Is Ceres Differentiated?

The dwarf planet Ceres is the largest body in the asteroid belt, and has a density of 2.0-2.3 g/cm3, and a dark non-icy surface with signs of hydrated minerals [1-3]. As opposed to a differentiated internal structure with a nonporous rocky core and a water ice mantle [1-3], there are arguments for a slightly differentiated or undifferentiated porous interior with a mineral composition similar to CI/CM carbonaceous chondrites. Ceres' shape and dimensions reported in [3] may imply a slightly differentiated or even undifferentiated interior. Ceres' internal pressures (< ~150 MPa) are insufficient to significantly reduce porosity and microporosity of accreted chondritic materials. Thus, there is no need for abundant ice to be present to account for the density of Ceres. An existence of a rocky layer atop a water mantle is unfavorable owing to gravitational instability [1]. Observed hydrated surface materials are not consistent with the unaltered nature of the rocky layer modeled in [1]. If such a layer sunk into the water mantle, subsequent sublimation of an icy shell would have led to abundant surface salt deposits, which are not observed. Later accumulation of a layer of hydrated minerals at the surface may not be consistent with the ice-bearing surfaces of such bodies as Callisto, and with cosmic dust accumulation rates on inner solar system bodies. Therefore, the surface material could represent hydrated planetesimals from which Ceres accreted, and/or reflects in situ aqueous alteration, which is less likely. Ceres could have formed from pervasively aqueously processed low-density asteroids (largely C- and G-types) in which 26Al had largely decayed. Abundant water ice may not have been accreted. The weak heat from 40K and 232Th decay would not have caused mineral dehydration and density stratification of the interior [1]. However, evaporation of limited aqueous solutions and warm ice sublimation caused water redistribution in the porous interior, some re-condensation and escape to space. Corresponding formation of light-toned salts may account for Ceres' elevated albedo compared to carbonaceous chondrites [cf. 1]. It is also possible that Ceres and spectrally similar asteroids are rich in low-density C-H-O-N species compared to CI/CM chondrites, which are depleted in them compared to comets. Refs.: [1] McCord T.B., Sotin C. (2005) J. Geophys. Res. 110, E05009. [2] Thomas P.C. et al. (2005) Nature 437, 224-226. [3] Carry B. et al. (2008) Astron. Astrophys. 478, 235-244.