After many billions of years, we end up with a moon whose two hemispheres look like total opposites. Over the course of many millions of years, the darker side absorbed more sunlight and got even darker, meanwhile the bright side only got brighter. The two hemispheres may have once looked similar, with one side being only slightly darker than the other.
This sublimation has resulted in the darker side getting darker and the brighter side getting brighter. This heating of the darker area causes any ice there to sublime out, eventually retreating to the colder side of the moon. The length of its temperature cycles allows the darker region to absorb more sunlight and become noticeably warmer than the brighter hemisphere. With such a slow rotation, Iapetus also experiences long cycles in its surface temperature. Iapetus has a fairly long rotation, with the moon taking approximately 79 days to rotate once. It is very likely that thermal segregation is responsible for the dark material on Iapetus. In 2007, NASA’s Cassini spacecraft discovered a new, more likely explanation. Volcanic eruptions could also supply darker materials by ejecting vast amounts of hydrocarbons onto the surface as well. Another explanation is that there might be ice volcanoes on Iapetus that have distributed darker material. If Iapetus formed with its two contrasting hemispheres, we would expect newly formed craters to be brighter or darker depending on what hemisphere they are on, yet craters on the dark hemisphere are also dark, which suggests that the material is being renewed over time. If this hypothesis is true, then the dark material is constantly being renewed over time, which could explain the lack of bright impact craters. Iapetus may be sweeping up darker particles from another nearby moon called Phoebe. Scientists have proposed numerous explanations.
Since Giovanni Cassini first discovered Iapetus, scientists have wondered why the moon has a bright side and a dark side. The Two Sides Of Iapetus Images of the two hemispheres of Iapetus placed together to illustrate the difference in brightness.