Sensitivity of leakage neutrons to the abundance and depth distribution of lunar subsurface water

Water on the Moon has received increasing attention due to its importance in planetary science and the utilization of space resources. Future lunar rover missions are poised to conduct explorations, specifically focusing on locating water. Neutron spectroscopy is a powerful technique for estimating subsurface water content. In this study, lunar surface neutrons induced by galactic cosmic rays were investigated through Monte Carlo simulation. This effort aims to yield insights pertinent to in-situ water search explorations utilizing neutron spectrometers. The sensitivity of the leakage neutron intensity to the depth profile of subsurface water within the top 1.5 m soil was obtained via calculations based on a lunar surface model, featuring a localized concentration of water-rich soil. Computational outcomes underscore the potential of neutron observations to provide data on the depth profile of subsurface water under specific circumstances. Notably, in scenarios where a thin and shallow water-rich layer, approximately $\lesssim$20 cm thick and located $\lesssim$50 cm deep, is assumable within lunar soil of density 1.6 g/cm³, a combination of thermal, epithermal, and fast neutron measurements enables concurrent estimation of water abundance and depth. To accurately understand the subsurface water abundance and depth across exploration areas along the rover’s path, a comprehensive assessment of leakage neutrons in a wide energy range becomes indispensable.