LABORATORY EXPERIMENTS AND GEOMORPHOLOGICAL ANALYSIS TO TEST THE HYPOTHESIS THAT SHORT-LIVED, DEBRIS-FLOW-LIKE PROCESSES OCCURRED ON VESTA AND CERES

The discovery of pitted terrain (Denevi+2012), curvilinear gullies and lobate deposits (Scully+2015) in Vestan impact craters indicated that, surprisingly, Vesta might contain a significant amount of volatiles, at least on a local scale. Similar features are also observed on ice-rich Ceres. Scully+2015 hypothesize that impacts heated localized deposits of subsurface water ice, releasing liquid water onto the crater walls. This liquid water would be unstable, but transiently present for a sufficient time (~10s mins) to form curvilinear gullies and lobate deposits via a debris-flow-like process. Vaporization of the liquid water could also form pitted terrain. We evaluate this hypothesis via lab experiments and geomorphological analysis. For our lab experiments, we connect a main vacuum chamber to an antechamber via a large valve. We place the sample container in the antechamber, and vary the contents: pure water, brine, and pure water or brine mixed with glass beads. By opening the valve, we suddenly expose the liquid water/brine to pressures of 10^-4-10^-5 torr, simulating the sudden release of the liquid onto the impact crater walls. Conditions are near/below the triple point, and we measure the time it takes for the liquid to evaporate/freeze. Upon opening the value, pure water and pure water or brine mixed with glass beads rapidly change state to a solid and a gas. No liquid remains after ~4 secs. For NaCl brines, there is rapid evaporation/freezing of the top of the liquid once the value is opened. Then a solid lid forms on top of the brine, which remains liquid below. The underlying brine periodically breaches the solid lid, and freezes. No underlying liquid brine remains after ≥55 mins. A brine forming curvilinear gullies and lobate deposits is plausible on Ceres, because there are numerous salts available (DeSanctis+2016) to mix with impact-melted water. On Vesta, perhaps pre-impact water/rock interactions produce limited amounts of salts and brines. We are also searching our mosaics of ten Vestan and Cerean impact craters for geomorphological indicators for the presence of volatiles, including: channels, fractures whose formation is associated with volatiles, pits possibly formed by sublimation, mounds/domes formed in a pingo-like process and lobate landslides/ejecta interpreted to be rich in water ice.