A qualitative study of the retention and release of volatile gases in JSC-1A lunar soil simulant at room temperature under ultrahigh vacuum (UHV) conditions

Abstract

We conducted a qualitative study to simulate the flux of volatile gases expected to occur at the lunar surface due to cometary impact or lunar outgassing events. A small sample cell containing 8.8g of JSC-1A lunar soil simulant in a vacuum system with a base pressure of 1.5×10−8Torr was exposed to various gases using dynamic pressure dosing at room temperature to observe any retention of those gases as a function of the exposure times, temperatures and pressures used. Gases included pure argon, a five-component gas mixture (H2, He, Ne, N2, Ar), a simulated Mars atmospheric mixture (CO2, N2, Ar, CO, O2), and a simulated Titan mixture (N2, CH4). Results at exposure pressures of approximately 1.5×10−8Torr above background showed no observable retention of rare gases, slight retention of molecular gases, but surface retention of the triatomic gas CO2 occurred at room temperature with a time to reach equilibrium of greater than 10min, which was an unanticipated result. Despite several bakeouts and months under ultrahigh vacuum (UHV) conditions, trace levels of atmospheric gases continued to evolve from the simulant. Mechanical and optical probing of the simulant surface increased this latent gas evolution, particularly for CO2 and CO, with some evidence also for the release of CH4. We assert our results are, by analogy, applicable to protocols and instrumentation needed for conducting analytical chemistry aboard future landed lunar missions.