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Phoebe (moon)

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Phoebe closeup cassini NASA.jpg
Discovered by W.H. Pickering
Discovery date 17 March 1899 & 16 August 1898
Saturn IX
Adjectives Phoebean
Orbital characteristics[1]
12955759 km
Eccentricity 0.1562415
550.564636 d
Inclination 173.04° (to the ecliptic)
151.78° (to Saturn's equator)
Satellite of Saturn
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]
0.038–0.050 m/s2[1]
≈ 0.10 km/s
0.38675 d (9 h 16 min 55.2 s) [2]
152.14° [3]
Albedo 0.06

Phoebe (/ˈfb/;[4] Greek: Φοίβη) is an irregular satellite of Saturn a little over 200 km in diameter. 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.


The satellite is named after Phoebe, a Titan in Greek mythology that was associated with the Moon.[7] It is also designated Saturn IX in some scientific literature. 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[edit]

Phoebe's orbit is retrogade, i.e. it orbits Saturn opposite to Saturn's rotation. 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 regular moons except 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[edit]

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 the 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 (−198.2 °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 1,640 ft) thick. In addition, quantities of carbon dioxide have been detected on the surface, a finding that has never been replicated for 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, slightly non-equilibrium shape.[14]

Material displaced from Phoebe's surface by microscopic meteor impacts may be responsible for the dark surfaces of Hyperion.[note 1] Debris from the biggest impacts may have been the origin of the other moons of Phoebe's group (the Norse group)—all of which are less than 10 km in diameter.


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 showing Phoebe's once spherical shape

Spacecraft flybys[edit]

Unlike Saturn's other moons, Phoebe was not favorably placed for the Voyager probes. Voyager 2 transmitted a single image of Phoebe in September 1981, although the distance of 2.2 million kilometres and low resolution meant that relatively little could be learned from it.

Cassini passed within 2,068 kilometres (1,285 mi) of Phoebe on 11 June 2004, returning many high-resolution images, which revealed a scarred surface. Because Voyager 2 had not been able to produce any decent images of Phoebe, obtaining them was a priority for the Cassini mission[10] and its flight path was deliberately designed to take it close by, otherwise Cassini would likely not have returned images much better than Voyager's. Due to its short rotation period of approximately 9 hours, 56 minutes, Cassini was able to map virtually the entire surface of Phoebe. The close flyby enabled the mass of Phoebe to be determined with an uncertainty of only 1 in 500.[15]

Phoebe ring[edit]

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

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 infrared Spitzer Space Telescope.

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

See also[edit]


  1. ^ The composition implied by spectra does not seem to support the earlier suggestion that Phoebe could be the source of the dark material deposited on Iapetus.


  1. ^ a b c d e Thomas, P. C. (July 2010). "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission". Icarus 208 (1): 395–401. Bibcode:2010Icar..208..395T. doi:10.1016/j.icarus.2010.01.025.  edit
  2. ^ Seidelmann, P. K.; Abalakin, V. K.; Bursa, M.; Davies, M. E.; de Bergh, C.; Lieske, J. H.; Oberst, J.; Simon, J. L.; Standish, E. M.; Stooke, P.; and Thomas, P. C.Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000
  3. ^ Porco CC et al. (2005-02-25). "Cassini Imaging Science: Initial Results on Phoebe and Iapetus". Science 307 (5713): 1237–1242. Bibcode:2005Sci...307.1237P. doi:10.1126/science.1107981. PMID 15731440. 
  4. ^ In US dictionary transcription, US dict: fē′·bē.
  5. ^ Pickering EC (1899-03-17). "A New Satellite of Saturn". 49. Harvard College Observatory Bulletin. 
  6. ^ Pickering EC (1899-03-23). "A New Satellite of Saturn". Astronomical Journal 20 (458): 13. Bibcode:1899AJ.....20...13P. doi:10.1086/103076. 
  7. ^ a b Pickering EC (1899-04-10). "A New Satellite of Saturn". Astrophysical Journal 9 (4): 274–276. Bibcode:1899ApJ.....9..274P. doi:10.1086/140590. 
  8. ^ Pickering EC (1899-04-29). "A New Satellite of Saturn". Astronomische Nachrichten 149 (10): 189–192. Bibcode:1899AN....149..189P. doi:10.1002/asna.18991491003.  (same as above)
  9. ^ "A Ninth Satellite to Saturn". The Observatory 22 (278): 158–159. April 1899. Bibcode:1899Obs....22..158. 
  10. ^ a b Martinez, Carolina; Brown, Dwayne (2004-06-09). "Cassini Spacecraft Near First Stop in Historic Saturn Tour". Mission News. NASA. Retrieved 2008-03-02. 
  11. ^ Features on Saturn's moon Phoebe given names, Spaceflight Now, February 24, 2005
  12. ^ Johnson, Torrence V.; Lunine, Jonathan I. (2005). "Saturn's moon Phoebe as a captured body from the outer Solar System". Nature 435 (7038): 69–71. Bibcode:2005Natur.435...69J. doi:10.1038/nature03384. PMID 15875015. 
  13. ^ Martinez, C. (May 6, 2005). "Scientists Discover Pluto Kin Is a Member of Saturn Family". Cassini–Huygens News Releases. 
  14. ^ JPL/NASA, 2012 Apr 26. Cassini Finds Saturn Moon Has Planet-Like Qualities
  15. ^ Roth et al., AAS Paper 05-311
  16. ^ Verbiscer, Anne; Skrutskie, Michael; Hamilton, Douglas (2009-10-07). "Saturn’s largest ring". Nature 461 (7267): 1098–100. Bibcode:2009Natur.461.1098V. doi:10.1038/nature08515. PMID 19812546. 
  17. ^ "The King of Rings". NASA, Spitzer Space Telescope center. 2009-10-07. Retrieved October 7, 2009. 
  18. ^ Cowen, Rob (October 6, 2009). "Largest known planetary ring discovered". Science News. 
  19. ^ Largest ring in solar system found around Saturn, New Scientist
  20. ^ Mason, J.; Martinez, M.; Balthasar, H. (2009-12-10). "Cassini Closes In On The Centuries-old Mystery Of Saturn's Moon Iapetus". CICLOPS website newsroom. Space Science Institute. Retrieved 2009-12-22. 
  21. ^ Denk, T. et al. (2009-12-10). "Iapetus: Unique Surface Properties and a Global Color Dichotomy from Cassini Imaging". Science (AAAS) 326 (5964): 435–439. Bibcode:2010Sci...327..435D. doi:10.1126/science.1177088. PMID 20007863. Retrieved 2009-12-19. 
  22. ^ Spencer, J. R.; Denk, T. (2009-12-10). "Formation of Iapetus’ Extreme Albedo Dichotomy by Exogenically Triggered Thermal Ice Migration". Science (AAAS) 326 (5964): 432–435. Bibcode:2010Sci...327..432S. doi:10.1126/science.1177132. PMID 20007862. Retrieved 2009-12-19. 

External links[edit]