Most of Saturn's moons have very bright surfaces, but Phoebe's albedo is very low (.05), as dark as lampblack. All of Saturn's moons except for Phoebe and Iapetus orbit very nearly in the plane of Saturn's equator. Phoebe's orbit is retrograde, inclined almost 175°, and is highly eccentric. The plane of its orbit is much closer to the solar system's ecliptic than it is to Saturn's. Phoebe is also unusual in that it does not rotate synchronously as all the other moons of Saturn except Hyperion do, and it has a very low density (0.7 g/cm3). All this suggests that it may be a captured asteroid with a composition unmodified since the time it was formed in the outer Solar System. It resembles the common class of dark carbonaceous asteroids, which are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula. Since they are so small they never heated up sufficiently to change chemical composition. If this is the case Phoebe is the first such object that has been photographed at close enough range to show shape and surface brightness.
Material knocked off of Phoebe's surface by microscopic meteor impacts may be responsible for the dark surfaces of Hyperion and the leading hemisphere of Iapetus.
- Orbital radius: 12,952,000 km
- Diameter: 220 km
- Mass: 4.0*1018 kg
- Density: 0.7 g/cm3
- Rotational period: 0.4 days
- Orbital period: 550.48 days
- Orbital inclination: 175.3°