Denk, T., Neukum, G., Roatsch, Th., Burns, J. A., Helfenstein, P., Porco, C. C., et al. (2005). "Surface colors of Iapetus and Hyperion as derived from Cassini ISS data, and implications for the global albedo dichotomy origin." American Astronomical Society, DPS meeting #37, #39.07.
Since June 2004, various multi-color images (0.3-1.0 μm) of Iapetus were taken by the Cassini camera. These observations cover almost exclusively the leading side, including the dark hemisphere (Cassini Regio). Highest-resolution color data at 1.0 km/pxl show northern parts of the leading hemisphere, others at 4.3 km/pxl or worse cover the full leading side and reach areas somewhat beyond the 180 degrees W meridian. For Hyperion, one useful 4-color data set (9.4 km/pxl) is currently available.|
Between 0 degrees W and 180 degrees W longitude, the dark Cassini Regio has a reddish and uniform color. However, dark material extending towards the trailing side shows a different color. The transition zones at mid-latitudes are characterized by an areal separation of bright icy and dark material. Within the southern bright terrain, the 180 degrees W meridian marks a rather sharp color boundary, with the trailing-side material showing flatter spectra. Hyperion material shows spectral slopes in between the dark and bright material spectral slopes on Iapetus. However, areas of similar brightness show much redder spectra.
Data interpretation: (1) Exogenic material on retrograde orbits might have reddened the *whole* leading side, but has presumably not darkened it very much. The same source might have caused the reddish color on brighter Hyperion. (2) Darkening of Iapetus by thermal segregation of ice particles as suggested by Spencer (LPSC, 2005) might be consistent with the local dark/bright pattern within the transition zones at mid-latitudes. These two processes, if significant, should be ongoing today and might be responsible for the observed absence of bright-ray or bright-floor craters within Cassini Regio. (3) The color difference within Cassini Regio was not predicted by any model and awaits an explanation. More data, especially from the bright trailing side, is needed to set further constraints. Possibly, up to three different processes caused the albedo dichotomy as visible on Iapetus today?