Coupled Effects of Grain Size and Metallic Iron on Spectral and Polarimetric Properties of Lunar Simulants

Abstract

The lunar surface is heterogeneous in grain size and metallic iron abundance due to diverse histories of space weathering. The spectral and polarimetric properties of the lunar regolith are strongly affected by both grain size and metallic iron particles. However, because physical and chemical alteration processes are closely coupled in their effects on optical properties, their combined influence remains poorly constrained. In this study, lunar simulants (JSC-1A and KLS-1) were sieved into two bulk-type and three size-separated fractions, and hydrogen reduction was conducted to produce metallic iron particles on grain surfaces. The spectral and polarimetric properties of these samples were then systematically measured. After reduction, reflectance decreases while the degree of positive polarization increases, consistent with Umov's law. In addition, reflectance contrasts among grain sizes are reduced because fine grains preferentially darken, whereas the polarization–albedo relation remains sensitive to grain size. The reflectance of bulk-type fractions is intermediate, unlike that of natural bulk lunar soils, which resembles the finest fractions. In contrast, the polarization–albedo behavior of bulk-type fractions is close to that of the finest fraction. These differences from natural lunar soils suggest that agglutinates, which are absent in our reduced samples, may be important for the optical properties of natural bulk lunar soils. Further comparisons using natural lunar soils will help clarify the relative roles of agglutinates and grain-surface weathering products in controlling the optical properties of lunar regolith.