Iapetus is a moon of Saturn. It has two unusual properties. One hemisphere is much darker than the other. The current theory as to why this is, is that there is thawing and freezing of water on the moon and the ice preferentially has moved from one hemisphere to another over a period of time. The unusual anomaly is used in the book ftl to indicate a rare element (Astatine) has been deposited on it. This was from a long past passage of our solar system through a nebulous cloud that has deposited the element on the moon, only one hemisphere facing the front of the cloud. That’s what science fiction is all about.
The other usual property is that there is an equatorial ridge around the moon.