Janus (moon)

Short description: Moon of Saturn

Janus

Janus as imaged by Cassini on 7 April 2010: highest-resolution full-disk image to date

Discovery
Discovered by| Audouin Dollfus
Discovery date| 15 December 1966
Designations

Designation

| Saturn X
Pronunciation| /ˈdʒeɪnəs/[1]
Named after| Jānus
Adjectives| Janian /ˈdʒeɪniən/[2][3]
Orbital characteristics [4]
Epoch 31 December 2003 (JD 2 453 005.5)

Semi-major axis

| 151460±10 km
Eccentricity| 0.0068
Orbital period| 0.694660342 d
Inclination| 0.163°±0.004° to Saturn's equator
Satellite of| Saturn
Group| Co-orbital with Epimetheus
Physical characteristics
Dimensions| 203.4 × 185.8 × 149.0 km
(± 1.8 × 0.6 × 0.6 km)[5]: 2
Mean diameter| 178.0±1.0 km[5]: 2
Volume| 2953010±950 km3[6]: 4
Mass| (1.89388±0.00028)×1018 kg[lower-alpha 1]
Mean density| 0.6413±0.0002 g/cm3[6]: 4

surface gravity

| 0.0111–0.0169 m/s2[5]: 3

escape velocity

| 0.05 km/s at longest axis
to 0.058 km/s at poles
Rotation period| synchronous
Axial tilt| zero
Albedo| 0.71±0.02 (geometric) [7]
Physics| 76 K

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

## 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 Notes
* 7 References
* 7.1 Citations
* 7.2 Sources
* 8 External links

## History

### Discovery

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

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

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,[lower-alpha 2] 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.

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

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.

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 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.[19]

Named Janian craters  Name | Pronunciation | Greek

|
|

Castor | /ˈkæstər/ | Κάστωρ
Idas | /ˈaɪdəs/ | Ἴδας
Lynceus | /ˈlɪnsiːəs/ | Λυγκεύς
Phoibe (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.[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]

## Gallery

* 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).

## Notes

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

## References

### Citations

1. ↑ Janus (3rd ed.), Oxford University Press, September 2005, http://oed.com/search?searchType=dictionary&q=Janus (Subscription or UK public library membership required.)
2. ↑ JPL (ca. 2008) Cassini Equinox Mission: Janus
3. ↑ Carter (1919) The gates of Janus
4. ↑ Spitale Jacobson et al. 2006.
5. ↑ 5.0 5.1 5.2 Thomas & Helfenstein 2020.
6. ↑ 6.0 6.1 6.2 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. ↑ 17.0 17.1 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

