selden wrote:because nobody ever expected a body formed in space, with no weather, to have layers of materials of different types.
I think it was also the fact that nobody expected Phoebe to have that. Separation of terrain into bright and dark over a vertical distance has been seen on the Galilean satellites (which also have no atmosphere), but that's due to differential heating by the sun that melts ice and makes a lag deposit of dust on the surface.
What it looks like in Phoebe's case is that it's dark on the outside but bright on the inside. I'd hypothesise that Phoebe itself is an icy body, but the dark exterior is the result of billions of years of micrometeoroid impacts that are depositing darker material. Every now and then a big impact smacks through that and exposes the icy interior.
.