Phoebe
Cassini image of Phoebe
Discovery
Discovered byW.H. Pickering
Discovery date18 March 1899 (from photos taken 16 August 1898)
Designations
Designation
Saturn IX
Pronunciation/ˈfiːbi/ FEE-bee[1][2]
Named after
Φοίβη Phoíbē
AdjectivesPhoebean /fiːˈbiːən/ fee-BEE-ən[3]
Orbital characteristics
12,960,000 km
Eccentricity0.1562415
550.564636 d
Inclination173.04° (to the ecliptic)
151.78° (to Saturn's equator)
Satellite ofSaturn
GroupNorse group
Physical characteristics
Dimensions(218.8±2.8) × (217.0±1.2) 
× (203.6±0.6) km[4]
Mean radius
106.5±0.7 km[4][5]
Mass(8.3123±0.0162)×1018 kg[5]
Mean density
1.6428±0.0326 g/cm3[5]
0.038–0.050 m/s2[4]
≈ 0.10 km/s
9.2735 h (9 h 16 min 25 s ± 3 s)[6]
152.14° (to orbit)[7]
Albedo0.06
Temperature≈ 73(?) K

    Phoebe (/ˈfiːbi/ FEE-bee) is the most massive irregular satellite of Saturn with a mean diameter of 213 km (132 mi). It was discovered by William Henry Pickering on 18 March 1899[8] from photographic plates that had been taken starting on 16 August 1898 at the Boyden Station of the Carmen Alto Observatory near Arequipa, Peru, by DeLisle Stewart. It was the first satellite to be discovered photographically.

    Phoebe was the first target encountered upon the arrival of the Cassini spacecraft in the Saturn system in 2004, and is thus unusually well-studied for an irregular satellite of its size. Cassini's trajectory to Saturn and time of arrival were specifically chosen to permit this flyby.[9] After the encounter and its insertion into orbit, Cassini did not go much beyond the orbit of Iapetus.

    Phoebe is roughly spherical and has a differentiated interior. It was spherical and hot early in its history and was battered out of roundness by repeated impacts. It is believed to be a captured centaur that originated in the Kuiper belt.[10] Phoebe is the second largest retrograde satellite in the Solar System after Triton.[11]

    History

    Discovery

    Phoebe was discovered by William Henry Pickering on 17 March 1899[8] from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory near Arequipa, Peru, by DeLisle Stewart.[12][13][14][15][16][17] It was the first satellite to be discovered photographically.

    Naming

    Phoebe was named after Phoebe, a Titaness in Greek mythology associated with the Moon that was Saturn's sister.[15] 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 (see Named features).[18]

    Toby Owen of the University of Hawaiʻi at Mānoa, 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.

    Orbit

    Animation of Phoebe's orbit.
       Saturn ·    Phoebe ·   Titan

    Phoebe's orbit is retrograde; that is, 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 are expected to rotate synchronously as all the inner moons of Saturn do (except for Hyperion). See Saturn's satellites' families.

    Phoebe ring

    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[19] times the radius of Saturn; Phoebe orbits the planet at an average distance of 215 Saturn radii. The ring is about 40 times as thick as the diameter of the planet.[20] Because the ring's particles are presumed to have originated from micrometeoroid impacts on Phoebe, they should share its retrograde orbit,[21] which is opposite to the orbital motion of the next inner moon, Iapetus. Inwardly migrating ring material would thus strike Iapetus's leading hemisphere, contributing to its two-tone coloration.[22][23][24][25] Although very large, the ring is virtually invisible—it was discovered using NASA's infrared Spitzer Space Telescope.

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

    Physical characteristics

    Cassini's 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±1.4 km[4] (132 mi), approximately one-sixteenth that of the Moon. It is Saturn's ninth-largest moon, but it might be the eighth-most massive. Hyperion, another one of Saturn's moons, has a larger radius, but is less massive than Phoebe. Phoebe rotates every nine hours and 16 minutes, and completes a full orbit around Saturn in about 18 months. Its surface temperature is on average 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 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 think 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.[26][27] 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 spherical body early in its history, with a differentiated interior, before solidifying and being battered into its current, slightly non-equilibrium shape.[28]

    Named features

    Apart from one regio named after Phoebe's daughter, Leto, all named features are craters named after characters from the Greek legend of Jason and the Argonauts.[29]