* Fountain, J. W.; Larson, S. M. (1978). "Saturn's ring and nearby faint satellites". Icarus 36 (1): 92–106. doi:10.1016/0019-1035(78)90076-3. Bibcode: 1978Icar...36...92F.
* El Moutamid, M. (1 October 2015). "How Janus' Orbital Swap Affects the Edge of Saturn's A Ring?". Icarus 279: 125–140. doi:10.1016/j.icarus.2015.10.025. Bibcode: 2016Icar..279..125E.
* Gingerich, Owen (January 3, 1967). "Probable New Satellite of Saturn" (discovery). IAU Circular 1987: 1. Bibcode: 1967IAUC.1987....1D. http://www.cbat.eps.harvard.edu/iauc/01900/01987.html. Retrieved 2011-12-28.
* Gingerich, Owen (January 6, 1967). "Possible New Satellite of Saturn". IAU Circular 1991. http://www.cbat.eps.harvard.edu/iauc/01900/01991.html. Retrieved 2011-12-28.
* Gingerich, Owen (February 1, 1967). "Saturn X (Janus)" (naming Janus). IAU Circular 1995. http://www.cbat.eps.harvard.edu/iauc/01900/01995.html. Retrieved 2011-12-28.
* "PIA08170: The Dancing Moons". Photojournal. JPL/NASA. 2006-03-05. http://photojournal.jpl.nasa.gov/catalog/PIA08170.
* "PIA08328: Moon-Made Rings". Photojournal. JPL/NASA. 2006-10-11. http://photojournal.jpl.nasa.gov/catalog/PIA08328.
* "NASA Finds Saturn's Moons May Be Creating New Rings". Cassini Solstice Mission. JPL/NASA. October 11, 2006. http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20061011-3/.
* Marsden, Brian G. (October 25, 1979). "New Ring and Satellites of Saturn". IAU Circular 3417: 1. Bibcode: 1979IAUC.3417....1G. http://www.cbat.eps.harvard.edu/iauc/03400/03417.html. Retrieved 2011-12-29.
* Marsden, Brian G. (February 25, 1980). "Saturn". IAU Circular 3454. http://www.cbat.eps.harvard.edu/iauc/03400/03454.html. Retrieved 2011-12-29.
* Llibre, J.; Ollé, M. (11 November 2011). "The motion of Saturn coorbital satellites in the restricted three-body problem". Astronomy and Astrophysics 378 (3): 1087–1099. doi:10.1051/0004-6361:20011274. Bibcode: 2001A&A...378.1087L.
* Marsden, Brian G. (February 29, 1980). "1980 S 2". IAU Circular 3456: 3. Bibcode: 1980IAUC.3456....3S. http://www.cbat.eps.harvard.edu/iauc/03400/03456.html. Retrieved 2011-12-29.
* Marsden, Brian G. (September 30, 1983). "Satellites of Jupiter and Saturn". IAU Circular 3872. http://www.cbat.eps.harvard.edu/iauc/03800/03872.html. Retrieved 2011-12-23.
* "Saturn: Moons: Janus". Solar System Exploration: Planets. NASA. 4 Apr 2011. http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Janus.
* 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. doi:10.1086/505206. Bibcode: 2006AJ....132..692S.
* Thomas, P. C. (July 2010). "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission". Icarus 208 (1): 395–401. doi:10.1016/j.icarus.2010.01.025. Bibcode: 2010Icar..208..395T. http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf. 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. doi:10.1126/science.1134681. PMID 17289992. Bibcode: 2007Sci...315..815V. (supporting online material, table S1)
* Thomas, P. C.; Helfenstein, P. (July 2020). "The small inner satellites of Saturn: Shapes, structures and some implications". Icarus 344: 20. doi:10.1016/j.icarus.2019.06.016. 113355. Bibcode: 2020Icar..34413355T.
* Lainey, V.; Rambaux, N.; Cooper, N.; Dahoumane, R.; Zhang, Q. (February 2023). "Characterising the interior of five inner Saturnian moons using Cassini ISS data". Astronomy & Astrophysics 670: 6. doi:10.1051/0004-6361/202244757. L25. Bibcode: 2023A&A...670L..25L.

## External links

* 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. https://books.google.com/books?id=aU6vcy5L8GAC&pg=PP1.
* 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

