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| name = Janus
| name = Janus
| image = PIA12714 Janus crop.jpg
| image = PIA12714 Janus crop.jpg
| image_scale =
| caption = Janus as imaged by ''Cassini'' on 7 April 2010: highest-resolution full-disk image to date
| caption = Janus as imaged by ''Cassini'' on 7 April 2010: highest-resolution full-disk image to date
| pronounced = {{IPAc-en|ˈ|dʒ|eɪ|n|ə|s}}<ref>{{OED|Janus}}</ref>
| pronounced = {{IPAc-en|ˈ|dʒ|eɪ|n|ə|s}}<ref>{{OED|Janus}}</ref>
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| satellite_of = [[Saturn]]
| satellite_of = [[Saturn]]
| group = [[Co-orbital moon|Co-orbital]] with [[Epimetheus (moon)|Epimetheus]]
| group = [[Co-orbital moon|Co-orbital]] with [[Epimetheus (moon)|Epimetheus]]
| dimensions = 203&thinsp;×&thinsp;185&thinsp;×&thinsp;152.6 km {{sfn|Thomas|2010}}
| dimensions = {{val|203.4|x|185.8|x|149.0}}&nbsp;km<br/>(±&nbsp;{{val|1.8|x|0.6|x|0.6}}&nbsp;km){{sfn|Thomas & Helfenstein|2020}}{{rp|page=2}}
| mean_radius = {{val|89.5|1.4|u=km}} {{sfn|Thomas|2010}}
| mean_diameter = {{val|178.0|1.0|u=km}}{{sfn|Thomas & Helfenstein|2020}}{{rp|page=2}}
| volume = ≈&thinsp;{{val|3000000|u=km<sup>3</sup>}}
| volume = {{val|2953010|950|u=km3}}{{sfn|Lainey et al.|2023}}{{rp|page=4}}
| mass = {{val|1.89388|0.00028|e=18|u=kg}}{{efn|Calculated from the standard gravitational parameter ''GM'' {{=}} {{val|0.126403|0.000019|u=km<sup>3</sup>·s<sup>–2</sup>}} given by Lainey et al. (2023), divided by the [[gravitational constant]] ''G'' {{=}} {{val|6.6743|e=-2|u=km<sup>3</sup>·kg<sup>–1</sup>·s<sup>–2</sup>}}.{{sfn|Lainey et al.|2023}}}}
| mass = {{val|1.8975|0.0012|e=18|u=kg}} {{sfn|Thomas|2010}}
| density = {{val|0.63|0.03|u=g/cm<sup>3</sup>}} {{sfn|Thomas|2010}}
| density = {{val|0.6413|0.0002|u=g/cm3}}{{sfn|Lainey et al.|2023}}{{rp|page=4}}
| surface_grav = 0.011–0.017 [[Acceleration|m/s<sup>2</sup>]] {{sfn|Thomas|2010}}
| surface_grav = {{val|0.0111|-|0.0169|u=m/s2}}{{sfn|Thomas & Helfenstein|2020}}{{rp|page=3}}
| escape_velocity = {{V2|1.89388e-3|101.7|3}}&nbsp;km/s at longest axis<br/>to {{V2|1.89388e-3|74.5|3}}&nbsp;km/s at poles<!-- V2 template uses mass in terms of e+21 kg, radius in terms of km, and number of decimal places -->
| rotation = [[Synchronous rotation|synchronous]]
| rotation = [[Synchronous rotation|synchronous]]
| axial_tilt = zero
| axial_tilt = zero
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}}
}}


'''Janus''' {{IPAc-en|ˈ|dʒ|eɪ|n|ə|s}} is an [[inner satellite]] of [[Saturn]]. It is also known as '''Saturn X'''. It is named after the mythological [[Janus (mythology)|Janus]].
'''Janus''' {{IPAc-en|ˈ|dʒ|eɪ|n|ə|s}} is an [[inner satellite]] of [[Saturn]]. It is also known as '''Saturn X'''. It is named after the mythological [[Janus (mythology)|Janus]]. This natural satellite was first identified by Audouin Dollfus on December 15, 1966, although it had been unknowingly photographed earlier by Jean Texereau. Further observations led to the realization that Janus shares a unique orbital relationship with another moon, [[Epimetheus (moon)|Epimetheus]]. The discovery of these two moons' peculiar co-orbital configuration was later confirmed by Voyager 1 in 1980.


