Cassini image of Phoebe.

PHOEBE - MOON OF SATURN

Phoebe was the daughter of Uranus and Gaia. She married her brother Coeus and by him bore Leto and Asteria. In some legends she is the grandmother of Apollo and Artemis. Phoebe was discovered by William Pickering in 1898. It was the first moon to be found using photography, though its discovery was not widely accepted until Edward Barnard saw it and was able to confirm its existence in 1904.

Orbit

Phoebe is the outermost of Saturn's moons, orbiting nearly 13 million kilometres from the planet. Its distance from Saturn is equivalent to 215 saturnian radii. Phoebe orbits retrograde, that is, opposite to the rotation of Saturn and the orbits of the other moons. Phoebe's path about Saturn is also inclined by 175o. Variation in Phoebe's brightness reveals that it has a rotational period of about nine hours.

The orbits of Titan, Hyperion, Iapetus, and Phoebe.

Physical properties

Phoebe is on average 220 kilometres across and roughly spherical, but actually measures 230 x 220 x 210 kilometres.

Magnetic field

No magnetic field has been detected.

Atmosphere

No atmosphere has been detected.

Interior

Phoebe is too small a body to have formed a sphere, or too have retained sufficient heat to sustain a molten interior and form a layered structure like many of the larger moons and terrestrial planets.

Surface

Voyager 1 was too great a distance to get images of Phoebe's surface. Its closest approach took it to about 13 million kilometres of the satellite. Voyager 2 though at about 1.5 million kilometres was able to capture some hazy images of Phoebe's cratered and pitted surface. Phoebe is very likely a captured asteroid. It has a retrograde orbit and is also very far from Saturn. Phoebe's irregular shape, and low reflectivity (0.06), add weight to the argument.

Phoebe is reddish in colour and as such, resembles a C-type or carbonaceous chondrite asteroid.These comprise material condensed from the solar nebula during the early history of the Solar System. It was thought that Phoebe could be the source of material which coats the leading hemisphere of Iapetus and which mantles Hyperion entirely. The idea is proposed that dust and fine particles, thrown up by micrometeorite impacts, drift inwards towards Saturn. The innermost moons avoid contamination in this way because Titan probably sweeps up any material which isn't deposited first on Hyperion and Iapetus. Examination of Iapetus' dark regions suggest that they are probably volcanic and not a layer of dust which has blown off Phoebe.