Radiative transfer mixing models of meteoritic assemblages

A spectral mixing model for the expected conditions on asteroid surfaces based on Hapke's radiative transfer theory for intimate mixtures is presented. This model calculates the visible/near-infrared reflectance spectrum of an intimate mixture incorporating the modal mineralogy, the mineral chemistry, the particle size, and the degree of space weathering as factors, and includes an improved treatment for the spectral effects of coarse-grained Fe, Ni-metal. This model can reproduce the spectra of geologically plausible mixtures of silicate components and meteorites with known mineral abundances, chemistry, and particle sizes. We compare the model spectral effects of the coarse-grained Fe, Ni-metal naturally present on asteroid surfaces to the spectral effects of submicroscopic iron produced by lunar-style space weathering. The model spectral effects of large asteroid surface concentrations of coarse- grained Fe, Ni-metal are similar to the spectral effects of SMFe in some cases. We model the composition of asteroid 4 Vesta and find it to be consistent with a minimally space-weathered eucritic composition. Finally, we predict the spectral properties of some example of ordinary chondrite parent bodies. When modeled with particle sizes matching the optically dominant particle size of the lunar surface and minimal abundances of SMFe, these model ordinary chondrite parent bodies plot outside the S(IV) asteroid field on a Band area ratio versus Band I center plot. We find that a combination of larger particle sizes and increased SMFe abundances is the most effective way to move these models into the S(IV) field.