==History==
==History==


=== Discovery ===
=== Discovery ===
Janus was identified by [[Audouin Dollfus]] on 15 December 1966{{sfn|IAUC 1987}} and given the temporary designation '''{{nowrap|S/1966 S 2}}'''. Previously, {{ill|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,{{sfn|Fountain|Larson|1978}} Walker is now credited with the discovery of [[Epimetheus (moon)|Epimetheus]].{{sfn|IAUC 1991}} ''[[Voyager 1]]'' confirmed this orbital configuration in 1980.{{sfn|Solar System, NASA: Janus}} (See [[co-orbital moon]] for a more detailed description of their unique arrangement.)
Janus was identified by [[Audouin Dollfus]] on 15 December 1966{{sfn|IAUC 1987}} and given the temporary designation '''{{nowrap|S/1966 S 2}}'''. Previously, {{ill|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,{{sfn|Fountain|Larson|1978}} Walker is now credited with the discovery of [[Epimetheus (moon)|Epimetheus]].{{sfn|IAUC 1991}} ''[[Voyager 1]]'' confirmed this orbital configuration in 1980.{{sfn|Solar System, NASA: Janus}}


===Observational history===
===Observational history===
Janus was observed on subsequent occasions and given different [[provisional designation in astronomy|provisional designations]]. ''[[Pioneer 11]]''{{'s}} three energetic-particle detectors detected its "shadow" when the probe flew by Saturn on 1 September 1979 ('''{{nowrap|S/1979 S 2}}'''.{{sfn|IAUC 3417}}<!--- [[Tom Gehrels]] and [[James A. van Allen]] --->) Janus was observed by Dan Pascu on 19 February 1980 ('''{{nowrap|S/1980 S 1}}''',{{sfn|IAUC 3454}}) and then by John W. Fountain, Stephen M. Larson, [[Harold J. Reitsema]] and Bradford A. Smith on 23 February 1980 ('''{{nowrap|S/1980 S 2}}'''.{{sfn|IAUC 3456}})
Janus was observed on subsequent occasions and given different [[provisional designation in astronomy|provisional designations]]. ''[[Pioneer 11]]''{{'s}} three energetic-particle detectors detected its "shadow" when the probe flew by Saturn on 1 September 1979 ('''{{nowrap|S/1979 S 2}}''').{{sfn|IAUC 3417}}<!--- [[Tom Gehrels]] and [[James A. van Allen]] ---> Janus was observed by Dan Pascu on 19 February 1980 ('''{{nowrap|S/1980 S 1}}'''),{{sfn|IAUC 3454}} and then by John W. Fountain, Stephen M. Larson, [[Harold J. Reitsema]] and Bradford A. Smith on 23 February 1980 ('''{{nowrap|S/1980 S 2}}''').{{sfn|IAUC 3456}}


=== Name ===
=== Name ===
Janus is named after the two-faced [[Roman mythology|Roman]] god [[Janus (mythology)|Janus]]. Although the name was informally proposed soon after the initial 1966 discovery,{{sfn|IAUC 1995}} it was not officially adopted until 1983,{{efn|name=mythology}} when [[Epimetheus (moon)|Epimetheus]] was also named.
Janus is named after the two-faced [[Roman mythology|Roman]] god [[Janus (mythology)|Janus]]. Although the name was informally proposed soon after the initial 1966 discovery,{{sfn|IAUC 1995}} it was not officially adopted until 1983,{{efn|name=mythology}} when [[Epimetheus (moon)|Epimetheus]] was also named.

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


== Orbit ==
== Orbit ==
[[File:PIA08170 Epimetheus and Janus.jpg|thumb|left|[[Epimetheus (moon)|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.]]
[[File:PIA08170 Epimetheus and Janus.jpg|thumb|left|[[Epimetheus (moon)|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 (moon)|Epimetheus]]. Janus's [[semimajor axis|mean orbital radius]] from Saturn was, as of 2006, only 50&nbsp;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 [[Gravity assist|momentum exchange]] is such that the moons effectively swap orbits, never approaching closer than about 10,000&nbsp;km. At each encounter Janus's orbital radius changes by ~20&nbsp;km and Epimetheus's by ~80&nbsp;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,{{sfn|JPL/NASA: The Dancing Moons}} 2010, 2014, and 2018, and the next in 2022. This is the only such orbital configuration known in the [[Solar System]].{{sfn|El Moutamid et al|2015}}
Janus's orbit is co-orbital with that of [[Epimetheus (moon)|Epimetheus]]. Janus's [[semimajor axis|mean orbital radius]] from Saturn was, as of 2006, only 50&nbsp;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 in exchange the outer moon's are decreased. The timing and magnitude of the [[Gravity assist|momentum exchange]] is such that the moons effectively swap orbits, never approaching closer than about 10,000&nbsp;km. At each encounter Janus's orbital radius changes by ~20&nbsp;km and Epimetheus's by ~80&nbsp;km: Janus's orbit is less affected because it is four times as massive as Epimetheus.
[[File:Epimetheus-Janus_Orbit.png|thumb|250x250px|[[Rotating reference frame|Rotating-frame]] depiction of the [[Horseshoe orbit|horseshoe orbits]] of Janus and Epimetheus]]
The exchange takes place close to every four years; the last close approaches occurred in January 2006,{{sfn|JPL/NASA: The Dancing Moons}} 2010, 2014, 2018, and 2022 and the next in 2026. This is the only such orbital configuration known in the [[Solar System]].{{sfn|El Moutamid et al|2015}}


