Constraints on Ceres' Internal Structure and Evolution From Its Shape and Gravity Measured by the Dawn Spacecraft

Abstract

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.

Normal Faults on Ceres: Insights Into the Mechanical Properties and Thermal History of Nar Sulcus

“…At this strain rate, we note that all but one of the f s = 0.3 and only one of the f s = 0.56 curves in Figure 3 fall below F = 100 mW/m 2 . We estimate an upper bound of~30 vol.% mechanically silicate-like phases within the mechanical layer at Nar Sulcus, which is broadly consistent with Fu et al's (2017) and Ermakov et al's (2017) estimate of the global rock volume fraction within the upper layer of Ceres. We estimate an upper bound of~30 vol.% mechanically silicate-like phases within the mechanical layer at Nar Sulcus, which is broadly consistent with Fu et al's (2017) and Ermakov et al's (2017) estimate of the global rock volume fraction within the upper layer of Ceres.…”

Conditions for the Long‐Term Preservation of a Deep Brine Reservoir in Ceres

“…Strong phases are required to reproduce the strength of the crust, which Fu et al (2017) inferred to be a mixture of phyllosilicates, salt hydrates, and gas hydrates (i.e., clathrate hydrates). A mixture of 10 vol.% silicates, 20 vol.% hydrated salts, and 30 vol.% ice and 40 vol.% clathrates matches the average crustal density derived by Ermakov et al (2017) and is consistent with the strength estimates from Bland et al (2016) and Fu et al (2017). Geochemical modeling suggests there should be no more than 20 vol.% salts in the crust and the averaged density of these salts, composed of carbonates and chlorides, is about 2,200 kg/m 3 .…”

“…Dehydration then encompasses a large Geophysical Research Letters 10.1029/2018GL081473 volume of the mantle (>300 km, see also Castillo-Rogez & McCord, 2010) and thus is not consistent with the average mantle density of ∼2,400 kg/m 3 inferred by Ermakov et al (2017). Dehydration then encompasses a large Geophysical Research Letters 10.1029/2018GL081473 volume of the mantle (>300 km, see also Castillo-Rogez & McCord, 2010) and thus is not consistent with the average mantle density of ∼2,400 kg/m 3 inferred by Ermakov et al (2017).…”

“…Hanami Planum is also characterized by a thicker crust than average (∼50 km, Ermakov et al, 2017). In particular, the Hanami Planum region displays Ceres' most ancient terrains and mildly relaxed geological features in comparison to the surrounding planitiae.…”

The formation and evolution of bright spots on Ceres

“…By latitude, the faculae are relatively evenly distributed within the ±55 °latitude bounds of our study area and are prevalent throughout both Vendimia Planitia and Hanami Planum. There is no clear correlation between facula concentration and crustal thickness, although the extensive Cerealia Facula is located in the thickest portion of the crust ( Ermakov et al, 2017 ). Faculae appear associated with many, but not all (see Sections 3.1.1 and 3.2.1 ), of the least-degraded craters and with some older basins.…”