Mineralogical Variations within the S-Type Asteroid Class

Abstract

A systematic spectral analysis has been Carried out on a large subset (39 of 144) of the S-type asteroid population. The S-asteroid class includes a number of distinct compositional subtypes [designated S(I)-S(VII)] which exhibit surface silicate assemblages ranging from pure olivine (dunites) through olivine-pyroxene mixtures to pure pyroxene or pyroxene-feldspar mixtures (basalts). S-asteroid absorption bands are weaker than expected for pure mafic silicate assemblages, indicating the presence of an additional phase, most probably FeNi metal, although the abundance of metallic or feldspar components is not well constrained, The diversity within the S-class probably arises from several sources, including the coexistence of undifferentiated, partially differentiated, and fully differentiated bodies within the general S-asteroid population and the exposure of compositionally distinct units from within metamorphosed and partially and fully differentiated parent bodies. Partial differentiation within planetesimals appears to be an important source of this diversity. The surface assemblages of these subtypes include both analogues to known meteorite classes (e.g., pallasites, mesosiderites, ureilites, lodranites, brachinites, winonaites) and materials not sampled in our present meteorite collections. No specific ordinary chondrite parent bodies have been identified within the S-class, but silicate mineralogy provides a strong test for possible ordinary chondritic affinities. This test is failed by 75% of the S-asteroids. Only the S(IV)-subtype objects have silicates consistent with ordinary chondrites (OC). This subtype provides the only viable OC parent body candidates among the large main-belt S-asteroid population, although the individual objects remain to be evaluated. The S(IV) objects are concentrated near the 3:1 Kirkwood gap at 2.5 AU, and their ejecta can be readily injected into the associated chaotic region and rapidly converted into Earth-crossing orbits which may contribute to the high abundance of OC meteorites. S-asteroid absorption band depth correlates with asteroid diameter. It is relatively constant for objects larger than 100 km and increases steeply toward smaller sizes. This suggests that some equilibrium has been attained in the optical surfaces of S-asteroids larger than 100 km but not on smaller bodies.