The orbital relationship between Janus and Epimetheus can be understood in terms of the [[Three-body problem#Circular restricted three-body problem|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|date=December 2012}}
The orbital relationship between Janus and Epimetheus can be understood in terms of the [[Three-body problem#Circular restricted three-body problem|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.{{Sfn|Llibre and Ollé 2011|}}


== Physical characteristics ==
== Physical characteristics ==


Janus is extensively cratered with several [[Impact crater|craters]] larger than 30&nbsp;km, but has few linear features. Janus's surface appears to be older than [[Prometheus (moon)|Prometheus]]'s but younger than [[Pandora (moon)|Pandora]]'s.
Janus is extensively cratered with several [[Impact crater|craters]] larger than 30 km and has few linear features. Janus's surface appears to be older than [[Prometheus (moon)|Prometheus]]'s but younger than [[Pandora (moon)|Pandora]]'s. [[File:Animation_of_Epimetheus_orbit_-_Rotating_reference_frame.gif|right|thumb|250x250px|Animation of Epimetheus{{'s}} orbit – Rotating reference frame


{{legend2|Darkkhaki|Saturn}}{{·}}{{legend2|Lime|Janus}}{{·}}{{legend2|Magenta|Epimetheus}}]]
Janus has a very low [[density]] and relatively high [[albedo]], meaning that it is likely very icy and [[Porosity|porous]] (a [[rubble pile]]).
Janus has a very low [[density]] and relatively high [[albedo]], meaning that it is likely icy in composition and structurally a [[rubble pile]] object.


===Features===
===Features===
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== Interactions with rings ==
== 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–Huygens|''Cassini'' spacecraft]] in 2006. The ring has a radial extent of about 5000&nbsp;km.{{sfn|JPL/NASA: Moon-Made Rings}} Its source is particles blasted off their surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.{{sfn|JPL/NASA: Creating New 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–Huygens|''Cassini'' spacecraft]] in 2006. The ring has a radial extent of about 5000 km.{{sfn|JPL/NASA: Moon-Made Rings}} Its source is particles blasted off their surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.{{sfn|JPL/NASA: Creating New Rings}}


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.{{sfn|El Moutamid et al|2015}}
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.{{sfn|El Moutamid et al|2015}}
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File:Saturn's moons Janus and Prometheus PIA08192 (NASA).jpg|Janus and [[Prometheus (moon)|Prometheus]] lie above and below [[Rings of Saturn|Saturn's rings]] (2006-04-29).
File:Saturn's moons Janus and Prometheus PIA08192 (NASA).jpg|Janus and [[Prometheus (moon)|Prometheus]] lie above and below [[Rings of Saturn|Saturn's rings]] (2006-04-29).
File:PIA18353-SaturnRingsMoons-JanusTethys-20151027.jpg|Janus and [[Tethys (moon)|Tethys]] (foreground) near [[Rings of Saturn|Saturn's rings]] (2015-10-27).
File:PIA18353-SaturnRingsMoons-JanusTethys-20151027.jpg|Janus and [[Tethys (moon)|Tethys]] (foreground) near [[Rings of Saturn|Saturn's rings]] (2015-10-27).
File:Janus 2006 closeup by Cassinix2.jpg|Janus in front of Saturn as imaged by ''[[Cassini–Huygens|Cassini]]'' (2006-09-25).
File:Janus against Saturn PIA08296.jpg|Janus in front of Saturn as imaged by ''[[Cassini–Huygens|Cassini]]'' (2006-09-25).
File:Janus-PIA09872.jpg|Janus as imaged by ''[[Cassini–Huygens|Cassini]]'' (2008-02-20).
File:Janus-PIA09872.jpg|Janus as imaged by ''[[Cassini–Huygens|Cassini]]'' (2008-02-20).
File:PIA10447 Janus.jpg|Crescent Janus (2008-06-30).
File:PIA10447 Janus.jpg|Crescent Janus (2008-06-30).
</gallery>
</gallery>


