Phoebe

Phoebe

Cassini photomosaic; the large craters at the upper right and the lower left are Jason and Oileus. Bright streaks in crater walls (as at top) suggest the presence of ice below a mostly ice-depleted surface.
Discovery
Discovered by	W.H. Pickering
Discovery date	17 March 1899 / 16 August 1898
Designations
Alternate name(s)	Saturn IX
Adjective	Phoebean
Orbital characteristics[1]
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
Proper orbital elements
Physical characteristics
Dimensions	218.8×217×203.6 km[1]
Mean radius	106.5 ± 0.7 km[1]
Mass	(0.8292 ± 0.0010)×1019 kg[1]
Mean density	1.638 ± 0.033 g/cm³[1]
Equatorial surface gravity	0.038–0.050 m/s2[1]
Escape velocity	~0.10 km/s
Sidereal rotation period	0.386 75 d (9 h 16 min 55.2 s) [2]
Axial tilt	152.14° [3]
Albedo	0.06

Phoebe ( /ˈfiːbiː/;^[4] Greek: Φοίβη) is an irregular satellite of Saturn. It is thought to be a captured planetesimal from the Kuiper belt. It 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.^{[5][6][7][8][9]} It was the first satellite to be discovered photographically.

Phoebe was the first target encountered upon the arrival of the Cassini spacecraft 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.^[10] After the encounter and its insertion into orbit, Cassini would not go much beyond the orbit of Iapetus.

Name

The moon is named after Phoebe,^[7] 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^[11] (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.

Orbital characteristics

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 moderately 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.

Physical characteristics

Cassini spacecraft closeup of Phoebe from 13 June 2004; the crater Euphemus is at top center.

Named craters on Phoebe.

Phoebe is roughly spherical and has a diameter of 213 kilometres (132 mi), which is equal to about one-sixteenth 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 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 (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.^[12][13] Phoebe is the first such object to be imaged as anything other than a dot.

Despite its small size, Phoebe is thought to have been a hot, spherical body early in its history, with a differentiated interior, before solidifying and being battered into its current, slighlty non-equilibrium shape.^[14]

Material displaced from Phoebe's surface by microscopic meteor impacts may be responsible for the dark surfaces of Hyperion.^[15] 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.

Maps

Map of Phoebe's middle latitudes (143 kB). The higher latitudes have been clipped from the main map, but can be seen in the polar projections below.

Map of Phoebe's south polar region.

Map of Phoebe's north polar region.

3D map of Phoebe, showing its once spherical shape

Spacecraft flybys

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

Cassini passed within 2,068 kilometres (1,285 mi) of Phoebe on 11 June 2004, returning many high-resolution images of the moon and its scarred surface. The mission plan^[10] was designed to allow Cassini to perform a close flyby of Phoebe during its approach to Saturn; otherwise, due to Phoebe's distance from Saturn, the pictures returned would probably not have been much better than those from Voyager. Due to its rapid rotation period of approximately 9 hours, 56 minutes, Cassini was able to map virtually the entire surface of Phoebe.

Phoebe ring

Artist's impression of the Phoebe ring, which dwarfs the main rings.

Wikinews has related news: New ring discovered around Saturn, could explain dark side of its moon

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^[16] times the radius of Saturn; Phoebe orbits the planet at an average distance of 215 Saturn radii. The ring is about 20 times as thick as the diameter of the planet.^[17] Since the ring's particles are presumed to have originated from micrometeoroid impacts on Phoebe, they should share its retrograde orbit,^[18] 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.^{[19][20][21][22]} Although very large, the ring is virtually invisible—it was discovered using NASA's infra-red Spitzer Space Telescope.

Phoebe (with NGC 4179 in the lower right corner) as imaged with a 24" telescope.

See also

• Moons of Saturn
• List of geological features on Saturn's smaller moons

References

External links

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Wikimedia Commons has media related to: Phoebe

• Phoebe Profile at NASA's Solar System Exploration site
• Cassini–Huygens Multimedia: Images: Moons: Small Moons
• Cassini Pass Reveals Moon Secrets, BBC News, June 14, 2004
• The Planetary Society: Phoebe
• Asaravala, A.; Saturn's Odd Moon Out, Wired, (May 4, 2005)
• NASA: Natural Satellite Physical Parameters
• Cassini images of Phoebe
• Images of Phoebe at JPL's Planetary Photojournal
• Movie of Phoebe's rotation from the National Oceanic and Atmospheric Administration
• Phoebe basemap (December 2005) from Cassini images
• Phoebe atlas (March 2006) from Cassini images
• Phoebe nomenclature from the USGS planetary nomenclature page