First Passive Permittivity Measurement of Lunar Regolith Simulant at Terahertz Band

Abstract This study presents the first passive measurements of the relative polarization ratio (RPR) and apparent bulk permittivity (ABP) of lunar regolith simulants at approximately 280 GHz. We prepare anorthositic and basaltic simulants with both optically flat surfaces and surfaces with roughness equivalent to typical lunar surface roughness represented by the Chang’E-4 landing site and conduct passive measurements to verify whether the RPR and ABP follow Fresnel’s law. For flat-surfaced samples, the results confirm that the relationship between RPR, incidence angle, and ABP is consistent with the Fresnel-derived relationship. The ABP values are 1.91 and 2.40 for the anorthositic and basaltic simulant, respectively. Importantly, even for rough-surfaced samples, which incorporate surface scattering effects, the results demonstrate that the relationship between RPR, incidence angle, and ABP remains consistent with the Fresnel-derived relationship. This is the first experimental confirmation that passive observation can estimate the ABP of lunar regolith using the Fresnel-derived relationship, even with lunar surface roughness. Furthermore, the permittivity differences originating from the inherent bulk density and chemical composition of the samples are preserved despite the surface roughness. These findings suggest that passive polarimetric remote sensing, such as that employed by the TSUKIMI mission, can effectively estimate the ABP of the lunar surface and provide insights into identifying regions with potential for resource exploration.