The Geology of Vesta’s Dark Material

Abstract

Deposits of dark material appear on Vesta’s surface as features of relatively low-albedo (1-4). Mixed with the regolith and partially excavated by younger impacts, the material is exposed as individual layered outcrops in crater walls or ejecta patches, having been uncovered and broken up by the impact. The asymmetric distribution of dark material in impact craters and ejecta suggests non-continuous distribution in the local subsurface. The spectral characteristics of the dark material resemble that of Vesta’s regolith, though with somewhat attenuated absorption bands (3-5,6). Dark material is distributed unevenly across Vesta’s surface with clusters of all types of dark material exposures. On a local scale, some craters expose or are associated with dark material, while others in the immediate vicinity do not show evidence for dark material. While the variety of surface exposures of dark material and their different geological correlations with surface features, as well as their uneven distribution, indicate a globally inhomogeneous distribution in the subsurface, the dark material seems to be correlated with the rim and ejecta of the older Veneneia south polar basin structure. The origin of the dark material is still being debated, however, the geological analysis suggests that it is exogenic, from carbon-rich low-velocity impactors, rather than endogenic, from freshly exposed mafic material or melt, exposed or created by impacts (3-5,7).