Phoebe (moon)
Phoebe (/ˈfiːbi/ FEE-bee) is the most massive irregular satellite of Saturn with a mean diameter of 213 km (132 mi). It was discovered by William Henry Pickering on 18 March 1899[9] from photographic plates that had been taken by DeLisle Stewart starting on 16 August 1898 at the Boyden Station of the Carmen Alto Observatory near Arequipa, Peru. It was the first natural satellite to be discovered photographically.
Discovery
18 March 1899 (from photos taken 16 August 1898)
Saturn IX
Φοίβη Phoíbē
Phoebean /fiːˈbiːən/ fee-BEE-ən[2]
12 960 000 km
0.1562415
550.564636 d
173.04° (to the ecliptic)
151.78° (to Saturn's equator)
(218.8±2.8) × (217.0±1.2) × (203.6±0.6) km[4]
(8.3123±0.0162)×1018 kg[5]
1.6428±0.0326 g/cm3[5]
≈ 0.102 km/s
9.2735 h (9 h 16 min 25 s ± 3 s)[6]
152.14° (to orbit)[7]
0.100±0.005[8]
≈ 73(?) K
6.59±0.02[8]
Phoebe was the first target encountered upon the arrival of the Cassini spacecraft in the Saturn system in 2004, and is thus unusually well-studied for an irregular moon of its size. Cassini's trajectory to Saturn and time of arrival were specifically chosen to permit this flyby.[10] After the encounter and its insertion into orbit, Cassini did not go much beyond the orbit of Iapetus.
Phoebe is roughly spherical and has a differentiated interior. It was spherical and hot early in its history and was battered out of roundness by repeated impacts. There is some evidence that it may be a captured centaur that originated in the Kuiper belt.[11] Phoebe is the second-largest retrograde satellite in the Solar System after Triton.[12]
Map of Phoebe's middle latitudes. The higher latitudes have been clipped from the main map, but can be seen in the polar projections.
Map of Phoebe's south polar region
Map of Phoebe's north polar region
3D map showing Phoebe's once spherical shape
Formation[edit]
Phoebe formed in the Kuiper belt within three million years after the origin of the Solar System. This was early enough that sufficient radioactive material was available to melt it into a sphere and stay warm enough to have liquid water for tens of millions of years.[31]