Surface Lithologic Heterogeneity of Asteroid 4 Vesta

Abstract

High precision spectrophotometry of 4 Vesta, the third largest asteroid, was used to establish the surface composition of this body and to investigate mineralogical variations across its surface. The average surface of Vesta is analogous to howardite and/or polymict eucrite assemblages, regolith-derived members of the HED meteorite suite which consist of a eucrite matrix containing differing amounts of a diogenite component. Color and/or spectral changes, which exhibit a consistent relationship to rotational phasing, are evident in 16 of 18 studies of Vesta dating back to 1929, including the present work. After elimination of possible sources of spurious spectral or color changes, it is concluded that these variations arise from hemispheric variations in surface materials, and hence provide a means of spatially resolving subhemispheric compositional units on the surface of Vesta. The background surface of Vesta is a relatively dark howardite or polymict eucrite (pyroxene–plagioclase) assemblage with several compositionally distinct bright regions clustered in one hemisphere viewed around the maximum in Vesta's lightcurve. These include what appears to be an olivine-bearing unit (suggested name “Leslie Formation”) located near Vesta's equator which probably represents an impact basin (and/or its associated ejecta) that penetrated through the basaltic crust. Other high-albedo compositional units including an apparently low-calcium eucrite region and several diogenite (pyroxenite) regions, at least one located near the southern pole, may be smaller, shallower impact basins. By analogy to the eucrite meteorites, which represent surface flows or shallow intrusions and which constitute the major component of the regolith-derived howardites and polymict eucrites, we conclude that the howardite/polymict eucrite units represent a regolith-gardened original surface of Vesta. It is probable that the low albedo of the background surface on Vesta is due to an age-related darkening effect similar to that inferred from the Galileo images of Gaspra and Ida. This mechanism is consistent with the correlation of absorption feature intensity with the lightcurve. Vesta appears to have an old eucritic surface, darkened with age and represented among the meteorites by the regolith-derived howardites and polymict eucrites, on which several impacts on one hemisphere have exposed fresher brighter diogenite and olivine-bearing material. Based on qualitative analysis of the mineralogical variations as a function of rotation, a generalized lithologic map of Vesta was produced. There is reasonable control on the longitude of lithologic features but little control on their latitudes, except where specifically noted. In producing this map, discrete circular features were used on the plausible assumption that impacts have been the most important geologic process on this surface for most of the age of the solar system. The shape of the features has thus been assumed rather than derived and is subject to future revision as data improve.