Phoebe (moon)

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For other meanings see Phoebe.

**Phoebe**
Discovery

Discovered by	W.H. Pickering
Discovered in	1898
Orbital characteristics
Semimajor axis	km
Eccentricity	0.163
Orbital period	550 day 11 h 31 min
Inclination	175.3 °
Is a satellite of	Saturn
Physical characteristics
Mean diameter	220 km
Mass	18 kilogram>kg
Mean density	2.3 g/cm³
Surface gravity	m/s²
Rotation period	9 h 30 min
Axial tilt	26.183 °
Albedo	0.06
Atmosphere	none

Phoebe is a moon of Saturn. It was discovered by William Henry Pickering in 1899 from photographic plates that had been taken in August 1898 at Arequipa, Peru by DeLisle Stewart. It was the first satellite to be discovered photographically.

It is named after Phoebe, a Titan in Greek mythology. It is also designated Saturn IX.

Table of contents

1 Overview
2 Characteristics
3 Spacecraft flybys
4 See also
5 External links

Overview

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 substatially larger than any of the other moons orbiting at comparable distances. Phoebe and Iapetus are the only major moons in the Saturn region that do not orbit close to the plane of Saturn's equator.

All of Saturn's moons except for Phoebe and Iapetus orbit very nearly in the plane of Saturn's equator. Phoebe's orbit is retrograde, inclined almost 175 °, and is highly eccentric. The plane of its orbit is much closer to the solar system's ecliptic than it is to Saturn's. Phoebe is also unusual in that it does not rotate synchronously as all the other moons of Saturn except Hyperion do.

Characteristics

Phoebe is roughly spherical and has a diameter of 220 kilometers (about 137 miles), which is equal to about one-fifteenth of the diameter of Earth's moon. Phoebe rotates on its axis every nine hours and it completes a full orbit around Saturn in about 18 months. Its surface temperature is only 75K (-198°C).

Most of Saturn's moons have very bright surfaces, but Phoebe's albedo is very low (0.06), as dark as lampblack. The surface is extremely heavily scarred, with craters up to 80 kilometres across, one of which has walls 16 kilometres high.

Phoebe's dark colouring 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 the Cassini-Huygens space probe 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 meters (980 to 1,600 feet) thick. In addition, quantities of carbon dioxide have been detected on the surface, a finding which has never been replicated on an asteroid. It is estimated that Phoebe is about 50% rock, as opposed to the 35% or so that typifies Saturn's other moons. For these reasons, scientists are coming to believe that Phoebe is a captured Centaur, one of a number of icy planetoids from the Kuiper belt that orbit the Sun between Jupiter and Neptune. If this is the case, Phoebe is the first such object to be imaged as anything other than a dot.

Material displaced from Phoebe's surface by microscopic meteor impacts may be responsible for the dark surfaces of Hyperion and the leading hemisphere of Iapetus. The debris from the biggest impacts may also have been the building blocks of four of Saturn's moons - Skathi, Mundilfari, Suttungr, and Thrymr - all of which are less than 10 km in diameter and have similar orbits to Phoebe.

Spacecraft flybys

Closeup image of Phoebe from Cassini-Huygens

The Voyager 2 spacecraft passed by Phoebe in September 1981, although the long distance (2.2 million km) and low resolution meant that relatively little could be learned from the resulting images.

The Cassini spacecraft flew within 2,068 kilometers (about 1,285 miles) of Phoebe on June 11, 2004, returning many high-resolution images of the moon and its scarred surface.

External links