Polarization of Saturn's moon Iapetus III. Models of the bright and the dark sides

Context. Like many other atmosphereless solar system bodies, Iapetus exhibits a phenomenon of negative polarization at small phase angles, which can be modeled using theoretical approaches that consider interaction of light with a complex medium. Aims. To retrieve information on the nature of Iapetus' surface material, we carried out theoretical modeling analyses for the observed polarization of its two sides. Methods. We applied two light-scattering models. The first modeling approach is based on the utilization of the phenomenological single-particle scattering matrix parametrization using the double Henyey-Greenstein (2HG) scattering phase function to characterize the resulting multiple scattering by a medium composed of such discrete scatterers. With this approach we carried out radiative-transfer coherent-backscattering (RT-CB) computations for a random medium composed of phenomenological fundamental scatterers. The second model, called the multiple sphere T-matrix method, is based on the exact solutions of the Maxwell equations. Employing this method, we carried out simulations of the scattering and absorption properties of light by a medium represented by a spherical volume of randomly positioned monodisperse particles. The modeling entails physical characteristics of the particulate surface, such as the porosity of the medium; the number of constituent particles; the size, and optical properties of the scatterers. Results. While our RT-CB model suggests geometric albedo values in the neighborhood of 0.40 for Iapetus' trailing side and similar to 0.10 for the leading one, our T-matrix model retrieves particles of radius similar to 0.10 <= r <= 0.20 mu m for both Iapetus' leading and trailing surface materials.