MSL DAN Passive Data and Interpretations

In its passive mode of operation, The Mars Science Laboratory Dynamic Albedo of Neutrons experiment (DAN) detects low energy neutrons that are produced by two different sources on Mars. Neutrons are produced by the rover's Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) and by interactions of high energy galactic cosmic rays (GCR) within the atmosphere and regolith. As these neutrons propagate through the subsurface, their energies can be moderated by interactions with hydrogen nuclei. More hydrogen leads to greater moderation (thermalization) of the neutron population energies. The presence of high thermal neutron absorbing elements within the regolith also complicates the spectrum of the returning neutron population, as shown by Hardgrove et al. DAN measures the thermal and epithermal neutron populations leaking from the surface to infer the amount of water equivalent hydrogen (WEH) in the shallow regolith. Extensive modeling is performed using a Monte Carlo approach (MCNPX) to analyze DAN passive measurements at fixed locations and along rover traverse segments. DAN passive WEH estimates along Curiosity's traverse will be presented along with an analysis of trends in the data and a description of correlations between these results and the geologic characteristics of the surfaces traversed.