- HED meteorite
The HED meteorites are a grouping of
achondrite meteorite types, the:
*Howardite s
*Eucrite s
*Diogenite s
* "Dunite"
* "Olivine Diogenites" [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) ]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 fromradioisotope ratios. HED meteorites are differentiated meteorites, which were created by igneous processes in the crust of their parent asteroid.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 falls [ [http://tin.er.usgs.gov/meteor/metbull.php Meteoritical Bulletin Database] ] , which is about 60% of all
achondrite s.Lindstrom, Marilyn M.; Score, Roberta. [http://www-curator.jsc.nasa.gov/antmet/ppr/ppr.htm Populations, Pairing and Rare Meteorites in the U.S. Antarctic Meteorite Collection] . NASA Johnson Space Center.]It is thought that the method of transport from 4 Vesta to
Earth is as followsDrake, Michael J. [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001M%26PS...36..501D&db_key=AST&data_type=HTML&format=&high=4374b9c9ce01839 The eucrite/Vesta story] . "Meteoritics and Planetary Science", Vol. 36, p. 501 (2001).] :
# An impact on 4 Vesta ejected debris, creating small (10 kmdiameter or less)V-type asteroid s. Either the asteroidal chunks were ejected as such, or were formed from smaller debris. Some of these small asteroids formed theVesta family , while others were scattered somewhat further. 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).] This event is thought to have happened less than 1 billion years before now.Binzel, R.P.; et al. Geologic Mapping of Vesta from 1994 Hubble Space Telescope Images. "Icarus", Vol. 128, p. 95 (1997).] 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 ended up in the 3:1Kirkwood gap . This is an unstable region due to strong perturbations byJupiter , 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-typenear-Earth asteroid s such as e.g.3551 Verenia ,3908 Nyx , or4055 Magellan .
# Later smaller impacts on these near-earth objects dislodged rock-sized meteorites, some of which later struck Earth. On the basis ofcosmic 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.Eugster, O.; Michel, Th. [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1995GeCoA..59..177E&db_key=AST&data_type=HTML&format=&high=4374b9c9ce08038 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).]References
External links
* [http://www.psrd.hawaii.edu/Archive/Archive-Meteorites.html Meteorite articles, including discussions of HEDs, in Planetary Science Research Discoveries]
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