The Dynamic Albedo of Neutrons (DAN) instrument onboard Mars Science Laboratory (MSL) is the first spacecraft-based active neutron investigation. DAN uses neutron die-away, a standard technique used in active nuclear spectroscopy, to measure the abundance and depth distribution of hydrogen and neutron-absorbing elements in the top $\sim$0.5 m of the Mars subsurface. We present the first comprehensive study of the intrinsic variability in rover-based neutron die-away data using simulated DAN measurements with a range of geochemistries relevant to equatorial and high-latitude environments on Mars. Our analysis shows that for thermal neutron die-away, the total neutron counts exhibit the greatest variance, followed by the timing of neutron arrival. We present an analysis of the sensitivity of these properties for a variety of elemental compositions that might be observed by DAN or future instruments. We found that neutron die-away is most sensitive to variations in H content when the abundances of H, Cl, and Fe are relatively low (e.g., in equatorial regions including Gale crater). When the abundance of H is high (e.g., in poleward regions of Mars or icy bodies), neutron die-away is most sensitive to variations in neutron-absorbing elements (e.g., Fe and Cl). Using this understanding of neutron die-away sensitivities and variability, we performed an outlier analysis of DAN measurements acquired between sols 1-2080. We found that most outliers along the Curiosity traverse correspond with measurements having abnormally low or high bulk neutron absorption cross sections, and interpretations may include high-silica fracture-associated halos and felsic igneous rocks or sediments.
Recommended citation: Kerner, H. R., Hardgrove, C. J., Czarnecki, S., Gabriel, T. S. J., Mitrofanov, I. G., Litvak, M. L., Sanin, A. B., Lisov, D. I. (2019). "Analysis of active neutron measurements from the Mars Science Laboratory Dynamic Albedo of Neutrons instrument: Intrinsic variability, outliers, and implications for future investigations." Under Review.