Analysis of Active Neutron Measurements From the Mars Science Laboratory Dynamic Albedo of Neutrons Instrument: Intrinsic Variability, Outliers, and Implications for Future Investigations. Issue 5 (5th May 2020) - Kerner, H. R.
Abstract: The Dynamic Albedo of Neutrons (DAN) instrument onboard Mars Science Laboratory uses neutron die‐away, an active nuclear spectroscopy technique, to measure the abundance and depth distribution of hydrogen and neutron‐absorbing elements (e.g., Fe and Cl) in the top ∼0.5 m of the Mars subsurface. We examined the intrinsic variability in neutron die‐away data using simulated DAN measurements with a range of compositions relevant to equatorial and high‐latitude environments on Mars. Our analysis shows that the total neutron counts and timing of neutron arrival explain most variability in thermal neutron die‐away curves for a DAN‐like instrument configuration over homogeneous subsurfaces. We analyzed the variability in thermal neutron die‐away across a variety of elemental compositions that might be observed by DAN or future instruments. We found that when H concentration is low (e.g., in equatorial regions including Gale crater), neutron die‐away is most sensitive to variations in H. Conversely, when H concentration is high (e.g., in poleward regions of Mars or icy bodies), neutron die‐away is most sensitive to variations in neutron absorbers. We estimated that the H and neutron absorber concentrations in DAN measurements acquired Sols 1–2080 in Gale crater and found that most measurements had low H and moderate absorber content. We presented an outlier analysis of these measurements to identify targets with high potential scientific impact and found that most outliers along the traverse corresponded with abnormally low or high neutron absorber content measurements, some of which may be associated with high‐silica fracture‐associated halos and felsic igneous material. Plain Language Summary: The Dynamic Albedo of Neutrons (DAN) instrument on the Mars Science Laboratory (MSL) Curiosity rover uses active nuclear spectroscopy to measure the abundance and depth distribution of hydrogen, chlorine, and iron in the Martian subsurface. We presented the first comprehensive study of the ways in which active neutron spectroscopy measurements and the sensitivity vary with changes in elemental composition and found that this technique is most sensitive to changes in hydrogen when the abundances of these elements in the subsurface are relatively low, as in Gale Crater. In contrast, we found that when hydrogen abundance is relatively high (e.g., in poleward regions of Mars or on icy bodies), this technique is most sensitive to changes in iron and chlorine (and other neutron‐absorbing elements). In addition, we performed an outlier analysis of all DAN measurements acquired between Sols 1 and 2080 and found 46 outliers that correspond to rocks and soils with anomalous iron and chlorine content that may indicate targets of particular scientific interest. We plan to use the presented outlier detection method to quickly analyze new measurements as they are acquired by DAN on MSL and identify those with potentially high scientific value before the rover drives away from each site. Key Points: We analyzed the variability in active neutron data due to changes in H, Cl, and Fe on a planetary surface We used outlier detection methods to highlight anomalously low/high Fe and Cl content in Gale crater We found that active neutron measurements are most sensitive to variations in H content when H, Cl, and Fe contents are low