Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29..453b&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 453
Other
3
Asteroid Belts, Asteroids, Evolution (Development), Olivine, Silicates, Absorption Spectra, Iron Meteorites, Pyroxenes, Spectrum Analysis
Scientific paper
One serious problem in our current understanding of the asteroid belt is the explanation for the rarity of olivine-dominated metal-free silicate asteroids in the main belt. The complete differentiation of a chondritic parent body should result in a body with a Ni-Fe core, a thick olivine-dominated mantle, and a thin plagioclase/pyroxene crust. If some fraction of Fe meteorites are fragments of the cores of these differentiated asteroids, the disruption of these bodies should produce a number of exposed Ni-Fe cores (historically thought to correspond to type M), a large number of olivine-dominated metal-free asteroids (assumed to correspond to type A), and a number of pyroxene-rich asteroids (assumed to correspond to type V or J). The problem with this scenario is that while M asteroids are relatively abundant in the main belt, asteroids classified as A, V, or J are relatively rare. One possible explanation for this paradox is that A type asteroids (and V and J types) are very common at small sizes that have yet to be spectroscopically characterized. Another explanation is that A types (and V and J types) are actually very uncommon due to the formation of only a relatively small number of fully differentiated bodies that were subsequently collisionally disrupted. The asteroidal evidence for the existence of olivine-dominated metal-free silicate asteroids in the main belt is not very convincing. The explanation for this apparent problem may be just that these olivine-dominated metal-free asteroids (and pyroxene-rich objects not related to Vesta) are very small and just have not been spectroscopically observed. This would imply that almost all of the completely differentiated bodies were disrupted relatively early in the age of the solar system, which would account for the A type asteroids being collisionally broken down into small sizes. However, an equally valid argument is that bodies (or fragments of bodies) that completely differentiated (e.g., Vesta) are relatively rare and therefore A types are very uncommon. In this scenario, partially differentiated bodies would be very common, which implies that S asteroids are predominately asteroids that only partially differentiated and not the core-mantle boundaries of disrupted differentiated bodies.
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