Statistics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.595w&link_type=abstract
Meteoritics, vol. 30, no. 5, page 595
Statistics
11
Asteroid Belts, Meteorites, Sources, Parent Bodies, Eucrite, Hed, Ordinary Chondrite
Scientific paper
With rare exceptions, our sample of meteorites is very unrepresentative of the parent bodies from which they originated. This results from the complex sequence of steps necessary to bring a meteorite from a particular location on a parent body located in the asteroid belt to the surface of the Earth as a recoverable meteorite, and the numerous biases that have affected each of these steps. Consider the differentiated meteorites. There is general agreement that many, probably most, differentiated meteorites formed by melting and differentiation of a parental asteroid that originally had a chondritic composition. In the simplest form these differentiation processes would produce a parent body with a crust, a silicate mantle and a metallic core. About 83% of the 700 irons in our meteorite collections can be assigned to 13 large groups each corresponding to a separate parent body. Some 60 of the remaining 120 irons form 20 grouplets with 2-4 members, but the remaining 60 seem to each require its own parent body. Thus we need about 90 parent bodies just to account for the iron meteorites. Some of these irons are undoubtedly impact melts from chondritic parent bodies, but we are still left with about differentiated 70 bodies. It seems probable that sizable fragments of most of these crusts, mantles and cores are still in the asteroid belt. Despite this abundance of differentiated bodies there is only one case where the compositional and isotopic data are fully consistent with having meteorite samples from all levels within the parent body--the IIIAB irons in the core, the main-group pallasites in the mantle, and the HED meteorites (eucrites, howardites, diogenites) and (possibly) mesosiderites representing crustal and subcrustal regions. But the consensus seems to be that the HED samples originated on Vesta (a = 2.36 AU), largely because it has a well-defined basaltic spectrum, and there is evidence for small basaltic bodies occupying orbits extending towards the 3:1 resonance at 2.50 AU. But the pallasites and IIIAB irons are certainly not from Vesta, and the O-isotopic evidence for associating the HED meteorites with pallasites and IIIAB constitutes a much stronger genetic link than does the rough agreement in reflection spectra, and the low fall rate of HED meteorites does not require that they be next to a major escape channel from the asteroid belt. Because the potency of all potential heat sources increases with decreasing distance from the Sun, it seems likely that the HED parent body (and most of the 70 other bodies) formed in the inner part of the asteroid belt or was perturbed into and stored in this region following formation nearer to the Sun. If the 3:1 resonance is the chief escape channel from the asteroid belt, then the three parent bodies of the ordinary chondrites must be located near 2.5 AU. The large numbers of these bodies and their more-or-less uniform distribution among metamorphic grades indicates that, in contrast to all other groups of meteorites, we are sampling with similar efficiencies materials from all depths within the parent bodies. It seems likely that they originally formed relatively near 2.5 AU, and that they and their sibling asteroids probably account for many of the S asteroids. It is essential that sampling biases be properly allowed for when attempting to reconstitute meteorite parent bodies or to use meteorite fall statistics to test the validity of spectral identifications of asteroids. Other than the three ordinary chondrite asteroids, our sampling of these bodies is so incomplete that all such inferred relationships must be taken cum grano salis.
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