The Dark Side of Iapetus: Additional Evidence for an Exogenous Origin

Abstract

The saturnian satellite Iapetus presents one of the most unusual appearances of any object in the Solar System: one hemisphere is about 10 times as bright as the other. The origin of the dark hemisphere—which reflects only a few percent of the solar radiation falling on it—has historically been one of the great puzzles of planetary science. From a map produced from previously unstudied, archived Voyager images obtained in the near ultraviolet region of the spectrum, we show that the interface between the bright and dark material is gradual. The ultraviolet (0.34 μm) normal reflectance gradually changes from ∼0.55 to less than 0.03 over a distance of about 1500 km. We present the first color map of Iapetus, showing there is a gradual change in color at the interface between the two hemispheres. This change is highly correlated with the change in albedo. This result supports a spectral mixing model in which the dark and bright sides are the spectral endmembers, and the interface represents a progressive enrichment of a dark red chromophore. Our results are most consistent with a model originally proposed by Cruikshank et al . (1983, Icarus 53 90-104) and Bell et al. (1985, Icarus 61, 192-207). In this model, exogenously produced material from Phoebe impacts the leading side of Iapetus and volatizes the icy component to leave a dark red lag deposit consisting of material from Phoebe and a preexisting nonvolatile constituent that is similar to the D-type material found on Hyperion and D-type asteroids.