Saturn's Moon: Phoebe Rotation

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• published: 08 Jan 2010
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Phoebe is an irregular satellite of Saturn. It was discovered by William Henry Pickering on 17 March 1899 from photographic plates that had been taken starting on 16 August 1898 at Arequipa, Peru by DeLisle Stewart. It was the first satellite to be discovered photographically. Phoebe was the first target encountered upon the arrival of CassiniHuygens to the Saturn system in 2004, and is thus unusually well-studied for a natural satellite of its size. Cassini's trajectory to Saturn and time of arrival were specifically chosen to permit this flyby. After the encounter and its insertion orbit, Cassini would not go much beyond the orbit of Iapetus. The moon is named after Phoebe, a Titan in Greek mythology. It is also designated Saturn IX. The IAU nomenclature standards have stated that features on Phoebe are to be named after characters in the Greek myth of Jason and the Argonauts. In 2005, the IAU officially named 24 craters (Acastus, Admetus, Amphion, Butes, Calais, Canthus, Clytius, Erginus, Euphemus, Eurydamas, Eurytion, Eurytus, Hylas, Idmon, Iphitus, Jason, Mopsus, Nauplius, Oileus, Peleus, Phlias, Talaus, Telamon, and Zetes). Dr. Toby Owen of the University of Hawaii at Manoa, chairman of the International Astronomical Union Outer Solar System Task Group said: "We picked the legend of the Argonauts for Phoebe as it has some resonance with the exploration of the Saturn system by Cassini-Huygens. We can't say that our participating scientists include heroes like Hercules and Atalanta, but they do represent a wide, international spectrum of outstanding people who were willing to take the risk of joining this voyage to a distant realm in hopes of bringing back a grand prize." 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). See Saturn's satellites families. Phoebe is roughly spherical and has a diameter of 220 kilometres (140 mi), 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 75 K (-198°C). Most of Saturn's inner moons have very bright surfaces, but Phoebe's albedo is very low (0.06), as dark as lampblack. The Phoebean surface is extremely heavily scarred, with craters up to 80 kilometres across, one of which has walls 16 kilometres high. 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 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 metres (980 to 1,600 ft) 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 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. Phoebe is the first such object to be imaged as. Discovered by W.H. Pickering Discovery date 17 March 1899 / 16 August 1898 Designations Alternate name Saturn IX Adjective Phoebean Orbital characteristics Semi-major axis 12 955 759 km Eccentricity 0.156 241 5 Orbital period 550.564 636 d Inclination 173.04° (to the ecliptic) 151.78° (to Saturn's equator) Satellite of Saturn Physical characteristics Dimensions 230 x 220 x 210 km Mean radius 106.60 ± 1.00 km Mass (0.829 2 ± 0.001 0) × 1019 kg Mean density 1.634 2 ± 0.046 0 g/cm³ Equatorial surface gravity ~0.049 m/s2 Escape velocity ~0.10 km/s Sidereal rotation period 0.386 75 d (9 h 16 min 55.2 s) Axial tilt 152.14° Albedo 0.06