CURVILINEAR GULLIES, LOBATE DEPOSITS AND PITTED TERRAIN ON VESTA AND CERES FORMED BY TRANSIENT, BRINE-MOBILIZED FLOWS

Vesta is volatile-poor while Ceres has a volatile-rich crust, including salts [e.g. Fu+2017; De Sanctis+2016]. Unexpectedly, the discovery of pitted terrain [Denevi+2012], curvilinear gullies and lobate deposits [Scully+2015] in Vestan impact craters indicated that Vesta might contain locally significant amounts of volatiles. Similar features are also observed on Ceres [Sizemore+2017]. Scully+2015 hypothesize that localized deposits of subsurface water ice were heated by impacts, releasing liquid water onto the walls of newly formed impact craters: the unstable liquid water is proposed to be transiently present for tens of minutes and to form curvilinear gullies and lobate deposits via a debris-flow-like process. Vaporization could form pitted terrain [Denevi+2012]. We evaluate this hypothesis via (1) laboratory experiments and (2) by searching for other geomorphological indicators of the presence of volatiles.

Our lab set-up is a main cryogenic vacuum chamber and an antechamber, connected by a pneumatic valve. The main chamber is at a transient atmospheric pressure of 10^-4-10^-5 torr [Magni+2014]. Samples consist of: NaCl brine, pure water, NaCl brine or pure water mixed with glass beads or JSC-1A lunar simulant (broadly analogous to Vesta’s basaltic surface). We vary stirring/no stirring, stirring rate, concentration of NaCl salt, and grain size of the solid particles. Stirring provides motion, analogous to the debris-flow-like process proposed to occur. Samples are placed in the antechamber and are stable at the vapor pressure of water (~18 torr) prior to experiment initiation (consistent with liquid stability underneath the surfaces of Vesta/Ceres). By opening the valve, we expose the sample to 10^-4-10^-5 torr, simulating the sudden release of liquid onto crater walls. We measure the time it takes for the unstable liquid to boil/freeze, to test if liquid would be present long enough to form the curvilinear gullies and lobate deposits.

NaCl brine appears to enable flow longevity, and hence the formation of curvilinear gullies and lobate deposits. This is consistent with the availability of Cerean salts to mix with impact-melted water ice from the crust. Long-timescale water/rock interactions preceding the impacts may also produce restricted salts/brines on Vesta, similar to the work of Zandanel+2022.