Exploring ceres's unusual regolith porosity and its implications for volatile retention

Abstract

Asteroid Ceres, the only dwarf planet located in the inner solar system, shows unique surface mineralogy and geomorphology as observed by the Dawn mission. Of particular interest is understanding the role that upper regolith porosity plays in retaining volatiles and shaping Ceres' surface. Unfortunately, Ceres' near-surface porosity remains largely uncharacterized, compromising the ability to quantify volatile occurrence and identify the mechanisms for volatile retention at shallow depths, a topic of ongoing debate. Herein, we estimate Ceres' shallowsubsurface porosity by reinterpreting existing S- and X-band Earth-based radar observations combined with dielectric laboratory measurements of analog materials that have been recently suggested by spectral observations from the Dawn VIR spectrometer and in the far-ultraviolet from the Hubble telescope. Contrary to previous assumptions, our results suggest that Ceres' surface is more porous than the lunar regolith, with a bulk porosity ranging from ~53% to 72% or even higher in the top meter of the regolith, as opposed to ~39% to 50% for both bodies. The above suggests that Ceres' regolith is on average 15% more porous than the Moon, hence explaining its higher potential for volatile retention. We propose that lofting and gradual redeposition of fine particles by avalanches, continuous micrometeorite bombardment, and localized volatile outgassing are possible mechanisms for generating a globally high-porosity regolith. In addition to Ceres' proximity to the snowline of the early solar system, such a highly porous regolith may explain its efficiency at retaining volatiles at shallow depths into the present, as revealed by Dawn's GRaND observations.