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Harry Y. McSween Colloquium Abstract (Oct 14, 2015)

Dawn’s Exploration of Vesta and Ceres

The Dawn mission is characterizing two of only a few surviving intact protoplanets. The Dawn spacecraft, with its novel ion propulsion system, has completed its exploration of Vesta and is now in orbit around Ceres. The two massive asteroids could not be more different. Vesta (~500 km diameter) is a rocky body, with a large metallic core, an ultramafic mantle, and a basaltic crust. Large impacts have excavated samples of the crust and mantle which occur as abundant achondritic meteorites, and which are used to calibrate Dawn instruments and improve data interpretations. The mapped surface of Vesta reveals unexpected geologic features and contamination of the regolith by exogenic carbonaceous chondrite. Dawn data suggest that olivine was sequestered in the lower mantle, likely as a residue from incomplete melting to form an early magma ocean. The early petrologic evolution of Vesta was surprising complex. Ceres (~940 km diameter) is an icy body with a rocky interior. Craters are under-abundant relative to model expectations, and smooth features suggest terrane softening. Thermal alteration has produced a variety of hydrous minerals that resemble assemblages in altered carbonaceous chondrites, although no meteorite samples from Ceres have been recognized (or are expected). The results of spectral and geochemical analyses will be presented. Our own planet is thought to have accreted from differentiated protoplanets like Vesta and Ceres, and the Dawn mission allows us to travel back in time before the present solar system was assembled.