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T. H. Burbine (MIT), E. A. Cloutis (Univ. Winnipeg), S. J. Bus (MIT), A. Meibom (Univ. Hawai'i), R. P. Binzel (MIT)
One class of asteroids, the E-types, have relatively featureless spectra and very high visual albedos [>30%], which have been interpreted as indicating surfaces composed predominately of an essentially iron-free silicate (Zellner et al., 1977). The most obvious meteorite analog is the aubrites, which are igneous meteorites composed of essentially iron-free enstatite plus accessory phases such as Fe-Ni metal, troilite, forsteritic olivine, plagioclase feldspar, and diopside (Watters and Prinz, 1979). The only known near-Earth E-asteroid, 3103 Eger, has been postulated as the source body for at least some of the aubrites (Gaffey et al., 1992). However, the identification of an absorption feature in the 3 micron wavelength region of a number of main-belt E-asteroid spectra (Rivkin et al., 1995; Rivkin, 1997) has been interpreted as indicating hydrated minerals on the surfaces of some of these objects, which appears inconsistent with an igneous origin for E-type asteroids.
We have identified a new absorption feature in the visible spectra of a number of E-type asteroids (e.g., 64 Angelina, 424 Hungaria, 3103 Eger) that gives considerable insight into their composition. Each of these objects has an absorption feature from 0.44 to 0.51 microns, a sharp turnup in reflectance from 0.51 to 0.55 microns, a flat red-sloped spectral shape from 0.55 to 0.70 microns, and a shallow feature that extends past 0.92 microns. Numerous meteorite and mineral spectral databases were checked to try to find a spectral match to this feature. The best match is troilite (FeS), whose absorption features are consistent with both the visible and near-infrared (when available) spectral measurements of these objects.