INVESTIGATING VOLATILES ON VESTA AND CERES THROUGH VACUUM CHAMBER EXPERIMENTS

The Dawn orbiter observed pitted terrain, curvilinear gullies, and lobate deposits within impact craters on the asteroids Ceres and Vesta. These features are morphologically analogous to terrestrial features eroded by water, so they are hypothesized to form when subsurface ice deposits, uncovered and melted by the crater-forming impact, flow down slope in a debris-flow-like process (forming gullies and lobate deposits) and evaporate (forming pits). To test this hypothesis, we used a vacuum chamber to simulate the transient atmospheres created by crater-forming impacts, with pressures of 10^-5 torr. Liquid water is not stable at these conditions, so it will quickly freeze and/or evaporate. However, these processes do not happen instantaneously. Models suggest that water would need to exist in the liquid phase for at least ~30 minutes to create these features.

Temperature, pressure, mass, and video were recorded as samples of water were suddenly exposed to vacuum. Plateaus in the temperature are likely representative of the heat of fusion, which we used, along with video, to define the time of freezing. These observations were corroborated by, and supplemented with, analyses of the mass and pressure data for each trial. Freshwater was found to last as liquid for approximately four seconds. However, any potential water on Ceres, and perhaps Vesta, will most likely be a brine and filled with regolith particles. Thus, brines and liquids mixed with particles were also investigated to identify the bounds of the parameter space: the vacuum experiments were repeated with (1) NaCl solution at its eutectic composition (23.3% NaCl by mass) and (2) a slurry of glass particles with water filling the interstitial spaces to approximate a debris flow. The water in the slurry was found to last as liquid for approximately four seconds. However, the NaCl eutectic solution did not fully freeze until at least 55 minutes had elapsed. While more trials will be conducted with varying salts and other compositions, these preliminary results indicate that it is plausible for liquid water (brine) to exist long enough to drive formation of the observed features on Ceres and Vesta.