SPECTRAL MIXING MODELS OF S-TYPE ASTEROIDS

We present the results of mixing model determinations of S-type asteroid mineralogies using the Hapke spectral theory. The goal is to quantify uncertainties resulting from poorly constrained model parameters and to produce sets of model. compositions that we can analyze for trends that can be related to what is already known about S asteroids and the meteorites proposed to be derived from them. We present two different approaches to the selection of the albedo input parameter and show the resulting differences in model compositions. Similar grain sizes and composition patterns in terms of metal/silicate and olivine/pyroxene ratios were determined regardless of albedo approach. Encouraged by this agreement, we suggest that the two resulting sets of model compositions can be used to set limits on end-member abundances. Residual errors from model fits are comparable to those found by other compositional modeling attempts and, furthermore, tend to be located in the same spectral regions. These regions are (1) 0.4-0.7 mu m, where the silicate and metal component UV drop-off in the asteroid spectra is too steep to be modeled with the current end-members, and (2) 1.5-2.5 mu m, where the IR continuum turnover of the asteroid spectra is too flat to be fit with metallic spectrally red components included to match the IR continuum from 0.7 to 1.5 mu m We believe these systematic problems to be compositional information, as yet uninvestigated. More information is needed regarding the grain size conditions and the nature of opaque components present at asteroid surfaces before highly accurate compositional modeling is possible. The technique presented here is thus most useful in conjunction with other spectral deconvolution methods.