Iapetus colorations....an explanations
Even though is so far away from the Sun, because it faces it for such a long time (one orbit is 79 days long and it always presents the same face towards Saturn), Iapetus stange dark and clear patterns can be explained by dust spreading coming from Phoebe AND ice being transformed into gas (sublimated) and carried where it's cooler and deposited there, exposing more darker surface. There is a 70 degrees C difference between night and day, much more than for any other moon around Saturn.
An explanation for Iapetus.
VIMS analysis