Brightness and polarization opposition effects at low phase angles of the Saturnian satellites Tethys, Dione, and Rhea

The photometric and polarization phase dependences of the Saturnian moons Tethys, Dione, and Rhea are studied in a narrow domain of solar phase angles around opposition by using observations obtained with the 2-m RCC telescope of the National Astronomical Observatory at Rozhen, Bulgaria. The observations were made in the solar phase angle range from 5.7° to 0.01° and in three different wavelength regions, that is, the blue, red, and infrared color bands. After applying corrections for longitude variation of the brightness, the disk-integrated photometric phase curves revealed a steep non-linear intensity growth in the narrow backscatter domain of phase angles less than 2°. The measured polarization state was found to be negative all over the investigated region of the phase angles. The profiles of polarization-phase dependences are most probably irregular with a steep descent and minima at phase angles approximately equal to the angular width of the corresponding intensity peaks. Both the profiles of backscatter peak intensities and the observed polarization phase dependences provided evidence in favor of the “coherent backscatter mechanism” as being the main contributor to the opposition effect of all three Saturnian moons. Surprisingly, the polarization phase dependences demonstrated deep minima of negative polarization at small phase angles with the exception of Tethys’ leading hemisphere and the brightest area of Rhea’s leading side (100°W<θ<180°W), which were assumed to be due to high albedos and low contaminant abundances. Finally, the longitude dependences of the water–ice absorption bands, which are deduced from the Cassini VIMS observations of Rhea and Dione, were compared with the measured polarization in a narrow range of phase angles around 5°. This comparison suggested that the larger negative polarizations (in absolute values) correspond to areas with surface materials which most likely are strongly contaminated and composed of smaller regolith grains.