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Janus (moon)

January 26, 2023

"Saturn X" redirects here. For the spurious moon reported in 1905, see Themis (hypothetical moon).

Janus /ˈdʒeɪnəs/ is an inner satellite of Saturn. It is also known as Saturn X. It is named after the mythological Janus.

Janus
Janus as imaged by Cassini on 7 April 2010: highest-resolution full-disk image to date
Discovery
Discovered byAudouin Dollfus
Discovery date15 December 1966
Designations
Designation
Saturn X
Pronunciation/ˈdʒeɪnəs/[1]
Named after
Jānus
AdjectivesJanian /ˈdʒeɪniən/[2][3]
Orbital characteristics [4]
Epoch 31 December 2003 (JD 2 453 005.5)
151460±10 km
Eccentricity0.0068
0.694660342 d
Inclination0.163°±0.004° to Saturn's equator
Satellite ofSaturn
GroupCo-orbital with Epimetheus
Physical characteristics
Dimensions203 × 185 × 152.6 km [5]
Mean radius
89.5±1.4 km [5]
Volume≈ 3000000 km3
Mass(1.8975±0.0012)×1018 kg [5]
Mean density
0.63±0.03 g/cm3 [5]
0.011–0.017 m/s2 [5]
synchronous
zero
Albedo0.71±0.02 (geometric) [6]
Temperature76 K

History

Discovery

Janus was identified by Audouin Dollfus on 15 December 1966[7] and given the temporary designation S/1966 S 2. Previously, Jean Texereau [fr] had photographed Janus on 29 October 1966 without realising it. On 18 December, Richard Walker observed an object in the same orbit as Janus, but whose position could not be reconciled with the previous observations. Twelve years later, in October 1978, Stephen M. Larson and John W. Fountain realised that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits,[8] Walker is now credited with the discovery of Epimetheus.[9] Voyager 1 confirmed this orbital configuration in 1980.[10] (See co-orbital moon for a more detailed description of their unique arrangement.)

Observational history

Janus was observed on subsequent occasions and given different provisional designations. Pioneer 11's three energetic-particle detectors detected its "shadow" when the probe flew by Saturn on 1 September 1979 (S/1979 S 2.[11]) Janus was observed by Dan Pascu on 19 February 1980 (S/1980 S 1,[12]) and then by John W. Fountain, Stephen M. Larson, Harold J. Reitsema and Bradford A. Smith on 23 February 1980 (S/1980 S 2.[13])

Name

Janus is named after the two-faced Roman god Janus. Although the name was informally proposed soon after the initial 1966 discovery,[14] it was not officially adopted until 1983,[a] when Epimetheus was also named.

The Oxford English Dictionary lists the adjectival form of the moon's name as Janian.

Orbit

 
Epimetheus (lower left) and Janus (right) seen on 20 March 2006, two months after swapping orbits. The two moons appear close only because of foreshortening; in reality, Janus is about 40,000 km farther from Cassini than Epimetheus.

Janus's orbit is co-orbital with that of Epimetheus. Janus's mean orbital radius from Saturn was, as of 2006, only 50 km less than that of Epimetheus, a distance smaller than either moon's mean radius. In accordance with Kepler's laws of planetary motion, the closer orbit is completed more quickly. Because of the small difference, it is completed in only about 30 seconds less. Each day, the inner moon is an additional 0.25° farther around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction increases the inner moon's momentum and decreases that of the outer moon. This added momentum means that the inner moon's distance from Saturn and orbital period are increased, and the outer moon's are decreased. The timing and magnitude of the momentum exchange is such that the moons effectively swap orbits, never approaching closer than about 10,000 km. At each encounter Janus's orbital radius changes by ~20 km and Epimetheus's by ~80 km: Janus's orbit is less affected because it is four times as massive as Epimetheus. The exchange takes place close to every four years; the last close approaches occurred in January 2006,[15] 2010, 2014, and 2018, and the next in 2022. This is the only such orbital configuration known in the Solar System.[16]

The orbital relationship between Janus and Epimetheus can be understood in terms of the circular restricted three-body problem, as a case in which the two moons (the third body being Saturn) are similar in size to each other.[citation needed]

Physical characteristics

Janus is extensively cratered with several craters larger than 30 km, but has few linear features. Janus's surface appears to be older than Prometheus's but younger than Pandora's.

