The September 2007 Cassini flyby of Iapetus showed that a third process, thermal segregation, is probably the most responsible for Iapetus' dark hemisphere. Iapetus has a very slow rotation, longer than 79 days. Such a slow rotation means that the daily temperature cycle is very long, so long that the dark material can absorb heat from the sun and warm up. (The dark material absorbs more heat than the bright icy material.) This heating will cause any volatile, or icy, species within the dark material to sublime out, and retreat to colder regions on Iapetus. This sublimation of volatiles causes the dark material to become even darker -- and causes neighboring bright, cold regions to become even brighter. Iapetus may have experienced a (possibly small) influx of dark material from an external source, which could have warmed up and triggered this thermal segregation process.