sentences:  The redistribution of ice is facilitated by Iapetus's weak gravity, which means that at ambient temperatures a water molecule can migrate from one hemisphere to the other in just a few hops.However, a separate process of color segregation would be required to get the thermal feedback started.The initial dark material is thought to have been debris blasted by meteors off small outer moons in retrograde orbits and swept up by the leading hemisphere of Iapetus.