Janus has a very low density and relatively high albedo, meaning that it is likely very icy and porous (a rubble pile).

Features

Craters on Janus, like those on Epimetheus, are named after characters in the legend of Castor and Pollux.[17]

Named Janian craters
Name Pronunciation Greek
Castor /ˈkæstər/ Κάστωρ
Idas /ˈaɪdəs/ Ἴδας
Lynceus /ˈlɪnsiːəs/ Λυγκεύς
Phoebe (of Messenia) /ˈfɔɪbiː/ Φοίβη

Interactions with rings

A faint dust ring is present around the region occupied by the orbits of Janus and Epimetheus, as revealed by images taken in forward-scattered light by the Cassini spacecraft in 2006. The ring has a radial extent of about 5000 km.[18] Its source is particles blasted off their surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.[19]

Along with Epimetheus, Janus acts as a shepherd moon, maintaining the sharp outer edge of the A Ring in a 7:6 orbital resonance. The effect is more obvious when the more massive Janus is on the resonant (inner) orbit.[16]

Gallery

In popular culture

  • In the book Pushing Ice by Alastair Reynolds, Janus plays a major role. At the beginning of the book, it suddenly deviates from its normal orbit and accelerates out of the solar system.

See also

Notes

  1. ^ Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (confirms Janus, names Epimetheus, Telesto, Calypso) (mentioned in IAUC 3872: Satellites of Jupiter and Saturn, September 30, 1983)

References

  1. ^ "Janus". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  2. ^
  3. ^ Carter (1919) The gates of Janus
  4. ^ Spitale Jacobson et al. 2006.
  5. ^ a b c d e Thomas 2010.
  6. ^ Verbiscer French et al. 2007.
  7. ^ IAUC 1987.
  8. ^ Fountain & Larson 1978.
  9. ^ IAUC 1991.
  10. ^ Solar System, NASA: Janus.
  11. ^ IAUC 3417.
  12. ^ IAUC 3454.
  13. ^ IAUC 3456.
  14. ^ IAUC 1995.
  15. ^ JPL/NASA: The Dancing Moons.
  16. ^ a b El Moutamid et al 2015.
  17. ^
  18. ^ JPL/NASA: Moon-Made Rings.
  19. ^ JPL/NASA: Creating New Rings.

Bibliography

  • Fountain, J. W.; Larson, S. M. (1978). "Saturn's ring and nearby faint satellites". Icarus. 36 (1): 92–106. Bibcode:1978Icar...36...92F. doi:10.1016/0019-1035(78)90076-3.
  • El Moutamid, M.; et al. (1 October 2015). "How Janus' Orbital Swap Affects the Edge of Saturn's A Ring?". Icarus. 279: 125–140. arXiv:1510.00434. Bibcode:2016Icar..279..125E. doi:10.1016/j.icarus.2015.10.025. S2CID 51785280.
  • Gingerich, Owen (January 3, 1967). "Probable New Satellite of Saturn" (discovery). IAU Circular. 1987: 1. Bibcode:1967IAUC.1987....1D. Retrieved 2011-12-28.
  • Gingerich, Owen (January 6, 1967). "Possible New Satellite of Saturn". IAU Circular. 1991. Retrieved 2011-12-28.
  • Gingerich, Owen (February 1, 1967). "Saturn X (Janus)" (naming Janus). IAU Circular. 1995. Retrieved 2011-12-28.
  • "PIA08170: The Dancing Moons". Photojournal. JPL/NASA. 2006-03-05. Retrieved 2016-01-17.
  • "PIA08328: Moon-Made Rings". Photojournal. JPL/NASA. 2006-10-11. Retrieved 2011-12-29.
  • . Cassini Solstice Mission. JPL/NASA. October 11, 2006. Archived from the original on February 12, 2012. Retrieved 2011-12-29.
  • Marsden, Brian G. (October 25, 1979). "New Ring and Satellites of Saturn". IAU Circular. 3417: 1. Bibcode:1979IAUC.3417....1G. Retrieved 2011-12-29.
  • Marsden, Brian G. (February 25, 1980). "Saturn". IAU Circular. 3454. Retrieved 2011-12-29.
  • Marsden, Brian G. (February 29, 1980). "1980 S 2". IAU Circular. 3456: 3. Bibcode:1980IAUC.3456....3S. Retrieved 2011-12-29.
  • Marsden, Brian G. (September 30, 1983). "Satellites of Jupiter and Saturn". IAU Circular. 3872. Retrieved 2011-12-23.
  • . Solar System Exploration: Planets. NASA. 4 Apr 2011. Archived from the original on 26 November 2002. Retrieved 2011-12-29.
  • Spitale, J. N.; Jacobson, R. A.; Porco, C. C.; Owen, W. M. Jr. (2006). "The orbits of Saturn's small satellites derived from combined historic and Cassini imaging observations". The Astronomical Journal. 132 (2): 692–710. Bibcode:2006AJ....132..692S. doi:10.1086/505206. S2CID 26603974.
  • Thomas, P. C. (July 2010). (PDF). Icarus. 208 (1): 395–401. Bibcode:2010Icar..208..395T. doi:10.1016/j.icarus.2010.01.025. Archived from the original (PDF) on 2018-12-23. Retrieved 2015-09-04.
  • Verbiscer, A.; French, R.; Showalter, M.; Helfenstein, P. (9 February 2007). "Enceladus: Cosmic Graffiti Artist Caught in the Act". Science. 315 (5813): 815. Bibcode:2007Sci...315..815V. doi:10.1126/science.1134681. PMID 17289992. S2CID 21932253. (supporting online material, table S1)

