sentences:  Over geologic time scales, this would further darken Cassini Regio and brighten the rest of Iapetus, creating a positive feedback thermal runaway process of ever greater contrast in albedo, ending with all exposed ice being lost from Cassini Regio.It is estimated that over a period of one billion years at current temperatures, dark areas of Iapetus would lose about of ice to sublimation, while the bright regions would lose only , not considering the ice transferred from the dark regions.This model explains the distribution of light and dark areas, the absence of shades of grey, and the thinness of the dark material covering Cassini Regio.