Fire and ice: GRaND observations of Vesta and Ceres by Dawn
Abstract
In June of 2016, Dawn completed its primary mission to explore the asteroid (4) Vesta and the dwarf planet (1) Ceres, the largest bodies in the main belt. At both targets, Dawn's Gamma Ray and Neutron Detector (GRaND) acquired several months of global elemental mapping data. Gamma ray and neutron spectra were analyzed to determine the bulk elemental composition of the uppermost meter of the regolith within broad surface regions. Measurements of global Fe/Si, Fe/O, and K/Th ratios buttress the connection between Vesta and the Howardite, Eucrite and Diogenite (HED) meteorites. Their parent body underwent igneous differentiation to form an iron-rich core, ultramafic mantle, and basaltic crust. In some regions, GRaND measurements show that hydrogen is concentrated in Vesta's otherwise anhydrous, basaltic regolith. Multiple lines of evidence support exogenic delivery of hydrogen by the infall of carbonaceous chondrites. In comparison, the regolith of Ceres contains orders of magnitude more hydrogen, which must have originated from within. Near Ceres' equator, the elemental composition of the regolith resembles the aqueously altered CI/CM chondrites (based on H, Fe, K, C and compositional parameters). Increased concentrations of hydrogen observed by GRaND at high latitudes likely result from the presence of extensive near-surface water ice, as anticipated by Fanale and Salvail (1989). GRaND data provide evidence that the action of water altered Ceres on a global scale, resulting in widespread hydrated minerals and residual water ice that has survived to the present day. We describe how GRaND elemental measurements constrain chemical and physical processes that shaped Ceres and discuss implications for the origins of Ceres and similar main belt objects.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.
- Publication:
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AAS/Division for Planetary Sciences Meeting Abstracts #48
- Pub Date:
- October 2016
- Bibcode:
- 2016DPS....4840701P