FIRST RESULTS OF DAWN'S INVESTIGATION OF THE SURFACE COMPOSITION OF VESTA

By October 7, 2011, the Dawn spacecraft mission will have completed its Survey orbit (3000 km from the body’s center of mass) and commenced high-altitude (950 km) mapping of 4Vesta, a large asteroid that experienced igneous differentiation and is thought to be the source of howardite-eucrite-diogenite (HED) meteorites. Dawn’s three payload instruments – Framing Camera, Visible and Near-Infrared Spectrometer, and Gamma Ray and Neutron Detector – are sensitive to the mineralogical and geochemical compositions of Vesta’s surface. Preliminary spectral mapping has revealed prominent color variations that imply compositional differences and/or different degrees of space weathering. Survey spectroscopic results will provide better information on the global distribution of pyroxenes and olivine at spatial scales <700 m/pixel. Information on the presence of surficial OH/H₂O will be determined by spectroscopic data across 3 microns. Data on surface hydrogen may indicate the effects of space weathering, as on the Moon. Imaging and spectroscopic mapping at higher spatial resolution of 100-200 m, providing geologic context for the HEDs, will occur during October. Geochemical analysis results for Mg, Si, Fe, K, H, and Th to 300 km resolution by gamma-ray/neutron spectroscopy must await low-altitude (450 km) measurements beginning in December. Other elements not directly measured can be estimated from models of neutron absorption and constraints from HED mixing models. Together, the geochemical and mineralogical data should be sufficient to confirm Vesta as the source of HEDs, identify and map terranes dominated by the various volcanic and plutonic lithologies, search for materials not represented in the HED collection, and model the proportions of lithologies comprising the regolith and the crust. Spectroscopic mapping of a huge south polar structure and impact craters may reveal exposures of mantle or lower crustal materials. Dawn’s orbital data collected during a year in Vestan orbit, along with petrologic, geochemical, and isotopic data from HEDs, will also be used to constrain the bulk composition, thickness of the crust and mantle, mass of the core, chronology, and magmatic evolution of this lone surviving, large differentiated protoplanet.