An Impact Origin for Surface Minerals on Ceres

The dwarf planet Ceres is the largest body in the main asteroid belt with a hydrated dark rocky surface and an uncertain internal structure [1,2]. Spectra of Ceres in the near- and mid-infrared ranges show that surface materials may not contain abundant serpentine, saponite, sulfates, olivine, pyroxenes, and organic matter [2,3], which are common in carbonaceous chondrites. However, brucite, Mg carbonates, cronstedtite, and magnetite could be abundant and indicate aqueous processes [2,3]. The formation of abundant brucite, carbonates, and cronstedtite requires open-system low-temperature conditions characterized by elevated water/rock ratios and low fugacities of hydrogen and carbon dioxide. The observed mineralogy is more consistent with a near-surface origin than with a formation within Ceres or on planetesimals. The instability of aqueous solutions at the surface of Ceres implies mineral deposition during transient events of fluidal activity. But a warming of near-surface rocks by thermal processes in the interior requires dehydration of rocks, which is not consistent with the low density of Ceres. The lack of low-solubility sulfates in surface materials does not indicate percolation of interior fluids. Carbonate-bearing fluids may not percolate to the cold surface, especially if Ceres had undergone water-rock differentiation [1,4]. The lack of serpentine in surface materials does not indicate a formation of brucite through aqueous alteration of olivine-rich rocks. Though, the observed minerals could form in impact collisions of ice-rich targets and/or impactors. OH-bearing phases may condense from water-rich impact plumes [5]. Brucite and Mg carbonates could form through hydrolysis and carbonation of condensed MgO formed through evaporation of silicates. Apparently abundant carbonates may indicate an ample oxidation of organics. Ferric iron in magnetite and cronstedtite agrees with water-rich and oxidizing impact settings [5]. Turbulent and disequilibrium environments in impact plumes and surges could have led to deposition of minerals which typically do not form together (e.g., brucite and cronstedtite). Aqueous minerals could have formed in impact clouds, crater outflows, transient ice-covered crater lakes, and related hydrothermal systems. The observed clay-sized and spatially homogeneous surface materials [2] could be gravitationally sorted deposits of impact clouds and surges. The surface materials could have formed through impacts on an icy shell of a differentiated Ceres during the Late Heavy Bombardment (LHB) in the inner solar system, which affected may other asteroids [6]. However, mineral-forming processes during collisions of an undifferentiated and hydrated Ceres with water-rich bodies during LHB remain a possibility. A detection of fluidized crater outflows together with topography and composition of surface materials with Dawn will test this hypothesis. Refs: [1] McCord, T.B. et al. (2011) Space Sci. Rev. 163, 63-76. [2] Rivkin, A.S. et al. (2011) Space Sci. Rev. 101, 1-22. [3] Milliken, R.E., and Rivkin, A.S. (2009) Nature Geoscience 2, 258-261. [4] Castillo-Rogez, J.C., and McCord, T.B. (2010) Icarus 205, 443-459. [5] Gerasimov, M.V. et al. (2002) Deep-Sea Res. II 49, 995-1009. [6] Marchi, S. et al. (2013) Nature Geoscience, 6, 303-307.