== In popular culture ==
== In fiction ==
Janus is the location of the plot in the [[Alastair Reynolds]] science-fiction novel [[Pushing Ice]].
* 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 ==
⚫
{{notelist|notes=

* [[Saturn's moons in fiction]]

==Notes==

⚫
{{notes
| notes =


{{efn
{{efn
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== References ==
== References ==
=== Citations ===
{{reflist|20em}}
{{reflist|20em}}


==Bibliography==
=== Sources ===
{{More footnotes|date=January 2016}}
{{More footnotes|date=January 2016}}
* {{cite journal| doi = 10.1016/0019-1035(78)90076-3| last1 = Fountain| first1 = J. W.| last2 = Larson| first2 = S. M.| year = 1978| title = Saturn's ring and nearby faint satellites| journal = Icarus| volume = 36| issue = 1| pages = 92–106| bibcode = 1978Icar...36...92F
* {{cite journal| doi = 10.1016/0019-1035(78)90076-3| last1 = Fountain| first1 = J. W.| last2 = Larson| first2 = S. M.| year = 1978| title = Saturn's ring and nearby faint satellites| journal = Icarus| volume = 36| issue = 1| pages = 92–106| bibcode = 1978Icar...36...92F
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| journal = IAU Circular
| journal = IAU Circular
| volume = 1991
| volume = 1991
| page = 2
⚫
| url = http://www.cbat.eps.harvard.edu/iauc/01900/01991.html
| bibcode = 1967IAUC.1991....2W
⚫
| url = http://www.cbat.eps.harvard.edu/iauc/01900/01991.html
| access-date = 2011-12-28
| access-date = 2011-12-28
| ref = {{sfnRef|IAUC 1991}}
| ref = {{sfnRef|IAUC 1991}}
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| ref = {{sfnRef|IAUC 3454}}
| ref = {{sfnRef|IAUC 3454}}
}}
}}
* {{cite journal | doi=10.1051/0004-6361:20011274 | title=The motion of Saturn coorbital satellites in the restricted three-body problem | first1=J. | last1=Llibre | first2=M. | last2=Ollé | journal=Astronomy and Astrophysics | volume=378 | issue=3 |pages = 1087–1099| date=11 November 2011 | ref={{sfnRef|Llibre and Ollé 2011}} |bibcode = 2001A&A...378.1087L | hdl=2117/1193 | doi-access=free | hdl-access=free }}
* {{cite journal
* {{cite journal
| last = Marsden
| last = Marsden
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| ref = {{sfnRef|Solar System, NASA: Janus}}
| ref = {{sfnRef|Solar System, NASA: Janus}}
}}
}}
* {{cite journal| doi = 10.1086/505206| last1 = Spitale| first1 = J. N.| last2 = Jacobson| first2 = R. A.| last3 = Porco| first3 = C. C.| last4 = Owen| first4 = W. M., Jr.| year = 2006| title = The orbits of Saturn's small satellites derived from combined historic and ''Cassini'' imaging observations| journal = The Astronomical Journal| volume = 132| issue = 2| pages = 692–710| bibcode = 2006AJ....132..692S| s2cid = 26603974| ref = {{sfnRef|Spitale Jacobson et al.|2006}}| doi-access = free}}
* {{cite journal| doi = 10.1086/505206| last1 = Spitale| first1 = J. N.| last2 = Jacobson| first2 = R. A.| last3 = Porco| first3 = C. C.| last4 = Owen| first4 = W. M. Jr. | year = 2006| title = The orbits of Saturn's small satellites derived from combined historic and ''Cassini'' imaging observations| journal = The Astronomical Journal| volume = 132| issue = 2| pages = 692–710| bibcode = 2006AJ....132..692S| s2cid = 26603974| ref = {{sfnRef|Spitale Jacobson et al.|2006}}| doi-access = free}}
* {{cite journal| doi = 10.1016/j.icarus.2010.01.025| last1 = Thomas| first1 = P. C.| date = July 2010| title = Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission| journal = Icarus| volume = 208| issue = 1| pages = 395–401| url = http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| bibcode = 2010Icar..208..395T}}
* {{cite journal| doi = 10.1016/j.icarus.2010.01.025| last1 = Thomas| first1 = P. C.| date = July 2010| title = Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission| journal = Icarus| volume = 208| issue = 1| pages = 395–401| url = http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| bibcode = 2010Icar..208..395T| access-date = 2015-09-04| archive-date = 2018-12-23| archive-url = https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| url-status = dead}}
* {{cite journal| doi = 10.1126/science.1134681| last1 = Verbiscer| first1 = A.| last2 = French| first2 = R.| last3 = Showalter| first3 = M.| last4 = Helfenstein| first4 = P.| title = Enceladus: Cosmic Graffiti Artist Caught in the Act| journal = Science| volume = 315| issue = 5813| page = 815| date = 9 February 2007| pmid = 17289992| bibcode = 2007Sci...315..815V| s2cid = 21932253| ref = {{sfnRef|Verbiscer French et al.|2007}}}} (supporting online material, table S1)
* {{cite journal| doi = 10.1126/science.1134681| last1 = Verbiscer| first1 = A.| last2 = French| first2 = R.| last3 = Showalter| first3 = M.| last4 = Helfenstein| first4 = P.| title = Enceladus: Cosmic Graffiti Artist Caught in the Act| journal = Science| volume = 315| issue = 5813| page = 815| date = 9 February 2007| pmid = 17289992| bibcode = 2007Sci...315..815V| s2cid = 21932253| ref = {{sfnRef|Verbiscer French et al.|2007}}}} (supporting online material, table S1)
* {{cite journal
|ref = {{sfnRef|Thomas & Helfenstein|2020}}
|first1 = P. C. |last1 = Thomas
|first2 = P. |last2 = Helfenstein
|title = The small inner satellites of Saturn: Shapes, structures and some implications
|journal = Icarus
|date = July 2020
|volume = 344
|id = 113355
|pages = 20
|doi = 10.1016/j.icarus.2019.06.016
|bibcode = 2020Icar..34413355T|s2cid = 197474587 }}
* {{cite journal
|ref = {{sfnRef|Lainey et al.|2023}}
|first1 = V. |last1 = Lainey
|first2 = N. |last2 = Rambaux
|first3 = N. |last3 = Cooper
|first4 = R. |last4 = Dahoumane
|first5 = Q. |last5 = Zhang
|title = Characterising the interior of five inner Saturnian moons using Cassini ISS data
|journal = Astronomy & Astrophysics
|date = February 2023
|volume = 670
|id = L25
|pages = 6
|doi-access = free
|doi = 10.1051/0004-6361/202244757
|bibcode = 2023A&A...670L..25L}}


