Multi-band Polarimetry of the Lunar Surface. III. Polarization Phase Curve

Abstract

Polarimetry of the Moon reveals characteristics of the lunar regolith such as the size and composition of regolith grains, which are useful in understanding the surface properties and to determine landing sites for future lunar missions. As part of the Korea Pathfinder Lunar Orbiter (KPLO) mission, the wide-angle polarimetric camera (PolCam) will perform polarimetric measurements of the Moon for the first time from the lunar orbit. Because the degree of linear polarization is a function of the phase angle, it is necessary to observe the same region multiple times at various phase angles to construct the phase curve of polarization and derive the map of maximum polarization (Pmax). Any unexpected discontinuities of observing runs or an abrupt termination of the mission may cause the scientific goals of the PolCam to be unachievable. Here, we investigated how to recover the polarimetric phase curve when the observational data set is incomplete. Using ground-based polarimetry data, we explored the parameter space of the fitting formula of the phase curve and show that the formula can be reduced into a single parameter form. Assuming several cases of incomplete measurements, we constructed Pmax maps and investigated the allowable range of the phase-angle coverage. We show that Pmax can be estimated with a root-mean-square discrepancy less than 0.5%p when the maximum phase angle covered is greater than 90°.