Dielectric Properties of Vesta's Surface as Constrained by Dawn VIR Observations

摘要

Earth and orbital-based radar observations of asteroids provide a uniqueopportunity to characterize surface roughness and the dielectric properties oftheir surfaces, as well as potentially explore some of their shallow subsurfacephysical properties. If the dielectric and topographic properties of asteroid'ssurfaces are defined, one can constrain their surface textural characteristicsas well as potential subsurface volatile enrichment using the observed radarbackscatter. To achieve this objective, we establish the first dielectric modelof asteroid Vesta for the case of a dry, volatile-poor regolith--employing ananalogy to the dielectric properties of lunar soil, and adjusted for thesurface densities and temperatures deduced from Dawn's Visible and InfraRedmapping spectrometer (VIR). Our model suggests that the real part of thedielectric constant at the surface of Vesta is relatively constant, rangingfrom 2.3 to 2.5 from the night- to day-side of Vesta, while the loss tangentshows slight variation as a function of diurnal temperature, ranging from 0.006to 0.008. We estimate the surface porosity to be ~55% in the upper meter of theregolith, as derived from VIR observations. This is ~12% higher than previousestimation of porosity derived from previous Earth-based X- and S-band radarobservation. We suggest that the radar backscattering properties of asteroidVesta will be mainly driven by the changes in surface roughness rather thanpotential dielectric variations in the upper regolith in the X- and S-band.