An aqueously altered carbon-rich Ceres

Abstract

The surface mineralogy of dwarf planet Ceres appears to be dominated by products of rock–fluid interactions, such as phyllosilicates—some of which are NH4-bearing—and carbonates1–3. Elemental concentrations derived from the inferred mineral mixing fractions, however, do not match measurements of H, C, K and Fe on Ceres4. A complicating factor in assessing Ceres’s unique surface composition is the secular accretion of asteroids typical of chondritic compositions. Here we show that Ceres’s mineral and elemental data can be explained by the presence of carbonaceous chondritic-like materials (~50–60 vol%), possibly due to infalling asteroids, admixed with aqueously altered endogenic materials that contain higher-than-chondritic concentrations of carbon. We find that Ceres’s surface may contain up to 20 wt% of carbon, which is more than five times higher than in carbonaceous chondrites. The coexistence of phyllosilicates, magnetite, carbonates and a high carbon content implies rock–water alteration played an important role in promoting widespread carbon chemistry. These findings unveil pathways for the synthesis of organic matter, with implications for their transport across the Solar System. Infrared and neutron spectroscopic observations by Dawn give contrasting results on the elemental composition of Ceres’s surface, which can be reconciled by assuming that Ceres’s surface contains ~20 wt% of carbon, coming from impacts by carbonaceous asteroids and/or generated by extensive aqueous alteration.