Photometry and polarimetry of particulate surfaces and aerosol particles over a wide range of phase angles

Abstract

We present results of photometric and polarimetric laboratory measurements of particulate surfaces undergoing different degrees of compression and aerosol particles at a phase angle range from 7 to 150°. Nine samples of natural materials with different albedo and particle sizes were measured in red ( λ=0.63 μm) and blue ( λ=0.44 μm) light using two different instruments: Kharkov's large-phase-angle photopolarimeter for measurements of surfaces and Amsterdam's polar nephelometer for particles in air. We compare phase curves for particulate surfaces with the corresponding measurements for single particles. Our measurements suggest that the maximum of positive polarization, which is observed for particulate surfaces at large phase angles, is mainly due to the contribution of single-particle scattering. We observe an increase in the polarization degree at large phase angles with increased sample compression. We have found the negative polarization at small phase angles to be strengthened sharply by surface compression. This is partially a geometric effect at slanting view (70°), which is explained via double scattering. For photometric data, we carried out a comparison between experimental data and the Hapke photometric model. The comparison of measured photometric phase-angle dependencies with the model reveals several model shortcomings. The principle shortcoming is the spectral dependence of the model parameters that should be independent of wavelength.