Anatomy of a partially differentiated asteroid: A "NEAR"-sighted view of acapulcoites and lodranites

With the insertion of the NEAR spacecraft into orbit around the asteroid 43 3 Eros, a new stage of asteroid exploration has begun, with the return of s patially resolved mineralogical, geological, and chemical information from specific asteroids. In this paper, we outline the features of a partially d ifferentiated asteroid using the acapulcoite and lodranite meteorites as a guide. Such a body would likely contain a variety of lithologies, including chondritic rocks largely unaltered by melting, residues of partial melts, and basaltic and Fe,Ni-FeS rocks crystallized from the partial melts. The p resence or nature of the regolith remains largely unconstrained, Mineralogi cal and isotopic heterogeneities preserved in these rocks suggest spatial s eparation of these rock types by distances in excess of 200 m. Spacecraft-b ased remote sensing instrumentation should detect substantial spectral hete rogeneity (most prominently manifested in the band area ratio) and chemical heterogeneity. Observation of these various lithologies in situ would prov ide important new constraints on the processes involved during the earliest differentiation of asteroids.