== External links ==
== External links ==
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* [https://web.archive.org/web/20021126213902/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Janus Janus Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [https://web.archive.org/web/20021126213902/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Janus Janus Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://www.planetary.org/explore/topics/saturn/janus.html The Planetary Society: Janus]
* [http://www.planetary.org/explore/topics/saturn/janus.html The Planetary Society: Janus]
* {{cite book|last1=Murray|first1=Carl D.|last2=Dermott|first2=Stanley F.|title=Solar System Dynamics|url=https://books.google.com/books?id=aU6vcy5L8GAC&pg=PP1|year=1999|publisher=Cambridge University Press|isbn=978-0-521-57597-3}}
* {{cite book|last1=Murray|first1=Carl D.|last2=Dermott|first2=Stanley F.|title=Solar System Dynamics|url=https://books.google.com/books?id=aU6vcy5L8GAC&pg=PP1|year=1999|publisher=Cambridge University Press|isbn=978-0-521-57597-3 |author1-link=Carl D. Murray }}
* [https://web.archive.org/web/20140903190527/http://ssdbook.maths.qmw.ac.uk/animations/Coorbital.mov QuickTime illustration of co-orbital motion] from Murray and Dermott
* [https://web.archive.org/web/20140903190527/http://ssdbook.maths.qmw.ac.uk/animations/Coorbital.mov QuickTime illustration of co-orbital motion] from Murray and Dermott
* [https://web.archive.org/web/20061002211034/http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2103 ''Cassini'' image] of Janus and Epimetheus near the time of their orbital swap.
* [https://web.archive.org/web/20061002211034/http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2103 ''Cassini'' image] of Janus and Epimetheus near the time of their orbital swap.
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[[Category:Co-orbital moons]]
[[Category:Co-orbital moons]]
[[Category:Astronomical objects discovered in 1966|19661215]]
[[Category:Astronomical objects discovered in 1966|19661215]]
[[Category:Moons with a prograde orbit]]

