ABSTRACT

Water ice trapped in permanently shadowed regions (PSRs) near the lunar poles is an important research target for understanding the distribution of light elements in the Moon. Herein, we examined the relationship between the near-infrared water absorption band depth and water ice content in mineral powders with a low ice content; these minerals serve as analogs to the regolith in the lunar PSRs. Four base minerals—olivine, plagioclase, clinopyroxene, and a mixture of these—were prepared at two grain size fractions. We constructed calibration lines based on the correlation between the water ice content ranging from 0.3 to 2.2 wt.% and 1.5-µm water absorption band depth. Results show that the calibration-line gradients, which are key parameters for determining the water ice content based on the absorption band depth, depend on the mineral species and grain size. The calibration-line gradients increased with increasing mineral grain size and correlated with variations in 1.5-µm reflectance among the dry mineral samples. The calibration-line gradients and reflectance values decrease in the order of clinopyroxene, plagioclase, mixture, and olivine. Combining the effects of mineral reflectance and grain size, we establish a predictive relationship for estimating the water ice content based on the observed 1.5-µm absorption bands. The proposed relationship provides a practical method to determine the ice content for future in situ landing explorations of the lunar PSRs, even when the exact regolith composition is unknown.