Iapetus: 4.5 Billion Years of Contamination by Phoebe Dust
Douglas P. Hamilton (University of Maryland)
One of the strangest satellites in the Solar System is Saturn's tidally-locked Iapetus which has a bright white trailing hemisphere and a jet-black leading face. It has long been suspected that dark dusty debris, originating from Saturn's outermost satellite Phoebe and brought inward by Poynting-Robertson drag, is responsible for Iapetus' striking albedo asymmetry. The Phoebe-dust model is very compelling because it naturally explains why the leading face of Iapetus, the side that is receiving dust from Phoebe, is dark. The model has not gained universal acceptance, however, primarily due to the following dynamical problems: i) the distribution of dark material on Iapetus does not precisely match predicted contours of constant dust flux from Phoebe, ii) there are dark-floored craters in Iapetus' high-albedo hemisphere, and iii) Iapetus' North pole is brighter than parts of the trailing hemisphere. These problems are greatly reduced with the realization that Iapetus took nearly a billion years to become tidally locked to Saturn.
I suggest the following scenario for the origin of the black/while dichotomy on Iapetus. Phoebe was probably captured early in the Solar System's history, well before Iapetus' spin slowed to its present synchronous rate. While Iapetus was spinning rapidly, dust from Phoebe accumulated at all longitudes on Iapetus uniformly. The accumulation was greatest near Iapetus' equator and decreased with roughly a cosine dependence toward the poles where the dust flux was lowest. After Iapetus became tidally locked, its trailing side was shielded from Phoebe dust, and volatile ice accumulated there burying the dark Phoebe material. Large impacts on the trailing side have dredged up some ancient Phoebe debris, creating the dark-floored craters. In addition, impacts have mixed dark debris with icy material, thereby lowering the albedo of the trailing side. Iapetus' poles are the brightest parts of the satellite simply because little Phoebe dust ever accreted there. Since the distribution of Iapetus' dark material depends on the flux of Phoebe dust over 4.5 billion years, it is not surprising that the distribution does not precisely match predictions which consider only the present dynamical configurations of the two satellites.