Evidence for high porosity in the top few meters of Asteroid Ceres’ desiccated regolith

Abstract:

Dawn is the first spacecraft to visit Asteroid Ceres and spent nearly four years in orbit mapping Ceres’ topography with optical framing cameras (FC), identifying surface minerals with its onboard visible and infrared mapping spectrometer (VIR), and assessing the depth and quantity of ice beneath the surface using its onboard neutron and gamma ray spectrometer (GRaND). The widespread occurrence of extensive landslides, aqueously altered minerals and strong signatures of subsurface hydrogen occurrence have built a strong case for the existence of a water-ice table buried a meter or more at the equator and surfacing at the poles. The compaction of the upper few meters of dry regolith have not been thoroughly explored, however, especially considering that active landslide processes could be launching avalanche-like dust clouds that can settle with very loose packing. Given that Ceres has a much weaker gravitational environment than on the Moon, much more ice and an entirely different surface composition (carbonaceous rather than basaltic), many of our typical assumptions about lunar-like surface density for the regolith may therefore not be appropriate for Ceres.

Near-surface porosity can be inferred from radar observations when surface topography, wavelength-scale surface roughness and dielectric properties of the upper meter of regolith are constrained, but the last remains largely unknown for asteroids. To establish a dielectric map of Ceres’ surface, we measured the laboratory complex dielectric properties of Ceres regolith analog mixtures comprised of ~95 wt% anthracite (hard coal, a graphitized amorphous carbon) in desiccated form and mixed with water-ice. Earth-based S- and X-band radar observations of Ceres from 1984-2018 consistently measure a radar albedo of 0.037 ± 0.01 that suggests 1 ≤ ε_r' ≤ (2.2 ± 0.3). Laboratory dielectric characterization of anthracite suggests that the uppermost meter of Ceres’ dry regolith has a maximum compaction density of ≤ 0.53 (+0.08, –0.14) g cm^-3 in the uppermost meter. With a solid density of 1.4-1.7 g cm^-3 for anthracite coal, the porosity of Ceres’ regolith is calculated to be 63-78%, much looser than lunar soil. We also suggest that Ceres’ buried ice table is detectable by X- and S-band radar at latitudes >±60°.