5:15 PM - 7:15 PM
[PPS08-P07] Hydrogen-Bearing Lunar Regolith Simulants: A Tool for Revisiting Previous Mission Data
Keywords:moon, regolith, simulant, volatile
The potential presence of water ice on the Moon has significant implications for both scientific research and the sustainability of human activities on the lunar surface. Recent remote sensing data suggest the presence of water ice in permanently shadowed regions (PSRs) near the lunar poles; however, uncertainties remain regarding its horizontal distribution, abundance, and depth of concentration. To address these uncertainties, re-examining previous data using different methods can be effective. Among the available data, neutron spectroscopy conducted by LP-NS and LEND has observed suppression of epithermal neutron flux, uniquely detecting subsurface hydrogen (Feldman et al., 1998; Sanin et al., 2017).
While neutron spectroscopy strongly supports the presence of water ice, complexities remain in its interpretation. Previous studies have largely assumed a homogeneous distribution of subsurface hydrogen in numerical simulations. However, other studies suggest that different subsurface water ice distributions (e.g., single-layer, two-layer, or wet-over-dry stratigraphy) can result in variations in the detected neutron flux (Lawrence et al., 2011). Therefore, it is crucial to thoroughly understand how neutron flux depends on subsurface water ice distribution, not only through conventional numerical simulations but also via experimental validation. Despite this need, experimental studies on neutron spectroscopy remain limited, and existing regolith simulants are not specifically designed for such applications. To address this gap, this study focuses on developing hydrogen-bearing regolith simulants optimized for neutron spectroscopy experiments.
Neutron spectroscopy is sensitive to hydrogen atoms rather than water molecules, meaning that hydrogen content must be carefully measured, including structurally bound water within minerals. Additionally, the instability of water ice at room temperature necessitates a method to control hydrogen content. Furthermore, to isolate the effects of hydrogen on epithermal and thermal neutron flux, other variables influencing neutron flux (e.g., Fe and Ti abundance) must be carefully controlled. Thus, the regolith simulant prototypes were designed to meet the following key requirements: accurately assessing water-equivalent hydrogen (WEH), maintaining consistent WEH content during measurements, and controlling the macroscopic absorption cross-section.
The WEH for each ingredient was quantified using the Loss on Ignition (LOI) method. To ensure WEH stability over a wider temperature range, unstable water ice was replaced with a hydroxide mineral. This substitution allows samples with identical WEH content to be reused multiple times, improving the reliability of data comparisons across experiments. Additionally, the chemical composition of the simulants was successfully optimized to match the estimated macroscopic absorption cross-section of the lunar south pole regolith.
While neutron spectroscopy strongly supports the presence of water ice, complexities remain in its interpretation. Previous studies have largely assumed a homogeneous distribution of subsurface hydrogen in numerical simulations. However, other studies suggest that different subsurface water ice distributions (e.g., single-layer, two-layer, or wet-over-dry stratigraphy) can result in variations in the detected neutron flux (Lawrence et al., 2011). Therefore, it is crucial to thoroughly understand how neutron flux depends on subsurface water ice distribution, not only through conventional numerical simulations but also via experimental validation. Despite this need, experimental studies on neutron spectroscopy remain limited, and existing regolith simulants are not specifically designed for such applications. To address this gap, this study focuses on developing hydrogen-bearing regolith simulants optimized for neutron spectroscopy experiments.
Neutron spectroscopy is sensitive to hydrogen atoms rather than water molecules, meaning that hydrogen content must be carefully measured, including structurally bound water within minerals. Additionally, the instability of water ice at room temperature necessitates a method to control hydrogen content. Furthermore, to isolate the effects of hydrogen on epithermal and thermal neutron flux, other variables influencing neutron flux (e.g., Fe and Ti abundance) must be carefully controlled. Thus, the regolith simulant prototypes were designed to meet the following key requirements: accurately assessing water-equivalent hydrogen (WEH), maintaining consistent WEH content during measurements, and controlling the macroscopic absorption cross-section.
The WEH for each ingredient was quantified using the Loss on Ignition (LOI) method. To ensure WEH stability over a wider temperature range, unstable water ice was replaced with a hydroxide mineral. This substitution allows samples with identical WEH content to be reused multiple times, improving the reliability of data comparisons across experiments. Additionally, the chemical composition of the simulants was successfully optimized to match the estimated macroscopic absorption cross-section of the lunar south pole regolith.