External links

  Media related to Janus (moon) at Wikimedia Commons

Listen to this article (4 minutes)
 
This audio file was created from a revision of this article dated 13 January 2010 (2010-01-13), and does not reflect subsequent edits.
(Audio help · More spoken articles)
  • by NASA's Solar System Exploration
  • The Planetary Society: Janus
  • Murray, Carl D.; Dermott, Stanley F. (1999). Solar System Dynamics. Cambridge University Press. ISBN 978-0-521-57597-3.
  • from Murray and Dermott
  • of Janus and Epimetheus near the time of their orbital swap.
  • Janus nomenclature from the USGS planetary nomenclature page

January 26, 2023
janus, moon, saturn, redirects, here, spurious, moon, reported, 1905, themis, hypothetical, moon, janus, inner, satellite, saturn, also, known, saturn, named, after, mythological, janus, janusjanus, imaged, cassini, april, 2010, highest, resolution, full, disk. Saturn X redirects here For the spurious moon reported in 1905 see Themis hypothetical moon Janus ˈ dʒ eɪ n e s is an inner satellite of Saturn It is also known as Saturn X It is named after the mythological Janus JanusJanus as imaged by Cassini on 7 April 2010 highest resolution full disk image to dateDiscoveryDiscovered byAudouin DollfusDiscovery date15 December 1966DesignationsDesignationSaturn XPronunciation ˈ dʒ eɪ n e s 1 Named afterJanusAdjectivesJanian ˈ dʒ eɪ n i e n 2 3 Orbital characteristics 4 Epoch 31 December 2003 JD 2 453 005 5 Semi major axis151460 10 kmEccentricity0 0068Orbital period sidereal 0 694660 342 dInclination0 163 0 004 to Saturn s equatorSatellite ofSaturnGroupCo orbital with EpimetheusPhysical characteristicsDimensions203 185 152 6 km 5 Mean radius89 5 1 4 km 5 Volume 3000 000 km3Mass 1 8975 0 0012 1018 kg 5 Mean density0 63 0 03 g cm3 5 Surface gravity0 011 0 017 m s2 5 Synodic rotation periodsynchronousAxial tiltzeroAlbedo0 71 0 02 geometric 6 Temperature76 K Contents 1 History 1 1 Discovery 1 2 Observational history 1 3 Name 2 Orbit 3 Physical characteristics 3 1 Features 4 Interactions with rings 5 Gallery 6 In popular culture 7 See also 8 Notes 9 References 10 Bibliography 11 External linksHistory EditDiscovery Edit Janus was identified by Audouin Dollfus on 15 December 1966 7 and given the temporary designation S 1966 S 2 Previously Jean Texereau fr had photographed Janus on 29 October 1966 without realising it On 18 December Richard Walker observed an object in the same orbit as Janus but whose position could not be reconciled with the previous observations Twelve years later in October 1978 Stephen M Larson and John W Fountain realised that the 1966 observations were best explained by two distinct objects Janus and Epimetheus sharing very similar orbits 8 Walker is now credited with the discovery of Epimetheus 9 Voyager 1 confirmed this orbital configuration in 1980 10 See co orbital moon for a more detailed description of their unique arrangement Observational history Edit Janus was observed on subsequent occasions and given different provisional designations Pioneer 11 s three energetic particle detectors detected its shadow when the probe flew by Saturn on 1 September 1979 S 1979 S 2 11 Janus was observed by Dan Pascu on 19 February 1980 S 1980 S 1 12 and then by John W Fountain Stephen M Larson Harold J Reitsema and Bradford A Smith on 23 February 1980 S 1980 S 2 13 Name Edit Janus is named after the two faced Roman god Janus Although the name was informally proposed soon after the initial 1966 discovery 14 it was not officially adopted until 1983 a when Epimetheus was also named The Oxford English Dictionary lists the adjectival form of the moon s name as Janian Orbit Edit Epimetheus lower left and Janus right seen on 20 March 