* v
* t
* e

Saturn

* Outline of Saturn

Geography|

* Dragon Storm
* Great White Spot
* Hexagon
* Magnetosphere
* Rings

Moons|

* S/2009 S 1
* Ring moonlets
* Pan
* Daphnis
* Atlas
* Prometheus
* S/2004 S 6
* S/2004 S 4
* S/2004 S 3
* Pandora
* Epimetheus
* Janus
* Aegaeon
* Mimas
* Methone
* Anthe
* Pallene
* Enceladus
* Tethys
* Telesto
* Calypso
* Dione
* Helene
* Polydeuces
* Rhea
* Titan
* Hyperion
* Iapetus
* Kiviuq
* Ijiraq
* Phoebe
* Paaliaq
* Skathi
* S/2004 S 37
* Albiorix
* S/2007 S 2
* S/2004 S 29
* Bebhionn
* Erriapus
* Skoll
* S/2004 S 31
* Siarnaq
* Tarqeq
* S/2004 S 13
* Greip
* Hyrrokkin
* Jarnsaxa
* Tarvos
* Mundilfari
* S/2006 S 1
* S/2004 S 17
* Bergelmir
* Narvi
* S/2004 S 20
* Suttungr
* Hati
* S/2004 S 12
* S/2004 S 27
* Farbauti
* Thrymr
* S/2004 S 30
* Aegir
* S/2007 S 3
* Bestla
* S/2004 S 7
* S/2004 S 22
* S/2004 S 23
* S/2004 S 25
* S/2004 S 32
* S/2006 S 3
* S/2004 S 38
* Fenrir
* S/2004 S 28
* Surtur
* Kari
* S/2004 S 35
* Ymir
* S/2004 S 21
* S/2004 S 24
* Loge
* S/2004 S 36
* S/2004 S 39
* S/2004 S 33
* S/2004 S 34
* Fornjot
* S/2004 S 26

Astronomy|

* Delta Octantis
* Saturn-crossing minor planets

Exploration|

* Cassini–Huygens (Huygens)
* timeline
* retirement
* Pioneer 11
* Voyager program
* Voyager 1
* Voyager 2

Related|

* Fiction
* Moons
* The Day the Earth Smiled
* In Saturn's Rings (2018 documentary)

* v
* t
* e

Moons of Saturn

Listed in approximately increasing distance from Saturn.

Ring shepherds|

* S/2009 S 1
* Ring moonlets
* Pan
* Daphnis
* Atlas
* Prometheus
* Pandora

Co-orbitals|

* Epimetheus
* Janus

G Ring|

* Aegaeon

Alkyonides|

* Methone
* Anthe
* Pallene

Inner large
(with trojans)|

* Mimas
* Enceladus
* Tethys
* Telesto
* Calypso
* Dione
* Helene
* Polydeuces

Outer large|

* Rhea
* Titan
* Hyperion
* Iapetus

Inuit group|

* Kiviuq
* Ijiraq
* Paaliaq
* S/2004 S 29
* S/2004 S 31
* Siarnaq
* Tarqeq

Norse group|

* Phoebe
* Skathi
* S/2004 S 37
* S/2007 S 2
* Skoll
* S/2004 S 13(?)
* Greip
* Hyrrokkin
* Jarnsaxa
* Mundilfari
* S/2006 S 1
* S/2004 S 17(?)
* Bergelmir
* Narvi
* S/2004 S 20
* Suttungr
* Hati
* S/2004 S 12(?)
* S/2004 S 27
* Farbauti
* Thrymr
* S/2004 S 30
* Aegir
* S/2007 S 3(?)
* Bestla
* S/2004 S 7(?)
* S/2004 S 22(?)
* S/2004 S 23(?)
* S/2004 S 25(?)
* S/2004 S 32(?)
* S/2006 S 3
* S/2004 S 38(?)
* Fenrir
* S/2004 S 28(?)
* Surtur
* Kari
* S/2004 S 35(?)
* Ymir
* S/2004 S 21(?)
* Loge
* S/2004 S 36(?)
* S/2004 S 39(?)
* S/2004 S 33(?)
* S/2004 S 34(?)
* Fornjot
* S/2004 S 26(?)

Gallic group|

* Albiorix
* Bebhionn
* Erriapus
* Tarvos

Prograde outer satellites|

* S/2004 S 24

* Rings of Saturn
* Cassini–Huygens
* Themis
* Chiron
* S/2004 S 6
* S/2004 S 4
* S/2004 S 3
* In fiction

0.00

(0 votes)

Original source: https://en.wikipedia.org/wiki/Janus (moon). Read more

|

Retrieved from "https://handwiki.org/wiki/index.php?title=Astronomy:Janus_(moon)&oldid=176994"
*[v]: View this template
*[t]: Discuss this template
*[e]: Edit this template