Relationship of E-type Apollo asteroid 3103 (1982 BB) to the enstatite achondrite meteorites and the Hungaria asteroids

Abstract

Visible and near-infrared observations confirm that Apollo asteroid 3103 (1982 BB) has a reflectance spectrum consistent with its previous assignment to taxonomic type E. It has been generally believed that the E-type asteroids are analogous (and perhaps even genetically related) to the enstatite achondrite meteorites (aubrites). Correlated variations of previously published visible and thermal infrared lightcurves and the lack of rotational color variations in the present data indicate that this is an elongated object with no substantial surface albedo variations. Apollo asteroid 3103 (1982 BB) is presently in an orbital resonance (3:5) with Earth and appears to be a relatively long-lived member of the Earth-approaching population. Its orbit is consistent with both the fall dates and long cosmic ray exposure ages of the aubrites. Combined with the rarity of E-type asteroids in the near-Earth asteroid population, this indicates the 3103 (1982 BB) is probably either a major or the primary immediate source body of the aubrites. The aphelion location and the mineralogy of 3103 (1982 BB) indicate that it was most probably derived from the Hungaria region at the innermost edge (1.9 AU) of the asteroid belt, although the exact dynamical mechanism for producing its present orbit is not clear. These results represent the first plausible direct link of a set of meteorites (aubrites or enstatite achondrites) back to a particular source region in the asteroid belt. This link allows the composition of the solar nebula to be constrained at that heliocentric distance during the time interval when the parent assemblage of the aubrites was isolated from contact with the nebular gas by accretion into its parent planetesimal.