Online Astronomy eText: Satellites (Moons)
The Satellites (Moons) of Saturn: Phoebe Link for sharing this page on Facebook

Phoebe imaged by Cassini in 2004 (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod040630)

     In late 2009 a very dark dust ring was found to be enveloping the orbit of Phoebe. The dark (carbon-rich?) particles in the ring absorb the heat of the Sun and radiate heat in the far infrared, allowing the partial image of the heat radiated shown below, along with a simulation of the size and position of the ring relative to Saturn. It has long been suspected that collisions with dark dust grains knocked off of Phoebe might explain the dark leading edge of Iapetus. The discovery of Phoebe's dust ring makes that explanation seem considerably more likely to be correct.

An infrared view of Phoebe's dust ring by the Spitzer Telescope, superimposed on a model of the ring
(NASA/JPL-Caltech/Univ. of Virginia, apod091013)

Data for Phoebe

Discovered by William Pickering in 1898
Named after one of the daughters of Uranus and Gaea
Orbital size 12,950,000 km (about 800,000 miles) (four times larger than that of Iapetus)
Orbital eccentricity 16%
Orbital inclination 175 degrees (relative to Ecliptic plane)
Orbital period -550.48 days (retrograde)
Rotation period 9.4 hours
Diameter 220 km (135 miles)
Mass around a million times less than Earth
Density probably about that of water (Composition probably mostly ice and carbon compounds)
Surface gravity perhaps 1/300 of Earth's
Albedo (reflectivity) 5%
Probably captured carbonaceous asteroid or Kuiper Belt Object