sentences:  The dark region is named Cassini Regio, and the bright region is divided into Roncevaux Terra north of the equator, and Saragossa Terra south of it.Before optical observations could be made by deep space probes, theories about the reason for this dichotomy included an asteroid shearing off part of the moon's crust.The original dark material is believed to have come from outside Iapetus, but now it consists principally of lag from the sublimation of ice from the warmer areas of Iapetus's surface.