ejects debris, creating small (10 km diameter or less) V-type asteroids. Either the asteroidal chunks are ejected as such, or form from smaller debris. Some of these small asteroids form the Vesta family, while others are scattered somewhat further. [4] This event is thought to have happened less than 1 billion years before present.[5] There is an enormous impact crater on 4 Vesta covering much of the southern hemisphere which is the best candidate for the site of this impact. The amount of rock that was excavated there is many times more than enough to account for all known V-type asteroids.
- Some of the more far-flung asteroid debris ends up in the 3:1 Kirkwood gap. This is an unstable region due to strong perturbations by Jupiter, and asteroids which end up here get ejected onto far different orbits on a timescale of about 100 million years. Some of these bodies are perturbed into near-Earth orbits forming the small V-type near-Earth asteroids such as e.g. 3551 Verenia, 3908 Nyx, or 4055 Magellan.
- Later smaller impacts on these near-earth objects dislodge rock-sized meteorites, some of which later strike Earth. On the basis of cosmic ray exposure measurements, it is thought that most HED meteorites arose from several distinct impact events of this kind, and spent from about 6 to 73 million years in space before striking the Earth.[6]
[edit] References
- ^ http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1502.pdf OLIVINE DIOGENITE NWA 1459: PLUMBING THE DEPTHS OF 4 VESTA, A. J. Irving, et. al., Lunar and Planetary Science XXXIV (2003)
- ^ Lindstrom, Marilyn M.; Score, Roberta. Populations, Pairing and Rare Meteorites in the U.S. Antarctic Meteorite Collection. NASA Johnson Space Center.
- ^ Drake, Michael J. The eucrite/Vesta story. Meteoritics and Planetary Science, Vol. 36, p. 501 (2001).
- ^ Binzel, R. P.; Xu, S. Chips off of asteroid 4 Vesta: Evidence for the parent body of basaltic achondrite meteorites. Science, Vol. 260, 186 (1993).
- ^ Binzel, R.P.; et al. Geologic Mapping of Vesta from 1994 Hubble Space Telescope Images. Icarus, Vol. 128, p. 95 (1997).
- ^ Eugster, O.; Michel, Th. Common asteroid break-up events of eucrites, diogenites, and howardites, and cosmic-ray production rates for noble gases in achondrites Geochemica et Cosmochimica Acta, Vol. 59, p. 177 (1995).