    Regios
    NamePronunciationGreekCoordinatesDiameter (km)Approval DateNamed AfterRef
    Leto Regio/ˈliːtoʊ/Λητώ60°00′N 20°00′W / 60.0°N 20.0°W / 60.0; -20.0 (Leto)952000Daughter of Phoebe in Greek mythologyWGPSN
    Named Phoebean craters
    NamePronunciationGreekCoordinatesDiameter (km)Approval DateNamed AfterRef
    Acastus/əˈkæstəs/Ἄκαστος9°36′N 148°30′W / 9.6°N 148.5°W / 9.6; -148.5 (Acastus)342006Son of Thessalian king Pelias, took part in the Calydonian boar huntWGPSN
    Admetus/ædˈmiːtəs/Ἄδμητος11°24′N 39°06′W / 11.4°N 39.1°W / 11.4; -39.1 (Admetus)582006Founder and king of Pherae in ThessalyWGPSN
    Amphion/æmˈfaɪ.ɒn/Ἀμφῑ́ων27°00′S 1°48′W / 27.0°S 1.8°W / -27.0; -1.8 (Amphion)182006Son of Hyperasius and HypsoWGPSN
    Butes/ˈbjuːtiːz/Βούτης49°36′S 292°30′W / 49.6°S 292.5°W / -49.6; -292.5 (Butes)292006Son of Teleon, bee-masterWGPSN
    Calais/ˈkæliəs/Κάλαϊς38°42′S 225°24′W / 38.7°S 225.4°W / -38.7; -225.4 (Calais)312006Son of Boreas, the north windWGPSN
    Canthus/ˈkænθəs/Κάνθος69°36′S 342°12′W / 69.6°S 342.2°W / -69.6; -342.2 (Canthus)442006Son of Kanethos or Cerion, the only member of the expedition to die in combatWGPSN
    Clytius/ˈklɪtiəs, -ʃəs/Κλυτίος46°00′N 193°06′W / 46.0°N 193.1°W / 46.0; -193.1 (Clytius)522006Son of Eurytus, skilled archer who was killed by Apollo for challenging the god to a shooting matchWGPSN
    Erginus/ˈɜːrdʒɪnəs/Ἐργῖνος31°36′N 337°06′W / 31.6°N 337.1°W / 31.6; -337.1 (Erginus)382006Son of Neptune, helmsman of the Argo after the death of TiphysWGPSN
    Euphemus/juːˈfiːməs/Εὔφημος31°18′S 331°06′W / 31.3°S 331.1°W / -31.3; -331.1 (Euphemus)232006Son of Neptune and EuropaWGPSN
    Eurydamas/jʊˈrɪdəməs/Εὐρυδάμᾱς61°30′S 281°36′W / 61.5°S 281.6°W / -61.5; -281.6 (Eurydamas)192006Son of CtimenusWGPSN
    Eurytion/jʊˈrɪtiən/Εὐρυτίων30°24′S 8°00′W / 30.4°S 8.0°W / -30.4; -8.0 (Eurytion)142006Son of Kenethos or CerionWGPSN
    Eurytus/ˈjʊərɪtəs/Εὔρυτος39°42′S 177°12′W / 39.7°S 177.2°W / -39.7; -177.2 (Eurytus)892006Son of Mercury and AntianiraWGPSN
    Hylas/ˈhaɪləs/Ὕλας7°54′N 354°30′W / 7.9°N 354.5°W / 7.9; -354.5 (Hylas)302006Son of Theiodamas/Theodamas, king of the DryopesWGPSN
    Idmon/ˈɪdmɒn/Ἴδμων67°06′S 197°48′W / 67.1°S 197.8°W / -67.1; -197.8 (Idmon)612006Son of Apollo and the nymph Cyrene, or of Abas, a prophetWGPSN
    Iphitus/ˈɪfɪtəs/Ἴφιτος27°12′S 293°18′W / 27.2°S 293.3°W / -27.2; -293.3 (Iphitus)222006Son of Eurytus, Jason's host during his consultation with the Oracle at DelphiWGPSN
    Jason/ˈdʒeɪsən/Ἰάσων16°12′N 317°42′W / 16.2°N 317.7°W / 16.2; -317.7 (Jason)1012006The leading Argonaut, son of the Thessalian king Aeson, delivered the FleeceWGPSN
    Mopsus/ˈmɒpsəs/Μόψος6°36′N 109°06′W / 6.6°N 109.1°W / 6.6; -109.1 (Mopsus)372006Prophesying son of ApolloWGPSN
    Nauplius/ˈnɔːpliəs/Ναύπλιος31°30′N 241°30′W / 31.5°N 241.5°W / 31.5; -241.5 (Nauplius)242006Son of Neptune and Amymone, or of KlytoneosWGPSN
    Oileus/oʊˈaɪliːəs/Ὀϊλεύς77°06′S 96°54′W / 77.1°S 96.9°W / -77.1; -96.9 (Oileus)562006King of the Locrians, renowned for his courage in battleWGPSN
    Peleus/ˈpiːliːəs/Πηλεύς20°12′N 192°12′W / 20.2°N 192.2°W / 20.2; -192.2 (Peleus)442006Son of Aeacus, father of AchillesWGPSN
    Phlias/ˈflaɪəs/Φλίας1°36′N 359°06′W / 1.6°N 359.1°W / 1.6; -359.1 (Phlias)142006Son of DionysusWGPSN
    Talaus/ˈtæliəs/Ταλαός52°18′S 325°12′W / 52.3°S 325.2°W / -52.3; -325.2 (Talaus)152006Son of Teleon, or of Bias and PeroWGPSN
    Telamon/ˈtɛləmən/Τελαμών48°06′S 92°36′W / 48.1°S 92.6°W / -48.1; -92.6 (Telamon)282006Son of Aeacus, took part in the Calydonian boar huntWGPSN
    Zetes/ˈziːtiːz/Ζήτης20°00′S 223°00′W / 20.0°S 223.0°W / -20.0; -223.0 (Zetes)292006Son of Boreas, the north windWGPSN

