Geologic mapping of Vesta from 1994 Hubble Space Telescope images

We utilize the 56 images of Asteroid 4 Vesta obtained through four mineralogically diagnostic filters by the Hubble Space Telescope (Zellner et al. 1997, Icarus) to construct a geologic map. Vesta's surface is found to be geologically diverse and to be dichotomous at a hemispheric scale, The eastern hemisphere (longitude definition from Thomas et al. 1997, Icarus) is dominated by units interpreted to be impact excavated plutonic material composed of magnesium-rich and calcium-poor pyroxene. The mineralogy of this region is most analogous to diogenite meteorites. Several eastern hemisphere units which have the deepest and broadest 1 mu m absorption bands may contain a substantial olivine component. The locations of these units are consistent with a previous map based on rotationally resolved groundbased spectroscopy (Gaffey 1997, Icarus). The western hemisphere is dominated by units interpreted to consist of a single component of iron-rich and relatively calcium-rich pyroxene, analogous to surface basalts such as eucrite meteorites. We investigate the spectral properties of 20 representative surface regions and find a correlation between the 1 mu m absorption band depth and albedo: the units with lower albedos have shallower band depths. Such a relation could arise from differences in lithologies, differences in surface particle sizes, or from a weathering effect which decreases albedo and band depth over time. Because of Vesta's geologic diversity and hemispheric dichotomy, it is apparent that any major impact events, such as those possibly related to the formation of the Vesta family (Binzel and Xu 1993, Science 260, 186-191) did not globally resurface the planet, The spectral evidence for remnants of a basaltic crust implies that the oldest units on the surface date back to the time of their emplacement, most likely similar to 4.5 byr ago, If a weathering process exists which alters the albedo, then the lowest albedo units, such as the feature with the proposed name ''Olbers,'' most likely represent remnants of Vesta's ancient basaltic crust. (C) 1997 Academic Press.