Polar Regolith Environment Molecular Impact Simulation Experiment (PREMISE), 15-R8241
Edward L. Patrick
F. Scott Anderson
Inclusive Dates: 07/01/11 – 1/02/13
Background — Evidence for water found in the polar regions of the Moon (LRO/LCROSS) and Mercury (MESSENGER) has triggered a renaissance in the study of these regolith environments. In the case of the Moon, the surface has not been probed by mass spectrometry since the Apollo 17 mission more than 40 years ago. Water at the lunar surface provides an essential natural resource needed for extended human presence and also provides a source for the production of hydrogen (H2) and oxygen (O2) for fuel cells or as a spacecraft propellant. The motivation for this project was the interest in simulating the lunar surface environment and how water (H2O) and other volatiles (CO2, CH4, etc.) are trapped there, and also to determine the best sample handling protocol to use at the lunar surface to release those volatiles to a landed mass spectrometer instrument.
Approach — A quantity of JSC-1A lunar soil simulant was deposited into a custom-fabricated, gold-plated copper sample cell and placed within a vacuum system where the simulant was desiccated by bakeout in excess of 100°C and evacuated until a base pressure of 1x10-8 Torr was obtained. A residual gas analyzer (RGA) quadrupole mass spectrometer (QMS) with 0 to 300 dalton mass resolution was integrated to the system to provide a log of the simulated lunar exosphere appearing above the surface of the simulant. A UV laser was used to test laser ablation of the simulant surface as a means of producing detectable gas plumes at the mass spectrometer. The original intent was to cool the sample cell to cryogenic temperatures to simulate surface conditions at the lunar poles. However, after exhaustive tests, persistent atmospheric gas peaks appearing in the mass spectrum of the chamber at room temperature led to the conclusion that these gases were evolving from the lunar simulant. Laser ablation was attempted as a means to produce increased signal at the mass spectrometer, but the intensity of the ablation plume was so great as to require abandoning the method. Subsequent inspection of the laser-ablated surface with a flashlight produced an inexplicable jump in background pressure. Despite the presence of persistent gas peaks, pure gases and gas mixtures were introduced to the simulant through a leak valve to produce, in a typical experiment, chamber pressures of approximately 3x10-8 Torr for a total of 10 minutes. Some of the gases showed no persistence based upon mass spectrometer scans. However, other gases did show persistence and suggested that adsorption of the molecules took place at the surface of one or more mineral phases in the grains. All of these tests were performed at room temperature, suggesting that an increased residence time exists for some gases within the lunar regolith with a potential for affecting volatile evolution even at temperatures far above those found at the lunar poles.
Accomplishments — Results of tests exposing the simulant to various gases and to photon sources are currently pending publication. A preliminary design for a prototype lunar mass spectrometer was completed to support future lunar mission proposals, and the results have implications for future studies of the lunar surface using mass spectrometers. Results from this experiment supported submission of a proposal to the NASA Research Opportunities in Space and Earth Sciences (ROSES) Lunar Advanced Science and Exploration Research (LASER) program and will also contribute to the submission of another LASER study proposal in February 2014.