Carbon on Vesta and Ceres: Constraints from nuclear spectroscopy

The NASA Dawn mission's Gamma Ray and Neutron Detector (GRaND) mapped the elemental composition of (4) Vesta and the dwarf planet Ceres, providing constraints on their formation and interior evolution. Data from Vesta and Ceres were acquired under similar conditions and at the same relative distance (about 0.8 body radii altitude). This enabled direct comparison of cosmogenic neutron and gamma ray signatures sensitive to the elemental composition of the bulk regolith to depths of a few decimeters. Analyses of GRaND data show that Vesta's basaltic regolith is contaminated by a few hundred µg/g hydrogen from the infall of carbonaceous chondrite impactors, which implies that the regolith contains less than 1 mg/g C. In contrast, Ceres' regolith contains between 1.8-and 3.2-wt.% hydrogen in the form of aqueous alteration products and water ice. Carbon was detected in the analysis of the gamma ray spectrum above 4 MeV, which includes contributions from C, O, and Fe. Analyses of neutron data indicate Ceres' ice-free regolith contains higher concentrations of C than found in C-rich meteorites (>3.5 wt.%). The ice-free regolith contains more H and less Fe than CI carbonaceous chondrites, Ceres' closest meteorite analogs. The addition of 10-to 15-wt.% C as carbonates and organics can explain the difference between Ceres and the meteorites. Carbonates, organics, and graphitized carbon are found on the surface of Ceres [2-4]. Graphitization by ionizing radiation may limit the lifetime of organics on the surface. Thus, organics may be more pervasive than indicated by infrared spectroscopy [5]. Ceres and the CI chondrites likely formed from a common reservoir within the solar nebula. Consequently, super chondritic abundances of H and C within Ceres' regolith are consistent with many lines of evidence that show Ceres underwent ice-rock fractionation. The presence of carbon in a water-rich environment makes Ceres a desirable target for studies of prebiotic chemistry.