Latest revision as of 22:06, 22 September 2024

"Saturn X" redirects here. For the spurious moon reported in 1905, see Themis (hypothetical moon).
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
Dimensions203.4 × 185.8 × 149.0 km
(± 1.8 × 0.6 × 0.6 km)[5]: 2 
178.0±1.0 km[5]: 2 
Volume2953010±950 km3[6]: 4 
Mass(1.89388±0.00028)×1018 kg[a]
Mean density
0.6413±0.0002 g/cm3[6]: 4 
0.0111–0.0169 m/s2[5]: 3 
0.05 km/s at longest axis
to 0.058 km/s at poles
synchronous
zero
Albedo0.71±0.02 (geometric) [7]
Temperature76 K

Janus /ˈdʒeɪnəs/ is an inner satellite of Saturn. It is also known as Saturn X. It is named after the mythological Janus. This natural satellite was first identified by Audouin Dollfus on December 15, 1966, although it had been unknowingly photographed earlier by Jean Texereau. Further observations led to the realization that Janus shares a unique orbital relationship with another moon, Epimetheus. The discovery of these two moons' peculiar co-orbital configuration was later confirmed by Voyager 1 in 1980.

History

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Discovery

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Janus was identified by Audouin Dollfus on 15 December 1966[8] 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,[9] Walker is now credited with the discovery of Epimetheus.[10] Voyager 1 confirmed this orbital configuration in 1980.[11]

Observational history

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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).[12] Janus was observed by Dan Pascu on 19 February 1980 (S/1980 S 1),[13] and then by John W. Fountain, Stephen M. Larson, Harold J. Reitsema and Bradford A. Smith on 23 February 1980 (S/1980 S 2).[14]

Name

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Janus is named after the two-faced Roman god Janus. Although the name was informally proposed soon after the initial 1966 discovery,[15] it was not officially adopted until 1983,[b] when Epimetheus was also named.

Orbit

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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 in exchange 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.

Rotating-frame depiction of the horseshoe orbits of Janus and Epimetheus

The exchange takes place close to every four years; the last close approaches occurred in January 2006,[16] 2010, 2014, 2018, and 2022 and the next in 2026. This is the only such orbital configuration known in the Solar System.[17]

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.[18]

Physical characteristics

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

Animation of Epimetheus's orbit – Rotating reference frame   Saturn ·   Janus ·   Epimetheus

Janus has a very low density and relatively high albedo, meaning that it is likely icy in composition and structurally a rubble pile object.

Features

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Craters on Janus, like those on Epimetheus, are named after characters in the legend of Castor and Pollux.[19]

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

Interactions with rings

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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.[20] Its source is particles blasted off their surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.[21]

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.[17]

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In fiction

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Janus is the location of the plot in the Alastair Reynolds science-fiction novel Pushing Ice.

Notes

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  1. ^ Calculated from the standard gravitational parameter GM = 0.126403±0.000019 km3·s–2 given by Lainey et al. (2023), divided by the gravitational constant G = 6.6743×10−2 km3·kg–1·s–2.[6]
  2. ^ 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

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Citations

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  1. ^ "Janus". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  2. ^ JPL (ca. 2008) Cassini Equinox Mission: Janus
  3. ^ Carter (1919) The gates of Janus
  4. ^ Spitale Jacobson et al. 2006.
  5. ^ a b c Thomas & Helfenstein 2020.
  6. ^ a b c Lainey et al. 2023.
  7. ^ Verbiscer French et al. 2007.
  8. ^ IAUC 1987.
  9. ^ Fountain & Larson 1978.
  10. ^ IAUC 1991.
  11. ^ Solar System, NASA: Janus.
  12. ^ IAUC 3417.
  13. ^ IAUC 3454.
  14. ^ IAUC 3456.
  15. ^ IAUC 1995.
  16. ^ JPL/NASA: The Dancing Moons.
  17. ^ a b El Moutamid et al 2015.
  18. ^ Llibre and Ollé 2011.
  19. ^ USGS: Janus nomenclature
  20. ^ JPL/NASA: Moon-Made Rings.
  21. ^ JPL/NASA: Creating New Rings.

Sources

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Media related to Janus (moon) at Wikimedia Commons

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