Preliminary Results from Multi-band Polarimetric Observations of the Lunar Surface

Introduction: We carried out multi-band (U, B, V, R, and I passbands) polarimetric observations of the whole near side of the moon from the Lick observatory using a 15-cm reflecting telescope. Polarization of the light scattered by the lunar surface contains information on the mean particle size of the lunar regolith, which gradually decreases by continued micro-meteoroid impact over a long period and thus is an age indicator of the surface. We have constructed a map of mean particle sizes for the whole near side of the moon. We also found that the mean particle sizes of Renier Gamma are somewhat dissimilar to those predicted by OMAT (optical maturity) values, which may be a clue to understanding the nature and formation mechanism of the swirls. Background: The sunlight scattered on the lunar surface is partially linearly polarized [1]. The polarized light contains information on the lunar regolith, such as its mean particle size, composition and so on. Particularly, the mean particle size is an important parameter when designing lunar rovers and making artificial regolith for laboratory experiments. In addition, the mean particle size can be an indicator of maturity for the lunar regolith because micro-impacts make particle sizes smaller and have constantly occurred on the lunar surface [2, 3]. The mean particle size can be measured from the observations of the maximum polarization and albedo [4, 5, 6]. However, monochromatic polarimetry and photometry provide limited information regarding the properties of the regolith because the regolith consists of various minerals. Therefore, multi-band polarimetry provides more informative data for understanding the polarization properties of lunar regolith. Observations and Data Processing: We conducted multi-band polarimetry at Lick observatory from July 14 to Aug 1. We used a 15-cm aperture reflecting telescope and a 3.3k X 2.5k CCD with band-pass filters (U, B, V, R and I) and a polarization filter. Spatial resolution is 1.84km/pixel at the lunar center. Polarization degree highly depends on phase angle. One has to obtain as many phase angle data as possible to obtain a more precise measure of the polarization maximum. The