Space.com is carrying this summary of a more comprehensive look at the data provided by Cassini during the recent Phoebe flyby:

"All our evidence leads us to conclude, Phoebe's surface is made of water ice, water-bearing minerals, carbon dioxide, possible clays and primitive organic chemicals in patches at different locations on the surface," said Roger N. Clark, team member for the Visual and Infrared Mapping Spectrometer, U.S. Geological Survey in Denver. "We also see spectral signatures of materials we have not yet identified."

Big dirty slushball, in other words. Kind of weird to think this thing is probably what a comet turns into when it's not near the sun.