A GEOMORPHOLOGICAL AND EXPERIMENTAL ANALYSIS OF THE BEHAVIOR OF VOLATILES ON VESTA AND CERES

The Dawn mission explored the two largest asteroid-belt objects: Vesta (diameter ~526 km) and Ceres (diameter ~940 km). Prior to Dawn’s arrival, Vesta was thought to be depleted in volatiles while Ceres was predicted to be volatile rich (e.g. Russell & Raymond, 2011). However, the discovery of pitted terrain (Denevi et al., 2012) and curvilinear gullies and lobate deposits in impact craters on Vesta (Scully et al., 2015) indicated that Vesta, at least locally, might not be as depleted in volatiles as originally thought. Scully et al. (2015) hypothesized that localized deposits of subsurface water ice were heated by impacts, releasing liquid water onto the walls of newly formed impact craters. Under impact-induced transient atmospheric conditions, this short-lived liquid water was proposed to form curvilinear gullies and lobate deposits via a debris-flow-like process. Vaporization of this short-lived liquid water, at the ends of the curvilinear gullies and lobate deposits, was further suggested to contribute to the formation of the pitted terrain. Similar geomorphological features have been observed on Ceres (e.g. Sizemore et al., 2017), and there is abundant evidence for water ice within Ceres’ crust (e.g. Combe et al., 2016; Schmidt et al., 2017). Here we evaluate the hypothesis of Scully et al. (2015), using both geomorphological analyses and laboratory experiments about the behavior of liquid water under low-pressure conditions. We present results of our geomorphological analyses of curvilinear gullies, lobate deposits and pitted terrain in specific Vestan impact craters, compare these features to those within Cerean impact craters and discuss further geomorphological indicators for the presence of volatiles within the craters of interest. We also discuss our experimental set-up, and present the results of our initial experiments, in which water and brine alone, and water and brine mixed with a variety of particle sizes and shapes, are exposed to pressures of 10^-4-10^-5 torr, which are equivalent to the predicted transient atmospheric pressures on Vesta and Ceres. 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.