Remote Neutron Spectroscopy on Mars

Remote neutron spectroscopy is an important technique in planetary science that allows for classification of the amount of light elements in a planetary regolith. It is especially suited for studying hydrogen abundances and elements with high thermal neutron absorption cross sections in the top ~1 meter of regolith. The Mars Science Laboratory rover Curiosity carries the first rover based neutron spectrometer Dynamic Albedo of Neutrons (DAN) in Gale crater, Mars. As the DAN instrument operates in passive mode, it is sensitive to neutrons produced through Galactic Cosmic Ray interactions and neutrons generated by the rover's Multi-Mission Radioisotope Thermoelectric Generator. In this work, we develop an appropriate simulation strategy and data analysis methods to interpret passive data from the DAN instrument. Furthermore, the methods are used to estimate water equivalent hydrogen abundances in the shallow regolith of Gale crater along the traverse route of Curiosity from landing to the base of Mt. Sharp. Hydrogen is shown to have large variability on a scale of a few meters, much smaller than the spatial footprint of previous orbital investigations. Strong correlations between WEH content and surface properties are not observed. While in passive operation, DAN also observes diurnal variations in the martian neutron leakage fluxes. These diurnal variations are investigated and shown to possibly be a consequence of a combination of instrumental effects and environmental effects, most notably preferential shielding of alpha particles by the martian atmosphere leading to increased neutron production in the regolith as the surface atmospheric pressure changes throughout the sol.