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

Abstract

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 ~0.10 for the leading one, our T-matrix model retrieves particles of radius ~0.10 ≤ r ≤ 0.20 μm for both Iapetus' leading and trailing surface materials.