Tamayo, D., Burns, J.A., Denk, T. (2009). "Dynamical Models for the Origin of Iapetus' Dark Material" American Astronomical Society, DPS meeting #41, #03.06.


Abstract
The stark albedo dichotomy on Iapetus has been known since 1671. Interestingly, recent Cassini ISS color observations have revealed a separate 'color dichotomy' -- color and slight albedo differences within the dark and within the bright terrains -- seemingly determined by Iapetus' orbital motion (Denk et al. 2009, Science, submitted). Spencer and Denk (2009, Science, submitted) have modeled how such a color dichotomy could result in thermally-driven runaway migration of water ice leading to the global albedo distribution observed today. This scenario seems very reasonable, but the (likely exogenous) source for the reddish material required to form the color dichotomy and initiate the runaway ice migration remains open.

We model dust particles from all the irregular moons as the source for the dark material by numerically integrating the effect of radiation forces on their orbits and calculating their cumulative probability of collision with Iapetus. This work is an extension of Burns et al. (1996) aimed at resolving the inconsistencies mentioned therein with regard to the distribution and supply of dark material. We evaluate Soter's model (1974) proposing Phoebe as the source of the dark material, as well as models where dust originates from irregular moons discovered more recently. Our calculations show that only particles on high-eccentricity orbits (induced by radiation pressure) are capable of striking Iapetus. We will discuss such a model's implications for the longitudinal coverage of dark material, as well as the importance of thermal processes for the latitudinal segregation of ice. Finally, we will address whether Phoebe and the outer irregular satellites can collectively account for a sufficient supply of material to darken and redden the leading-side polar areas relative to their trailing-side counterparts, thereby creating the color dichotomy. This would initiate the proposed thermally-driven migration process and lead to the presently observed global albedo distribution.