Hyperspectral polarimetry of eight Apollo soils

The reflected light from the lunar surface is polarized and contains perpendicular (I⊥) and parallel (I∥) branches. To provide supporting data for the first polarimetric camera (PolCam) on board the Korean Pathfinder lunar orbiter, in this work, we built a polarimeter and measured the polarized spectra for eight Apollo soils that span a wide range in composition and maturity. We found a linear correlation between reflectance R and the difference of perpendicular and parallel branches: R+a′⋅(I⊥−I∥)=b′, and b’ might be sensitive to the grain size of lunar soils. The regression coefficient b’ can be derived from both positive and negative polarization spectra and has little dependence on wavelength, thus it has great potential in estimating grain size for lunar soils. We also used radiative transfer equations to calculate the real index of optical constants and to reproduce the perpendicular and parallel polarized spectra for the lunar soils. We correlated polarimetry indexes including polarization degree (P=(I⊥−I∥)/(I⊥+I∥)) and the difference of the perpendicular and parallel branches (I⊥−I∥) with the abundances of FeO and TiO2 and soil maturity, and our result indicates that these two polarimetry indexes show dependence on both the compositions and soil maturity.