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The leading face of Saturn's moon Iapetus, Cassini Regio, has an albedo onlyone tenth that on its trailing side. The origin of this enigmatic dichotomy hasbeen debated for over forty years, but with new data, a clearer picture isemerging. Motivated by Cassini radar and imaging observations, we investigateSoter's model of dark exogenous dust striking an originally brighter Iapetus bymodeling the dynamics of the dark dust from the ring of the exterior retrogradesatellite Phoebe under the relevant perturbations. In particular, we study theparticles' probabilities of striking Iapetus, as well as their expected spatialdistribution on the Iapetian surface. We find that, of the long-lived particles(greater than about 5 microns), most particle sizes (greater than about 10microns) are virtually certain to strike Iapetus, and their calculateddistribution on the surface matches up well with Cassini Regio's extent in itslongitudinal span. The satellite's polar regions are observed to be bright,presumably because ice is deposited there. Thus, in the latitudinal directionwe estimate polar dust deposition rates to help constrain models of thermalmigration invoked to explain the bright poles (Spencer & Denk 2010). We alsoanalyze dust originating from other irregular outer moons, determining that asignificant fraction of that material will eventually coat Iapetus--perhapsexplaining why the spectrum of Iapetus' dark material differs somewhat fromthat of Phoebe. Finally we track the dust particles that do not strike Iapetus,and find that most land on Titan, with a smaller fraction hitting Hyperion. Ashas been previously conjectured, such exogenous dust, coupled with Hyperion'schaotic rotation, could produce Hyperion's roughly isotropic, moderate-albedosurface.

Finding the trigger to Iapetus' odd global albedo pattern: Dynamics of dust from Saturn's irregular satellites