Saturn's A and B rings exhibit two intriguing opposition phenomena, namely, the unusually narrow photometric and polarization opposition effects. Specifically, the intensity of the reflected light measured in the visible has a strong and very narrow peak centered at zero phase angle and superposed on a smooth photometric phase curve, while the reflected polarization, being zero at zero phase angle, becomes negative and almost reaches its maximum negative value at a small phase angle of several tenths of degree. The small angular width of both phenomena makes it difficult to explain them on the basis of the commonly used shadowing models. On the other hand, it is known from controlled laboratory experiments and theoretical studies that a strong and very narrow opposition peak in the reflected intensity can be produced by coherent back-scattering of light from powder-like layers composed of small regolithic grains. Recently, Mishchenko and Dlugach assumed that macroscopic particles of Saturn's rings are covered with a layer of submicrometer-sized regolithic ice grains and demonstrated that coherent backscattering of sunlight from this layer can explain the unusually narrow photometric opposition effect observed for Saturn's A and B rings in the visible. In this paper, we use a rigorous vector theory of coherent backscattering, developed recently for very small (pointlike) scattering grains by Ozrin, to show that for subwavelength-sized regolithic particles, the photometric opposition effect is accompanied by a polarization opposition effect of the same angular width. The common origin of the two coherent opposition phenomena was also demonstrated in recent controlled laboratory experiments by van Albada, van der Mark, and Lagendijk, although different terminology was exploited. These results may be considered an evidence that the polarization opposition effect observed for Saturn's rings in the visible has the same origin as the photometric opposition effect and is due to coherent backscattering of light from the regolithic layer composed of the submicrometer-sized ice grains. A remarkable opposition brightening similar to that for Saturn's rings has also been observed for some icy satellites and E-type asteroids. It is suggested that if the regolithic grains responsible for the observed opposition spikes are smaller that the wavelength, then the polarization opposition effect should also be observed for these objects at phase angles less than 1 degree.
