New evidence explains Saturn moon's black and white nature.
Basically it's a freeze-thaw action:
The new observations add support to a two-part explanation for Iapetus' appearance. First, as Iapetus treks around Saturn, its leading edge scoops up a thin coating of dark material, which amplifies sunlight absorption.
Sounds like my truck's windshield in winter. That kind of ramscoop effect can really impact your visibility. Given all the icy bits flying around Saturn, it's probably a constant feature.
Iapetus was one of those throwaway Titans, but I always loved that name. So vowelly.