Denk, T., Spencer, J. R. (2008). "Iapetus: A Two-step Explanation for its Unique Global Appearance" American Astronomical Society, DPS meeting #40, #61.04.


Abstract
All previous attempts to explain the unusual brightness dichotomy of Iapetus (Cassini 1673) failed to explain simultaneously (1) the precise alignment of the dark terrain (Cassini Regio) with respect to the leading side and (2) the complex transition zone between the dark and the bright terrain. Progress was made with the model by Spencer et al. (DPS 2005, 2007) who propose thermal segregation as the major physical effect to remove water ice from Cassini Regio, and to re-distribute it at high latitudes, while a dark non-ice residual remains. However, this process requires an a-priori albedo difference between the leading and the trailing side. This difference appears to be provided by the global color dichotomy detected in data from the Cassini imaging instrument (Denk et al., EGU 2006), with the leading side being generally somewhat redder and darker than the trailing side; even the dark material within Cassini Regio shows different colors. For the color dichotomy, the transition zone is not complex, but smooth and gradual, with the boundaries being close to the meridians at the sub-Saturn and anti-Saturn longitudes. The multiple earlier attempts to explain the brightness dichotomy and Cassini Regio by an exogenic influence of reddish dusty material preferentially on retrograde orbits can now be re-visited as potential explanations for the color dichotomy, without the requirement to explain a complex transition terrain anymore.