GRAIN SIZES AND MINERAL COMPOSITIONS OF SURFACE REGOLITHS OF VESTA-LIKE ASTEROIDS

Visible and near-infrared reflectance spectra (0.4-1.0 mu m) of 21 Ves ta-like asteroids and 12 HED meteorite powders (<25 mu m in grain size ) have been compared, Each reflectance spectrum was deconvolved into m odified Gaussians and a constant background to allow comparisons of th e 1-mu m absorption band position and shape, The 1-mu m band centers a nd widths of Vesta-like asteroids and HED meteorites generally overlap , but asteroids have a slightly wider range of band centers and widths indicating a wider range of Fe, Mg, and Ca content of their pyroxenes . Many of the Vesta-like asteroids, however, have shallower 1-mu m abs orption bands than HED meteorites. The weaker 1-mu m band suggests tha t the surface regoliths of those asteroids may contain a larger amount of either fine grains or plagioclase than the laboratory-prepared HED meteorite powders. The most serious spectral difference between Vesta -like asteroids and HED meteorites is their visible absorption strengt h (reflectance drop-off shortward of similar to 0.7 mu m) relative to the 1-mu m band strength. Vesta-like asteroids tend to have stronger v isible absorption than HED meteorites. The difference in the visible a bsorption strength may be due to some minor transition elements (Ti, C r, etc.) in pyroxene, other unidentified mineral components, or space weathering processes. (C) 1995 Academic Press, Inc.