2006 two months after swapping orbits The two moons appear close only because of foreshortening in reality Janus is about 40 000 km farther from Cassini than Epimetheus Janus s orbit is co orbital with that of Epimetheus Janus s mean orbital radius from Saturn was as of 2006 only 50 km less than that of Epimetheus a distance smaller than either moon s mean radius In accordance with Kepler s laws of planetary motion the closer orbit is completed more quickly Because of the small difference it is completed in only about 30 seconds less Each day the inner moon is an additional 0 25 farther around Saturn than the outer moon As the inner moon catches up to the outer moon their mutual gravitational attraction increases the inner moon s momentum and decreases that of the outer moon This added momentum means that the inner moon s distance from Saturn and orbital period are increased and the outer moon s are decreased The timing and magnitude of the momentum exchange is such that the moons effectively swap orbits never approaching closer than about 10 000 km At each encounter Janus s orbital radius changes by 20 km and Epimetheus s by 80 km Janus s orbit is less affected because it is four times as massive as Epimetheus The exchange takes place close to every four years the last close approaches occurred in January 2006 15 2010 2014 and 2018 and the next in 2022 This is the only such orbital configuration known in the Solar System 16 The orbital relationship between Janus and Epimetheus can be understood in terms of the circular restricted three body problem as a case in which the two moons the third body being Saturn are similar in size to each other citation needed Physical characteristics EditJanus is extensively cratered with several craters larger than 30 km but has few linear features Janus s surface appears to be older than Prometheus s but younger than Pandora s Janus has a very low density and relatively high albedo meaning that it is likely very icy and porous a rubble pile Features Edit Craters on Janus like those on Epimetheus are named after characters in the legend of Castor and Pollux 17 Named Janian craters Name Pronunciation GreekCastor ˈ k ae s t er KastwrIdas ˈ aɪ d e s ἼdasLynceus ˈ l ɪ n s iː e s LygkeysPhoebe of Messenia ˈ f ɔɪ b iː FoibhInteractions with rings EditA faint dust ring is present around the region occupied by the orbits of Janus and Epimetheus as revealed by images taken in forward scattered light by the Cassini spacecraft in 2006 The ring has a radial extent of about 5000 km 18 Its source is particles blasted off their surfaces by meteoroid impacts which then form a diffuse ring around their orbital paths 19 Along with Epimetheus Janus acts as a shepherd moon maintaining the sharp outer edge of the A Ring in a 7 6 orbital resonance The effect is more obvious when the more massive Janus is on the resonant inner orbit 16 Gallery Edit Janus as viewed by Voyager 2 1981 08 25 Janus and Prometheus lie above and below Saturn s rings 2006 04 29 Janus and Tethys foreground near Saturn s rings 2015 10 27 Janus in front of Saturn as imaged by Cassini 2006 09 25 Janus as imaged by Cassini 2008 02 20 Crescent Janus 2008 06 30 In popular culture EditIn the book Pushing Ice by Alastair Reynolds Janus plays a major role At the beginning of the book it suddenly deviates from its normal orbit and accelerates out of the solar system See also EditSaturn s moons in fictionNotes Edit Transactions of the International Astronomical Union Vol XVIIIA 1982 confirms Janus names Epimetheus Telesto Calypso mentioned in IAUC 3872 Satellites of Jupiter and Saturn September 30 1983 References Edit Janus Oxford English Dictionary Online ed Oxford University Press