[X] My Assistant

[Guest_AlexBlackwell_*]

An interesting paper in the February 16, 2006, issue of Nature:

Iron meteorites as remnants of planetesimals formed in the terrestrial planet region
William F. Bottke, David Nesvorný, Robert E. Grimm, Alessandro Morbidelli and David P. O'Brien
Nature 439, 821-824 (2006)
doi:10.1038/nature04536
First paragraph
Supplementary Information

[Guest_BruceMoomaw_*]

Not having read the abstract itself till now, I was surprised to see that Bottke thinks Vesta itself may have originally formed in the inner Solar System and then wandered out into the Main Belt -- which I suppose could make sense as an explanation for that asteroid's atypicality.

However, one piece of evidence that Bottke mentions for his theory -- "Observational evidence... does not indicate that differentiated bodies or their fragments were ever common" in the Main Belt" -- meets a different explanation from M.J. Gaffey ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1223.pdf ):

"Thus it is plausible that many asteroid and meteorite parent bodies will have spent a significant portion of their heating history in the partial melting temperature interval [in the inner Main Belt]. This is consistent with the relatively high frequency of partially melted asteroids indicated by quantitative analysis of asteroid spectra. [Gaffey, unlike Bottke, regards many of the S-type asteroids as partially differentiated 'primitive achondrites' -- Moomaw.] But there is still a problem –- the reciprocal of that stated earlier -- with the meteorite statistics; particularly, why isn’t there more diversity among the partially differentiated assemblages in our meteorite collections, even if represented only by single specimens? As noted previously, it is now clear that meteorite fall statistics are controlled primarily by the locations of their parent bodies. At least 135 parent bodies are represented in the meteorite collections, but ~75% of all meteorites falling during the past several hundred thousand years derive from just three favorably parent bodies. One of these (6 Hebe) has already been identified. In the present epoch (the past few million years) it would appear that only one of the partially differentiated asteroids (the lodranite-acapulcoite parent body) has been near a favorable location. The high abundance of parent bodies sampled by only one or a few iron meteorites strongly suggests that the short collisional lifetimes of stones severely limit their ability to traverse significant distances to an escape hatch. So even though main belt asteroids derived from partially melted parent bodies are relatively abundant, the 'luck of the draw' apparently hasn’t resulted in such a body being in a favorable location recently.

"Thus there is no contradiction between the plethora of partial differentiated asteroids indicated by spectral studies and the rarity of similar assemblages in our meteorite collections. The identification of a partially [differentiated] asteroid in a location favorable for meteorite delivery would constitute a contradiction, but to date no such objects have been identified in our small sample."