The Impact of Thermal Neutron Scattering and Temperature Variations on Martian Water Content Estimates with Dynamic Albedo of Neutrons

The Dynamic Albedo of Neutrons (DAN) instrument onboard the Curiosity rover uses neutron interrogation to characterize the hydration and macroscopic thermal neutron absorption cross section of Martian soil. Its data analysis requires comparing the measured die-away curves to Monte Carlo simulations. Earlier DAN studies have provided water estimates that generally agree with other Curiosity instruments that heat samples to release water or directly measure chlorine, and have helped identify unusually dry dune sands and buried, silica-rich layers in Gale crater. Here, we investigate for the first time how using cross section data that account for low soil temperatures and thermal neutron scattering impact the estimates of the amount of water and neutron absorbers in the soil. We model a generic neutron generator and detector geometry and generate sets of simulated die-away curves using cross sections for realistic Martian soil temperatures and thermal neutron scattering with hydrogen bound in liquid water and in water ice. We find that using these low-temperature and thermal scattering cross sections generally leads to higher inferred water-equivalent hydrogen (WEH), larger formal WEH uncertainties, and systematic shifts in the inferred macroscopic thermal neutron absorption cross section. These results do not invalidate previous DAN-based estimates, which used the then-standard nuclear data and modeling assumptions; rather, they quantify a systematic component that can now be addressed using recently developed and benchmarked thermal scattering data. These results further demonstrate the importance of the availability and use of relevant hydrogen TSL data for future planetary missions using the same pulsed-neutron technique.