Phoebe

Saturn IX

Phoebe ("FEE bee") is the outermost of Saturn's known satellites. Phoebe is almost 4 times more distant from Saturn than its nearest neighbor (Iapetus).

        orbit:    12,952,000 km from Saturn
        diameter: 220 km
        mass:     4.0e18 kg

Phoebe is the daughter of Uranus and Gaia; grandmother of Apollo and Artemis.

Discovered by Pickering in 1898.

On June 11, 2004 Cassini passed close to Phoebe on its way into the Saturn system, giving us our first real look at it since Voyager and at much higher resolution.

Most of Saturn's moons are bright 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 inclined almost 175° (its north pole is in the opposite direction to Saturn's).

Phoebe's eccentric, retrograde orbit and unusual albedo indicates that it may be a captured asteroid or Kuiper Belt object.

Phoebe is also unusual in that it does not rotate synchronously as do all the other moons of Saturn except Hyperion.

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.

More about Phoebe

• from LANL
• first images from Cassini:
  • Battered Moon
  • The Face of Phoebe
  • Phoebe's Surprise
  • Crater Close-up
  • closest approach images
• Clues to Phoebe's Origins, a preliminary report

Open Issues

• Why is Phoebe in such an odd orbit?
• Is it really the source of the dark material on Hyperion and the leading hemisphere of Iapetus?
• Phoebe's density is only 0.7 which is anomalously low. What is its composition? Ice with an overlay of something dark is the initial guess from Cassini images. But stay tuned...