Denevi, Brett - Global Variability in Regolith Properties on Vesta

Abstract: 
The Dawn spacecraft spent nearly 14 months in orbit around 4 Vesta, mapping the asteroid with a framing camera (FC), a visible and infrared mapping spectrometer (VIR), and Gamma Ray and Neutron Detectors (GRaND). Despite differences in the depth of sensitivity of each instrument, most of our remote sensing information about Vesta comes from its regolith. Regolith is a mixture of local material comminuted by numerous small impacts, material excavated by distant impacts, plus a small contribution of exogenic material. Knowledge of the origin, depth, and mobility of Vesta’s regolith is thus of consequence for a contextual understanding of results from each of Dawn’s instruments.
We investigate Vesta’s regolith with several methods developed previously for the Moon and asteroids that can provide depth constraints, and taken together can provide a fuller picture of Vesta’s regolith. These morphologic methods include assessing the presence or absence of coherent material within a crater’s ejecta, examining crater walls, studying morphologic evidence for downslope movement and infilling of topographic lows with regolith, and documenting the preservation of small craters (formed mostly within the regolith) at different locations. The combined results from these methods are then compared to predictions of regolith depth based on the ejecta distribution expected for a model crater production function.
From these studies, we find evidence for substantial regional variability in Vesta’s regolith. A large equatorial region from ~100–240° E (Claudia system) contains a relative dearth of craters <10 km in diameter that have excavated blocky material, suggesting a regolith thickness >~1 km thick, or a more mobile regolith which can more rapidly bury blocks. This area also contains fewer craters that expose material resistant to erosion in their walls, and has a lower retention of small craters (<~300 m) that formed largely within the regolith. These features are consistent with a thicker-than-average regolith in the region, which is marked by a low albedo and relatively high hydrogen abundance. A thicker regolith in this region is consistent with the suggestion that the low albedo and high hydrogen are due to the accumulation of exogenic, carbonaceous debris in ancient, well-preserved regolith. This is in contrast to the Rheasilvia basin, with high albedo and low hydrogen, where we find a high concentration of blocky craters, exposures of material resistant to erosion in craters walls, and a larger population of small impact craters. These results are compared to numerical models of regolith production. Potential sites are identified where local bedrock may be exposed and the spectral properties of these sites are explored.