The geological nature of dark material on Vesta and implications for the subsurface structure

Abstract

a b s t r a c tDeposits of dark material appear on Vesta's surface as features of relatively low-albedo in the visible wavelength range of Dawn's camera and spectrometer. Mixed with the regolith and partially excavated by younger impacts, the material is exposed as individual layered outcrops in crater walls or ejecta patches, having been uncovered and broken up by the impact. Dark fans on crater walls and dark deposits on crater floors are the result of gravity-driven mass wasting triggered by steep slopes and impact seismicity. The fact that dark material is mixed with impact ejecta indicates that it has been processed together with the ejected material. Some small craters display continuous dark ejecta similar to lunar dark-halo impact craters, indicating that the impact excavated the material from beneath a higher-albedo surface. The asymmetric distribution of dark material in impact craters and ejecta suggests non-continuous distribution in the local subsurface. Some positive-relief dark edifices appear to be impact-sculpted hills with dark material distributed over the hill slopes. Dark features inside and outside of craters are in some places arranged as linear outcrops along scarps or as dark streaks perpendicular to the local topography. The spectral characteristics of the dark material resemble that of Vesta's regolith. Dark material is distributed unevenly across Vesta's surface with clusters of all types of dark material exposures. On a local scale, some craters expose or are associated with dark material, while others in the immediate vicinity do not show evidence for dark material. While the variety of surface exposures of dark material and their different geological correlations with surface features, as well as their uneven distribution, indicate a globally inhomogeneous distribution in the subsurface, the dark material seems to be correlated with the rim and ejecta of the older Veneneia south polar basin structure. The origin of the dark material is still being debated, however, the geological analysis suggests that it is exogenic, from carbon-rich low-velocity impactors, rather than endogenic, from freshly exposed mafic material or melt, exposed or created by impacts.

Global variations in regolith properties on asteroid Vesta from Dawn's low‐altitude mapping orbit

“…The region we interpret to have thicker regolith is host to a large fraction of the craters that have lowreflectance material in their walls and ejecta , including spectacular layering of lowreflectance material exposed in the wall of Marcia crater <1 km below the rim Jaumann et al 2014;Williams et al 2014). A regolith depth often >1 km in this area supports the idea that the complicated stratigraphy of high-and low-reflectance material in the region is at least in part due to layering within the regolith from impact mixing of exogenic carbonaceous chondrite material.…”

Mineralogical and spectral analysis of Vesta’s Gegania and Lucaria quadrangles and comparative analysis of their key features

“…This could be ascribed to the fact that the fracture allows for observation the Vesta subsurface. The CC material delivered by impacts could be more dispersed within the subsurface (Jaumann et al, 2014), and this would explain the lower CC content in this DU.…”

“…We discard the latter hypothesis since the exposed materials excavated from depths of a few hundred meters is often dark or brighter than the upper crust (Jaumann et al, 2014). Thus the probability to have a similar composition between surface and subsurface material is very low.…”

Olivine on Vesta as exogenous contaminants brought by impacts: Constraints from modeling Vesta's collisional history and from impact simulations

“…The possibility of an exogenous origin for the detected olivine is indirectly supported by the comparison of its geologic context with that of the dark material on Vesta, also identified as an exogenous contaminant (McCord et al 2012;Prettyman et al 2012;Reddy et al 2012;Turrini et al 2014;Jaumann et al 2014). As with the currently detected olivine-rich sites, the dark material appears both as surface deposits and as buried veneers, both in clear association with craters and their ejecta and as isolated spots (in some cases on topographic heights) not associated to specific craters (McCord et al 2012;Reddy et al 2012;Jaumann et al 2014). As also pointed out by Ruesch et al (2014a) for the case of the olivine deposits on Rheasilvia's rim, the existence of such isolated spots is a direct consequence of Vesta's low gravity, which allows for ballistic transport of the projectile's material on a global scale (the latter effect also enhanced by Vesta's fast rotation as pointed out by Jutzi et al 2013).…”