Surveying Vesta's styles of space weathering and surface mixing

Abstract

The Dawn spacecraft's mission at Vesta [1] has revealed a world that, although much smaller than the Moon or Mercury, has experienced planet-like processes and undergone a complicated geological evolution [2]. One fascinating characteristic of Vesta is the manner in which the regolith evolves in response to exposure to the space environment. In general, vestan space weathering is dominated by admixture of low-reflectance material delivered by carbonaceous chondrite (CC) impactors [3-7]. As a result, freshly exposed vestan basaltic material tends to become darker with time, and the strong absorption bands (near 1000 and 2000 nm) caused by ferrous iron in pyroxene become shallower. Darkening and decreased band contrast are hallmarks of lunar space weathering, however on the Moon these are accompanied by a strong increase in the continuum slope (reddening) [e.g., 8, 9]. The cause of the spectral changes on the Moon is the accumulation of micro- and nanophase metallic iron as a result of melting and vaporization by micrometeoroid bombardment and/or solar-wind sputtering [reviewed by 10]. We are conducting a survey of impact mixing and regolith maturation trends in different regions of Vesta. The goals are to document the range of space weathering styles on Vesta, and to examine how the observed trends can give clues to the composition of the material that is undergoing space weathering. The findings should help to further understanding of space weathering in the asteroid belt, and hence as a general phenomenon across the Solar System. Here we present results from two locations that illustrate Vesta's spectral diversity: Vibidia crater and near Oppia crater.