Porosity-filling Metamorphic Brines Explain Ceres’s Low Mantle Density

Recent work has sought to constrain the composition and makeup of the dwarf planet Ceres’s mantle, which has a relatively low density, between 2400 and 2800 kg m −3 , as inferred by observations by the Dawn mission. Explanations for this low density have ranged from a high fraction of porosity-filled brines to a high fraction of organic matter. We present a series of numerical thermodynamic models that yield the mineralogy and fluid composition in the mantle as a function of Ceres’s thermal evolution. We find that the resulting phase assemblage could have changed drastically since the formation of Ceres, as volatile-bearing minerals such as serpentine and carbonates would partially destabilize and release their volatiles as temperatures in the mantle reach their maximum about 3 Gyr after Ceres’s formation. These volatiles consist mainly of aqueous fluids containing Na + and HS − throughout the metamorphic evolution of Ceres and, in addition, high concentrations of CO 2 at high temperatures relatively recently. The predicted present-day phase assemblage in the mantle, consisting of partially devolatilized minerals and 13–30 vol% fluid-filled porosity, is consistent with the mantle densities inferred from Dawn. The metamorphic fluids generated in Ceres’s mantle may have replenished an ocean at the base of the crust and may even be the source of the Na 2 CO 3 and NaHCO 3 mineral deposits observed at Ceres’s surface.