Unfolding the neutron flux spectrum at the surface of Mars using the MSL-RAD and Odyssey-HEND data

Understanding the long-term radiation environment at the surface of Mars allows us to estimate the exposure to future robotic and crewed missions. Typically, the radiation environment includes charged particles (i.e., protons and heavier ions) and neutral particles (i.e., gamma rays and secondary neutrons). Previous studies used in-situ measurements, models, or both, to determine the characteristics of the radiation at Mars. For example, the Mars Science Laboratory instrument RAD, or Radiation Assessment Detector, has provided invaluable in-situ data since landing in 2012. However, the RAD instrument is only sensitive to neutrons with energies > ~6 MeV, and therefore misses what is expected to be a substantial flux of lower-energy neutrons. To address this gap, we have developed an approach to derive the surface neutron spectrum using the MSL RAD data augmented by the orbital data from High Energy Neutron Detector onboard Odyssey (neutron energy < ~10MeV). By using the power law fit we were able to find the neutron flux that can reproduce the measurements recorded by RAD and HEND. Our approach involves a series of Monte Carlo particle simulations to develop a set of atmospheric transmission functions, which enables us to convert the on-orbit HEND data to its corresponding surface neutron flux. The combined RAD - HEND data presents a unique opportunity to obtain a complete picture of the surface neutron environment.