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Ceres is the largest body in the asteroid belt with a radius of approximately 470 km. In part due to its large mass, Ceres more closely approaches hydrostatic equilibrium than major asteroids. Pre-Dawn mission shape observations of Ceres revealed a shape consistent with a hydrostatic ellipsoid of revolution. The Dawn spacecraft Framing Camera has been imaging Ceres since March 2015, which has led to high-resolution shape models of the dwarf planet, while the gravity field has been globally determined to a spherical harmonic degree 14 (equivalent to a spatial wavelength of 211 km) and locally to 18 (a wavelength of 164 km). We use these shape and gravity models to constrain Ceres' internal structure. We find a negative correlation and admittance between topography and gravity at degree 2 and order 2. Low admittances between spherical harmonic degrees 3 and 16 are well explained by Airy isostatic compensation mechanism. Different models of isostasy give crustal densities between 1,200 and 1,400 kg/m3 with our preferred model giving a crustal density of 1,287+70−87 kg/m3. The mantle density is constrained to be 2,434+5−8 kg/m3. We compute isostatic gravity anomaly and find evidence for mascon-like structures in the two biggest basins. The topographic power spectrum of Ceres and its latitude dependence suggest that viscous relaxation occurred at the long wavelengths (>246 km). Our density constraints combined with finite element modeling of viscous relaxation suggests that the rheology and density of the shallow surface are most consistent with a rock, ice, salt and clathrate mixture.

Ceres is the largest asteroid belt object, and the Dawn spacecraft observed Ceres since 2015. Dawn observed two morphologically distinct linear features on Ceres’s surface: secondary crater chains and pit chains. Pit chains provide unique insights into Ceres’s interior evolution. We interpret pit chains called the Samhain Catenae as the surface expression of subsurface fractures. Using the pit chains’ spacings, we estimate that the localized thickness of Ceres’s fractured, outer layer is approximately ≥58 km, at least ~14 km greater than the global average. We hypothesize that extensional stresses, induced by a region of upwelling material arising from convection/diapirism, formed the Samhain Catenae. We derive characteristics for this upwelling material, which can be used as constraints in future interior modeling studies. For example, its predicted location coincides with Hanami Planum, a high-elevation region with a negative residual gravity anomaly, which may be surficial evidence for this proposed region of upwelling material.

Dawn at Ceres Guest Investigator Program award [NNX15AI29G]; NSF Graduate Research Fellowship award [DGE-1143653]

Due to the small current obliquity of Ceres (e≈4°), permanently shadowed regions (PSRs) exist on the dwarf planet's surface. Since the existence and persistence of the PSRs depend on the obliquity, we compute the obliquity history over the last 3 Myr and find that it undergoes large oscillations with a period of 24.5 kyr and a maximum of emax≈19.6°. During periods of large obliquity, most of the present-day PSRs receive direct sunlight. Some craters in Ceres's polar regions possess bright crater floor deposits (BCFDs). We find an apparent correlation between BCFDs and the most persistent PSRs. In the north, only two PSRs remain at emax and they both contain BCFDs. In the south, one of the two only craters that remain in shadow at emax contains a BCFD. The location of BCFDs within persistent PSRs strongly suggests that BCFDs consist of volatiles accumulated in PSR cold traps: either water molecules trapped from the exosphere or exposed ground ice.