Low-energy impact distribution and the albedo dichotomy of Iapetus

We put forward a dynamical model for the distribution of dust falling on the surface of Iapetus. Theoretical distributions are obtained by solving the Saturn-Iapetus circular restricted three-body problem for low- and intermediate-energy particles entering the system and colliding with Iapetus. Comparison of the resulting impact densities with photometric data gives an empirical model for the boundary of the heavily impacted regions, which reproduces some, but not all, relevant features of the observed dark region. Comparison with the dust deposition rate assumed to trigger the thermal segregation process proposed by Spencer & Denk (2010) gives instead qualitatively good agreement. The relevant features are best reproduced for a uniform density of incoming particles within a relatively narrow range of intermediate energies. The corresponding incoming trajectories are direct and originate in a close external neighbourhood of Iapetus' orbit, at no more than 1.33 times the Saturn-Iapetus distance. The features of the population originating this flux, as shaped by the influence of Solar gravity, are briefly discussed.