    Maps

    Formation

    Phoebe formed in the Kuiper belt within three million years after the origin of the Solar System. This was early enough that sufficient radioactive material was available to melt it into a sphere and stay warm enough to have liquid water for tens of millions of years.[28]

    Observation and exploration

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

    Unlike Saturn's other moons, Phoebe was not favorably placed for the Voyager probes. Voyager 2 observed Phoebe for a few hours in September 1981. In the images, taken from a distance of 2.2 million kilometres at low phase angle, the size of Phoebe was approximately 11 pixels and showed bright spots on the otherwise dark surface.

    Cassini passed 2,068 kilometres (1,285 mi) from 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 high-quality images of Phoebe, obtaining them was a priority for the Cassini mission[9] 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. Because of Phoebe's short rotation period of approximately 9 hours, 17 minutes, Cassini was able to map virtually the entire surface of Phoebe. The close fly-by enabled the mass of Phoebe to be determined with an uncertainty of only 1 in 500.[30]

    See also

    Wikimedia Commons has media related to Phoebe.

    Notes

    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.

    References

    1. ↑ "Phoebe". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
    2. ↑ The Latin form is 'Phoēbē', with stress on the first 'e' (Phoebe), but no-one proposes that pronunciation for English
    3. ↑ "Phoebean". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
    4. 1 2 3 4 Thomas, P. C. (July 2010). "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission" (PDF). Icarus. 208 (1): 395–401. Bibcode:2010Icar..208..395T. doi:10.1016/j.icarus.2010.01.025. Archived from the original (PDF) on 2011-09-27. Retrieved 2015-09-04.
    5. 1 2 3 Jacobson, Robert. A. (1 November 2022). "The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole*". The Astronomical Journal. 164 (5): 199. doi:10.3847/1538-3881/ac90c9.
    6. ↑ Bauer, J.M.; Buratti, B.J.; Simonelli, D.P.; Owen, W.M. (2004). "Recovering the Rotational Lightcurve of Phoebe". The Astronomical Journal. 610 (1): L57–L60. Bibcode:2004ApJ...610L..57B. doi:10.1086/423131.
    7. ↑ Porco CC; et al. (2005-02-25). "Cassini Imaging Science: Initial Results on Phoebe and Iapetus" (PDF). Science. 307 (5713): 1237–1242. Bibcode:2005Sci...307.1237P. doi:10.1126/science.1107981. PMID 15731440. S2CID 20749556. Archived (PDF) from the original on 2018-07-19. Retrieved 2019-09-23.
    8. 1 2 Kovas, Charlie. "On This Day". What Happened on March 18, 1899. Unknown. Archived from the original on September 12, 2015. Retrieved September 21, 2017.
    9. 1 2 Martinez, Carolina; Brown, Dwayne (2004-06-09). "Cassini Spacecraft Near First Stop in Historic Saturn Tour". Mission News. NASA. Archived from the original on 2008-06-28. Retrieved 2008-03-02.
    10. ↑ Jewitt, David; Haghighipour, Nader (2007). "Irregular Satellites of the Planets: Products of Capture in the Early Solar System" (PDF). Annual Review of Astronomy and Astrophysics. 45 (1): 261–95. arXiv:astro-ph/0703059. Bibcode:2007ARA&A..45..261J. doi:10.1146/annurev.astro.44.051905.092459. S2CID 13282788. Archived from the original (PDF) on 2009-09-19.
