Diversity in the Iron Content of Olivine Rich Asteroids as Revealed by MGM Analyses of New SMASSIR Spectra

J. M. Sunshine (SAIC), R. P. Binzel, T. H. Burbine, S. J. Bus (MIT)

Two olivine-rich asteroids, 289 Nenetta (V13.9) and 863 Benkoela (V13.7), were observed in December and January using the new "Asteroid Grism" system and NSFCAM detector developed by Binzel for use at the IRTF at Mauna Kea. These data are part of a near-IR extension of the MIT Small Main-Belt Asteroid Spectroscopic Survey (SMASSIR). The combined SMASSIR data typically range from 0.4- to 1.7 tex2html_wrap_inline12 m.

The new SMASSIR spectra agree well with previous measurements (Cruikshank and Hartman, Science, 1984; Bell et al., LPSC, 1988). However, the SMASSIR data have significantly better signal to noise and higher spectral resolution than previous data. Visual comparison to laboratory spectra of olivines confirms that the SMASS data include many of the subtleties characteristic of the complex 1 tex2html_wrap_inline12 m olivine absorption feature. In particular, the SMASS data contain inflections consistent with the three major Fe tex2html_wrap_inline16 olivine absorptions.

The SMASSIR spectra were analyzed using the Modified Gaussian Model (MGM; Sunshine et al., JGR, 1990), a physically based framework for deconvolving spectra into their constituent absorption bands. However, even the SMASSIR data include a noise level such that simple application of the MGM results in absorptions that have properties which are inconsistent with the known variations among olivine absorptions. Nonetheless, when weak constraints based on the range of observed variations among olivine absorptions are applied to the modeling, physically plausible results are obtained.

MGM modeling using various constraints consistently indicates that Benkoela is forsteritic (Mg-rich). In sharp contrast, Nenetta is revealed to be more fayalitic (Fe-rich), with preliminary suggestions of a composition on the order of Fo60. It is therefore likely that the surfaces now revealed on Nenetta and Benkoela formed from diverse differentiation conditions. Continued application of the MGM to SMASSIR data should enhance our understanding of the petrologic and thermal evolution of asteroids.

This research sponsored by NASA PGG (W-91534) and NSF (AST-9530282).