There appears to be a layer of dark material covering the small moon Phoebe as revealed by the collapse of materials in the northern region of the small moon. Phoebe orbits Saturn outside the orbit of Iapetus and has been considered a possible candidate for the dark material also found covering one side of that unusual moon. What may have caused this transfer of materials is still a mystery… or that Phoebe had anything to do with that feature on Iapetus is still very much in question.
Some propose that Phoebe is actually a captured comet from the Kuiper belt (a region of small bodies orbiting the sun beyond Neptune of which Pluto is considered a member). If this is true, the images taken by cassini would be the only images of such bodies to be anything other than a single point of light.