Compositional maps of Saturn's moon Phoebe from imaging spectroscopy

Phoebe a Kuiper-belt refugee? Phoebe, the outermost large satellite of Saturn, is of particular interest because its unusual orbit suggests that it was gravitationally captured by Saturn, having formed outside the solar nebula where Saturn itself formed. The Cassini–Huygens spacecraft encountered Phoebe on 11 June 2004, and imaging spectroscopy from Cassini was used to detect iron, bound water, trapped CO2, phyllosilicates, organics, nitriles and cyanide compounds on Phoebe. The presence of all these compounds makes Phoebe one of the most compositionally diverse objects in our Solar System, consistent with a surface of cometary origin incorporating primitive materials from the outer Solar System. Further evidence on Phoebe's past comes from density measurements made by two other instrument systems on Cassini. Phoebe's composition is distinctly different from the ice-rich material that formed the intermediate-sized saturnian satellites, and is consistent with formation from the same material out of which Pluto and Triton (archetypical Kuiper-belt objects) formed.