Subscription or participating institution membership required JPL ca 2008 Cassini Equinox Mission Janus Carter 1919 The gates of Janus Spitale Jacobson et al 2006 a b c d e Thomas 2010 Verbiscer French et al 2007 IAUC 1987 Fountain amp Larson 1978 IAUC 1991 Solar System NASA Janus IAUC 3417 IAUC 3454 IAUC 3456 IAUC 1995 JPL NASA The Dancing Moons a b El Moutamid et al 2015 USGS Janus nomenclature JPL NASA Moon Made Rings JPL NASA Creating New Rings Bibliography EditThis article includes a list of general references but it lacks sufficient corresponding inline citations Please help to improve this article by introducing more precise citations January 2016 Learn how and when to remove this template message Fountain J W Larson S M 1978 Saturn s ring and nearby faint satellites Icarus 36 1 92 106 Bibcode 1978Icar 36 92F doi 10 1016 0019 1035 78 90076 3 El Moutamid M et al 1 October 2015 How Janus Orbital Swap Affects the Edge of Saturn s A Ring Icarus 279 125 140 arXiv 1510 00434 Bibcode 2016Icar 279 125E doi 10 1016 j icarus 2015 10 025 S2CID 51785280 Gingerich Owen January 3 1967 Probable New Satellite of Saturn discovery IAU Circular 1987 1 Bibcode 1967IAUC 1987 1D Retrieved 2011 12 28 Gingerich Owen January 6 1967 Possible New Satellite of Saturn IAU Circular 1991 Retrieved 2011 12 28 Gingerich Owen February 1 1967 Saturn X Janus naming Janus IAU Circular 1995 Retrieved 2011 12 28 PIA08170 The Dancing Moons Photojournal JPL NASA 2006 03 05 Retrieved 2016 01 17 PIA08328 Moon Made Rings Photojournal JPL NASA 2006 10 11 Retrieved 2011 12 29 NASA Finds Saturn s Moons May Be Creating New Rings Cassini Solstice Mission JPL NASA October 11 2006 Archived from the original on February 12 2012 Retrieved 2011 12 29 Marsden Brian G October 25 1979 New Ring and Satellites of Saturn IAU Circular 3417 1 Bibcode 1979IAUC 3417 1G Retrieved 2011 12 29 Marsden Brian G February 25 1980 Saturn IAU Circular 3454 Retrieved 2011 12 29 Marsden Brian G February 29 1980 1980 S 2 IAU Circular 3456 3 Bibcode 1980IAUC 3456 3S Retrieved 2011 12 29 Marsden Brian G September 30 1983 Satellites of Jupiter and Saturn IAU Circular 3872 Retrieved 2011 12 23 Saturn Moons Janus Solar System Exploration Planets NASA 4 Apr 2011 Archived from the original on 26 November 2002 Retrieved 2011 12 29 Spitale J N Jacobson R A Porco C C Owen W M Jr 2006 The orbits of Saturn s small satellites derived from combined historic and Cassini imaging observations The Astronomical Journal 132 2 692 710 Bibcode 2006AJ 132 692S doi 10 1086 505206 S2CID 26603974 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 2018 12 23 Retrieved 2015 09 04 Verbiscer A French R Showalter M Helfenstein P 9 February 2007 Enceladus Cosmic Graffiti Artist Caught in the Act Science 315 5813 815 Bibcode 2007Sci 315 815V doi 10 1126 science 1134681 PMID 17289992 S2CID 21932253 supporting online material table S1 External links Edit Media related to Janus moon at Wikimedia Commons Listen to this article 4 minutes source source This audio file was created from a revision of this article dated 13 January 2010 2010 01 13 and does not reflect subsequent edits Audio help More spoken articles Janus Profile by NASA s Solar System Exploration The Planetary Society Janus Murray Carl D Dermott Stanley F 1999 Solar System Dynamics Cambridge University Press ISBN 978 0 521 57597 3 QuickTime illustration of co orbital motion from Murray and Dermott Cassini image of Janus and Epimetheus near the time of their orbital swap Janus nomenclature from the USGS planetary nomenclature page Retrieved from https en wikipedia org w index php title Janus moon amp oldid 1126020861, wikipedia, wiki, book, books, library,

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