Paper No. 110-4
Presentation Time: 8:55 AM
ELEMENTAL CONSTRAINTS ON CERES' EVOLUTION
The Gamma Ray and Neutron Detector (GRaND) onboard the Dawn spacecraft globally mapped the elemental composition of Ceres’ regolith, providing new information about interior processes that shaped Ceres. The bombardment of surface materials by galactic cosmic rays makes gamma rays and neutrons. Gamma rays are also made by the decay of natural radioelements (K, Th, U) found in silicate minerals. At low altitude, GRaND measures the leakage spectrum of gamma rays and neutrons, which are sensitive to the composition of the uppermost meter of the regolith within broad spatial regions (several 100 km in scale). The data show that Ceres’ regolith is hydrogen rich, with an equatorial composition (Fe, K, C) similar to that of aqueously altered CI/CM chondrites. These meteorites contain hydrogen in the form of water and hydroxyl bound to phyllosilicate minerals with concentrations similar to that observed by GRaND. This is consistent with the inference by Dawn’s Visible and Infrared (VIR) Spectrometer that phyllosilicates are a major component of the surface. At high latitudes, the concentration of hydrogen is elevated, likely due to pervasive water ice very near the surface, as predicted by ice stability modeling (Fanale and Salvail, 1989). The elemental data support global-scale alteration of accreted material by the action of water within Ceres’ interior; however, due to Ceres’ large size, various processes such as convection may have separated brine-rich liquid from solid residues, resulting in chemical fractionation. If so, then the elemental composition of the regolith should deviate from altered carbonaceous chondrites, which formed in an environment with limited fluid flow. In this presentation, we describe the latest elemental measurements of Ceres by GRaND and show how these measurements constrain regolith processes and interior evolution.
Fanale, F. P. and J. R. Salvail (1989), The water regime of asteroid (1) Ceres, Icarus, Vol. 82, Issue 1, pp. 97-110, doi:10.1016/0019-1035(89)90026-2.