    11. ↑ "Planetary Satellite Physical Parameters". Archived from the original on 2021-09-04. Retrieved 2020-06-09.
    12. ↑ Pickering EC (1899-03-17). "A New Satellite of Saturn". Harvard College Observatory Bulletin. 9 (221): 456. doi:10.1126/science.9.221.456. PMID 17844472. S2CID 30496398. Archived from the original on 2016-03-03. Retrieved 2004-07-14.
    13. ↑ Pickering, E. C. (24 March 1899). "A New Satellite of Saturn". Science. 9 (221): 456. doi:10.1126/science.9.221.456. PMID 17844472. S2CID 30496398.
    14. ↑ Pickering EC (1899-03-23). "A New Satellite of Saturn". Astronomical Journal. 20 (458): 13. Bibcode:1899AJ.....20...13P. doi:10.1086/103076. Archived from the original on 2013-11-12. Retrieved 2004-07-14.
    15. 1 2 Pickering EC (1899-04-10). "A New Satellite of Saturn". Astrophysical Journal. 9 (4): 274–276. Bibcode:1899ApJ.....9..274P. doi:10.1086/140590. PMID 17844472. Archived from the original on 2016-04-11. Retrieved 2004-07-14.
    16. ↑ 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. Archived from the original on 2013-11-12. Retrieved 2004-07-14. (same as above)
    17. ↑ "A Ninth Satellite to Saturn". The Observatory. 22 (278): 158–159. April 1899. Bibcode:1899Obs....22..158. Archived from the original on 2013-11-12. Retrieved 2004-07-14.
    18. ↑ Features on Saturn's moon Phoebe given names Archived 2016-01-10 at the Wayback Machine, Spaceflight Now, February 24, 2005
    19. ↑ 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. S2CID 4349726.
    20. ↑ "The King of Rings". NASA, Spitzer Space Telescope center. 2009-10-07. Archived from the original on 2009-10-10. Retrieved October 7, 2009.
    21. ↑ Cowen, Rob (October 6, 2009). "Largest known planetary ring discovered". Science News. Archived from the original on August 22, 2011. Retrieved October 6, 2009.
    22. ↑ Largest ring in solar system found around Saturn Archived 2015-05-18 at the Wayback Machine, New Scientist
    23. ↑ 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. Archived from the original on 2012-02-03. Retrieved 2009-12-22.
    24. ↑ Denk, T.; et al. (2009-12-10). "Iapetus: Unique Surface Properties and a Global Color Dichotomy from Cassini Imaging". Science. 327 (5964): 435–439. Bibcode:2010Sci...327..435D. doi:10.1126/science.1177088. PMID 20007863. S2CID 165865.
    25. ↑ Spencer, J. R.; Denk, T. (2009-12-10). "Formation of Iapetus' Extreme Albedo Dichotomy by Exogenically Triggered Thermal Ice Migration". Science. 327 (5964): 432–435. Bibcode:2010Sci...327..432S. CiteSeerX 10.1.1.651.4218. doi:10.1126/science.1177132. PMID 20007862. S2CID 20663944.
    26. ↑ 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. S2CID 4390697.
    27. ↑ Martinez, C. (May 6, 2005). "Scientists Discover Pluto Kin Is a Member of Saturn Family". Cassini–Huygens News Releases. Archived from the original on May 1, 2008.
    28. 1 2 Jia-Rui C. Cook and Dwayne Brown (2012-04-26). "Cassini Finds Saturn Moon Has Planet-Like Qualities". JPL/NASA. Archived from the original on 2012-04-27.
    29. ↑ Phoebean craters Archived 2020-09-25 at the Wayback Machine, USGS
    30. ↑ Roth et al., AAS Paper 05-311
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