Phoebe belongs to the Norse group: a large group of retrograde irregular satellites of Saturn. Their semi-major axes range between 12 and 24 Gm, their inclinations between 136° and 175° and their eccentricities between 0.13 and 0.77.
Discovery
Phoebe was discovered by William Henry Pickering on 17 March 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory near Arequipa, Peru, by DeLisle Stewart.
Phoebe was the first satellite to be discovered photographically.
Naming
Phoebe the moon is named after Phoebe, a Titan in Greek mythology.
In Greek mythology Phoebe was one of the original Titans, who were one set of sons and daughters of Uranus and Gaia.
Stats
Diameter (mean): 213 km
Semi-major axis: 12,955,759 km
Orbital Period: -550.56 days
Orbit
For more than 100 years, Phoebe was Saturn's outermost known moon, until the discovery of several smaller moons in 2000. Phoebe is almost 4 times more distant from Saturn than its nearest major neighbor, Iapetus, and is substantially larger than any of the other moons orbiting planets at comparable distances.
All of Saturn's moons up to Iapetus orbit very nearly in the plane of Saturn's equator. The outer irregular satellites follow fairly to highly eccentric orbits, and none is expected to rotate synchronously as all the inner moons of Saturn do (except for Hyperion).
Formation
Most of Saturn's inner moons have very bright surfaces, but Phoebe's albedo is very low (0.06), as dark as lampblack.
Phoebe's dark coloring initially led to scientists surmising that it was a captured asteroid, as it resembled the common class of dark carbonaceous asteroids. These are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula with little modification since then.
However, images from Cassini indicate that Phoebe's craters show a considerable variation in brightness, which indicate the presence of large quantities of ice below a relatively thin blanket of dark surface deposits some 300 to 500 metres thick. In addition, quantities of carbon dioxide have been detected on the surface, a finding which has never been replicated on an asteroid.
Morevover, it is estimated that Phoebe is about 50% rock, as opposed to the 35% or so that typifies Saturn's inner moons.
For these reasons, scientists are coming to believe that Phoebe is in fact a captured centaur, one of a number of icy planetoids from the Kuiper belt that orbit the Sun between Jupiter and Neptune.
Physical characteristics
Phoebe is roughly spherical and has a diameter of 213 kilometres, which is equal to about one-fifteenth of the diameter of Earth's Moon.
The Phoebean surface is extremely heavily scarred, with craters up to 80 kilometres across, one of which has walls 16 kilometres high.
Material displaced from Phoebe's surface by microscopic meteor impacts may be responsible for the dark surfaces of Hyperion. Debris from the biggest impacts may have been the building blocks of the other moons of Phoebe's group — all of which are less than 10 km in diameter.
Phoebe ring
The Phoebe ring is one of the rings of Saturn. This ring is tilted 27 degrees from Saturn's equatorial plane (and the other rings). It extends from at least 128 to 207 times the radius of Saturn.
The diameter of the ring is equivalent to 300 Saturns lined up side to side. The ring is thick too -- it's about 20 times as thick as the diameter of the planet.
Since the ring's particles are presumed to have originated from micrometeoroid impacts on Phoebe, they should share its retrograde orbit, which is opposite to the orbital motion of the next inner moon, Iapetus.
Inwardly migrating ring material would thus strike Iapetus's leading hemisphere, and is suspected to have triggered the processes that led to the two-tone coloration of that moon.
Although very large, the ring is virtually invisible — it was discovered using NASA's infra-red Spitzer Space Telescope.
Why did it take so long to find something so big? The answer is that the ring is very tenuous, made up of a sparse collection of ice and dust particles. If you could transport yourself to the ring, you wouldn't even know you were there because the particles are so far apart. There's not a lot of sunlight out at Saturn, so this small density of particles doesn't reflect much visible light. Spitzer was able to spot the band because it sees infrared light, or heat radiation, from objects. Even though the ring material is very cold, it still gives off heat that can Spitzer can see.
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