SCALLOPED AND DEGRADED CRATERS AS GEOMORPHOLOGICAL EVIDENCE FOR PERVASIVE GROUND ICE ON CERES AS SEEN BY THE DAWN SPACECRAFT

Five decades of observations of Ceres’ albedo, surface composition, shape and density by Earth and space based telescopes suggest that Ceres is comprised of both silicates and several tens of percent water ice (McCord et al., 2010). Ceres’ bulk density of ~2100 kg/m³ (McCord and Sortin, 2005), the detection of OH and water emissions from the Herschel Space Observatory (Küppers et al., 2014), and recent Dawn observations of young craters containing high albedo spots all support this conclusion.

We report initial geomorphological evidence for sublimative, evaporative, and flow like processes within a wide range of craters on Ceres. We interpret these features as indications of a significant water ice component in Ceres’ surface or near subsurface. The craters we describe display a number of features indicative of the aforementioned processes, which include: asymmetrically scalloped rims that are morphologically reminiscent of scalloped terrain on Mars (Zanetti et al., 2010), degraded and recessed rims characterized by a high degree of mass wasting, and partial or completely circumferential pits located near the crater rims, which are followed by higher elevation material lobes towards the crater centers. These lobes appear similar to talus lobe and rampart features found in terrestrial glaciated terrains (Humlum, 1982). In some high latitude craters (~60° N and S), we observe lobate flows that emanate both inwardly and outwardly from “breached” rims that bear a striking similarity to terrestrial rock glaciers (Haeberli et al., 2006). Many of these high latitude craters also display symmetrical conical domes that frequently occur in clusters both on the crater floors and inward facing rims, and in some cases show evidence for high albedo or activity. These features could be due to local melt and extrusion via hydrologic gradients, forming domes similar to pingos (MacKay, 1998).

The global distribution, along with the latitudinal/regional variation in the diversity and prevalence of these craters suggest that ground ice is a key parameter of the geology on Ceres. It also suggests that ice content within the surface and near subsurface is either spatially varied and/or activated by energetic events.