Improved Modeling of Lunar Prospector Neutron Spectrometer Data: Implications for Hydrogen Content at Lunar Poles

Introduction: In recent years, a number of studies [1,2,3] have been published using data from the Lunar Prospector Neutron Spectrometer (LP-NS) to identify and quantify anomalously high hydrogen abundances near the poles of the Moon, which may be in the form of water ice. If water ice is indeed present at the lunar poles in high enough abundance, this could provide a critical resource for future lunar exploration. It is therefore important that we continue to better understand the nature of these deposits and the interpretation of the LP measurements. In a recent study, Hodges [4] questioned the interpretation that the measured epithermal neutron signals are from hydrogen and suggested that the neutron data can be equally explained by variations in the abundances of other elements such as Si or Ca. Additionally, a recent study by Maurice et al., [5] has shown that, in part, variations of epithermal neutrons in lunar equatorial regions are likely due to composition variations of the rare-earth elements Gd and Sm. To address these issues, we have completed an effort to model the LP neutron data with a single neutron transport code (MCNPX) for the first time [6] (previous LP modeling efforts used multiple codes for different energy regimes of the neutron transport). The goal of this modeling effort is to understand the variation of epithermal neutrons for the full range of lunar soil compositions. These models, along with other information about lunar geochemistry, will enable us to better understand the nature of the lunar polar deposits Compare MCNPX Models to Measured LP-NS Data: MCNPX is a three-dimensional particle transport code that can model the entire neutron transport process starting from the initial cosmic ray protons to the full-energy neutron transport and detection. MCNPX has been used extensively for other planetary science applications [e.g., 7] and has been validated with the Apollo 17 drill core neutron density measurements using no free parameters [8]. Figure 1 shows the comparison between the measured and modeled neutron data from the Cd and Sn covered neutron sensors (Cd covered sensors measure epithermal neutrons and Sn covered sensors measure thermal plus epithermal). The solid lines are histograms of global LP neutron data and the triangles show the range of modeled values for various dry lunar soils that have been normalized to ferroan anorthosite types soils. As seen, the modeled soil values cover almost the full range of measured data in absolute counts. One exception where the model does not match the data is for low epithermal neutron counts, which are discussed next.