The Surface of 4 Vesta: a petrologist's view

Introduction: How do HED meteorites constrain the data to be returned from 4 Vesta by the Dawn Discovery Mission? Vesta has long been equated with the " eucrite parent body " in its various iterations and the spectroscopic similarities between Vesta and HED meteorites are compelling. The assumption is made that Vesta is a fully differentiated body. In common with other terrestrial planets, the surface of the 'asteroid' Vesta (more realistically a planetoid) has provinces with distinct spectral characteristics. On the Earth, Moon, Mars and Venus, dichotomies between highlands and lowlands provinces are clearly delineated in existing planetary maps. On Vesta, the presence of a dichotomy is manifested as the huge elevation difference between the south polar region and the northern hemisphere when compared to an ideal spherical body. Petrologically, the uplands and lowlands provinces of Vesta have variable spectral signatures. These lithologies are assumed to correspond to the dominant lithic components in the basaltic achondrite meteorites (eucrites and diogenites) but the regolith of 4 Vesta is better represented spectrally by polymict breccias, especially howardites, that contain additional lithic components. The howardite group of meteorites provide the optimum sample suite for constraining the spectral data. The Major Provinces of Vesta: The (Southern) Lowlands province is a huge crater (multiring basin with central uplift?) that may have excavated as much as 10-20% of the original body and presumably exposes material from the Vestan mantle and perhaps even the core in its central uplift. The impact(s) that produced this depression must have triggered magmatism with associated thermal metamorphism and systematically reset the ages of several isotopic systems. For example, young crystallization ages associated with some cumulate eucrites reflect late stage magmatism associated with the Southern Lowlands impact. At present, limited spectral evidence exists for mantle material on the surface, but that may be the effect of Vesta's axis of rotation, relative to the observations so far. If the Southern Lowlands disect a thick planetary crust(~65km), then the area of mantle (+/-core) derived material may be geographically limited The (Northern) Uplands province presently encompasses much of the surface of Vesta. (Observations of Vesta suggest that other 100+km impact structures are present and may represent an intermediate 'province'.) The Uplands are significantly older then the Southern Lowlands. The age distribution of eucrites, between 4.5