An Ice-rich Mantle on Ceres from Dawn Mapping of Central Pit and Peak Crater Morphologies

Abstract:

Dawn Survey mapping of Ceres at ~400 m pixel scales reveals a rich variety of well-preserved impact crater morphologies. Pre-arrival studies predicted that impact craters might resemble those on the midsize icy moons of Saturn (of similar size and density to Ceres), and might constitute a direct observational test of whether Ceres had an ice-rich interior. Central peak craters occur down to 10-15 km diameter, consistent with those predictions. Freshest crater depths on Ceres are also consistent with those on ice-rich Dione and Tethys. Scaling from Saturn’s moons suggested that central pit craters would not occur on Ceres. Central pit craters do occur on Ceres at ~50-150 km diameters, consistent with gravity scaling from the larger Galilean satellites. Pit dimensions (~0.15 of crater diameter) are indeed very similar to those on Ganymede, though they appear to be ~50% shallower on Ceres. The lack of pits on Saturn moons may be the anomaly, related to low internal temperatures or paucity of non-ice materials. Crater morphologies on Ceres thus indicate that the rheology of the outer layers of Ceres are dominated by ice (or materials that behave similarly; the contribution from non-ice phases is indeterminate). Impact morphologies deviate from pit morphology at >200 km diameters, indicating either impact into the core or effects of viscous relaxation. Arcuate crater chains, some likely secondary chains, extend from the large crater rims. More subtle differences between craters on Ceres and Ganymede/Dione/Tethys are expected as resolution increases during the mission, and indeed floor fractures and ‘smooth’ embayment materials are observed on Dantu and Occator. “Bright spot” crater Occator itself has no superposed impact craters at 400m pixel scales and likely formed or was resurfaced <100 myr ago. The largest bright spot within Occator is located within the central pit (fainter bright material also occurs within the central pit of Dantu crater and on the central peak of Urvara crater). The association of the most prominent bright spots with central pit structures suggests that either uplift or localized extraction of compositionally distinct material is involved in bright spot formation in the most strongly uplifted zones of large craters on Ceres, the details of which (currently) remain obscure.