AN INVESTIGATION OF THE BEHAVIOR OF VOLATILES ON VESTA AND CERES

Prior to the Dawn mission, Vesta was thought to be depleted in volatiles while Ceres was predicted to be volatile rich (Russell & Raymond, 2011). Ceres was confirmed to be volatile rich, in the form of water ice and postulated clathrates (e.g. Combe+, 2016; Fu+, 2017; Schmidt+, 2017). However, the discovery of pitted terrain (Denevi+, 2012), curvilinear gullies and lobate deposits in Vestan impact craters (Scully+, 2015) indicated that Vesta, at least locally, might not be as depleted in volatiles as originally thought. Scully+ (2015) hypothesized that localized deposits of subsurface water ice were heated by impacts, releasing liquid water onto the walls of newly formed impact craters. This liquid water would not be stable, but was proposed to be transiently present for a sufficient time to form curvilinear gullies and lobate deposits via a debris-flow-like process. Moreover, vaporization of the liquid water was suggested to form the pitted terrain. Similar geomorphological features have also been observed on Ceres (Sizemore+, 2017). We evaluate the Scully+ (2015) hypothesis via laboratory experiments, which investigate the behavior of liquid water under Vestan/Cerean conditions shortly following an impact (e.g. transient atmospheric pressures of ~10^-4-10^-5 torr). We suddenly expose liquid water or brine to 10^-4-10^-5 torr, simulating the sudden release of liquid onto the crater walls. The liquid is near to triple point conditions and is not stable. Thus, we measure the time it takes for the liquid to be lost to the gaseous or solid states, to investigate if liquid water would be present long enough to form the pitted terrain, curvilinear gullies and lobate deposits. We test whether the liquid composition, and the presence or absence of particles of varying sizes and shapes, effects its longevity. We also present results of our geomorphological analyses of curvilinear gullies, lobate deposits and pitted terrain in specific Vestan impact craters: we compare these features to those on Ceres and discuss additional geomorphological indicators for the presence of volatiles. This work is funded by NASA ROSES DDAP under grant 16-DDAP16_2-0016. Part of this work is being carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. Government sponsorship acknowledged.