The Exogenous Origin of Lapetus' Dark Material

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Scientific paper

Recent observations by the Cassini and Spitzer spacecraft have reinvigorated interest in Iapetus, Saturn's enigmatic two-faced moon exhibiting a ten-fold albedo difference between its leading and trailing sides.
Soter (1974) proposed that dark dust, collisionally generated at the outer retrograde moon Phoebe, would migrate inward due to Poynting-Robertson drag and preferentially coat the leading side of the tidally locked Iapetus. Despite much debate in the intervening years, the recent discovery (Verbiscer et al. 2009) of the enormous diffuse dust ring generated near Phoebe seems a "smoking gun” supporting Soter's model. Burns et al. (1996) published a short analysis of the dynamics of such dust particles striking Iapetus, and Tosi et al. (2009) a simplified analysis that didn't include the important effects of radiation pressure, but a full analysis has not been carried out.
We numerically integrate dust-particle orbits including the effects of Poynting-Robertson drag, radiation pressure and solar tides. From the orbits we calculate cumulative probabilities of collision with Iapetus vs. other objects in the Saturn system, as well as global maps of the relative coverage on the surface of Iapetus as a function of latitude and longitude. We find that most particles of radius greater than 10 m strike Iapetus, while most less than 5 m are ejected from the system or crash into Saturn due to radiation pressure. We also find that the large eccentricities of the smaller particles most affected by radiation pressure act to extend longitudinal coverage onto Iapetus's trailing side, in agreement with the observed albedo distribution. We compare our results to data and images obtained through radar and by the Cassini spacecraft, the latter of which discovered bright craters in the dark terrain--showing the dark material to be a thin layer overlying an icy Iapetus (Ostro et al. 2006, Denk et al. 2009).

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