DAWN AT CERES & VESTA: CRATER MORPHOLOGIES & PROCESSES IN ICE & ROCK ON LOW-GRAVITY WORLDS

Dawn mapping of Vesta (to 20 m pixel scales) and Survey mapping of Ceres (currently down to ~400 m) reveals a rich variety of well-preserved impact crater morphologies. Ceres and Vesta are the smallest worlds on which we observe complex craters to date. Both Ceres and Vesta have surface gravities ~1/6^th that of the dry Moon, providing a unique opportunity to compare impact crater processes on dry (Vesta) and wet (Ceres) worlds under similar gravity regimes, and on relatively dry silicate worlds (Vesta v. Moon) in different gravity regimes. Complex craters are >65 km across on Vesta but transitional morphologies occur down to ~30 km, smaller than expected from simple g^-1 scaling. Crater depths are ~10% deeper on Vesta than the Moon, but rim heights may be 25-40% greater. Morphologies are otherwise indistinguishable, except for the virtual lack of impact melt/debris deposits on the floors of Vesta craters. Asymmetric textures and swirl patterns in Vesta ejecta are similar to those on lunar craters, but more common, probably due to the ubiquitous steep slopes on Vesta. Secondary craters form on Vesta but rarely as well-organized clusters or fields. Survey-orbit imaging of Ceres show well-organized secondary fields around larger craters, including spectacular sets of arcuate radiating crater chains similar to those seen occasionally on icy worlds, indicating that secondary crater formation approximately scales with gravity. Crater depths and simple-complex transitions on Ceres are all smaller than on Vesta but consistent with those of similar size/density Saturn moons Dione and Tethys, with some subtle differences. Smooth floor deposits are common on Ceres but absent on Dione/Tethys. These may be (currently unresolved) impact melt/debris sheets. Central peak heights and widths are comparable on Ceres and Dione/Tethys but the transition to central pit morphologies occurs at much smaller diameters on Ceres. These measurements confirm that the outer zone of Ceres is dominated rheologically by water ice (or a material that behaves in similar ways). The few large basins on Ceres are also different from Dione/Tethys in morphology but these may be related to a thinner or more compositionally heterogenous ice-rich mantle on Ceres.