Jump to content

Janus (moon)

Listen to this article
From Wikipedia, the free encyclopedia
"Saturn X" redirects here. For the spurious moon reported in 1905, seeThemis (hypothetical moon).
Janus
Janus as imaged byCassinion 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]
Epoch31 December 2003 (JD2 453 005.5)
151460±10 km
Eccentricity0.0068
0.694660342d
Inclination0.163°±0.004°to Saturn's equator
Satellite ofSaturn
GroupCo-orbitalwithEpimetheus
Physical characteristics
Dimensions203.4 × 185.8 × 149.0km
(±1.8 × 0.6 × 0.6km)[5]: 2 
178.0±1.0 km[5]: 2 
Volume2953010±950 km3[6]: 4 
Mass(1.89388±0.00028)×1018kg[a]
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]
Temperature76K

Janus/ˈdʒeɪnəs/is aninner satelliteofSaturn.It is also known asSaturn X.It is named after the mythologicalJanus.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[edit]

Discovery[edit]

Janus was identified byAudouin Dollfuson 15 December 1966[8]and given the temporary designationS/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 ofEpimetheus.[10]Voyager 1confirmed this orbital configuration in 1980.[11]

Observational history[edit]

Janus was observed on subsequent occasions and given differentprovisional designations.Pioneer 11'sthree 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. Reitsemaand Bradford A. Smith on 23 February 1980 (S/1980 S 2).[14]

Name[edit]

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

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 offoreshortening;in reality, Janus is about 40,000 km farther fromCassinithan Epimetheus.

Janus's orbit is co-orbital with that ofEpimetheus.Janus'smean orbital radiusfrom Saturn was, as of 2006, only 50 km less than that of Epimetheus, a distance smaller than either moon's mean radius. In accordance withKepler's laws of planetary motion,the closerorbitis 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 andorbital periodare increased, and in exchange the outer moon's are decreased. The timing and magnitude of themomentum exchangeis 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-framedepiction of thehorseshoe orbitsof 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 theSolar System.[17]

The orbital relationship between Janus and Epimetheus can be understood in terms of thecircular 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[edit]

Janus is extensively cratered with severalcraterslarger than 30 km and has few linear features. Janus's surface appears to be older thanPrometheus's but younger thanPandora's.

Animation of Epimetheus'sorbit – Rotating reference frameSaturn·Janus·Epimetheus

Janus has a very lowdensityand relatively highalbedo,meaning that it is likely icy in composition and structurally arubble pileobject.

Features[edit]

Craters on Janus, like those on Epimetheus, are named after characters in the legend ofCastor 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[edit]

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 theCassinispacecraftin 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 ashepherd moon,maintaining the sharp outer edge of theA Ringin a 7:6orbital resonance.The effect is more obvious when the more massive Janus is on the resonant (inner) orbit.[17]

Gallery[edit]

In fiction[edit]

Janus is the location of the plot in theAlastair Reynoldsscience-fiction novelPushing Ice.

Notes[edit]

  1. ^Calculated from the standard gravitational parameterGM=0.126403±0.000019km3·s–2given by Lainey et al. (2023), divided by thegravitational constantG=6.6743×10−2km3·kg–1·s–2.[6]
  2. ^Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (confirms Janus, names Epimetheus, Telesto, Calypso) (mentioned inIAUC 3872:Satellites of Jupiter and Saturn,September 30, 1983)

References[edit]

Citations[edit]

  1. ^"Janus".Oxford English Dictionary(Online ed.).Oxford University Press.(Subscription orparticipating institution membershiprequired.)
  2. ^JPL (ca. 2008)Cassini Equinox Mission: Janus
  3. ^Carter (1919)The gates of Janus
  4. ^Spitale Jacobson et al. 2006.
  5. ^abcThomas & Helfenstein 2020.
  6. ^abcLainey 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. ^abEl 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[edit]

External links[edit]

Media related toJanus (moon)at Wikimedia Commons

Listen to this article(4minutes)
Spoken Wikipedia icon
This audio filewas 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)
Retrieved from "https://en.wikipedia.org/w/index.php?title=Janus_(moon)&oldid=1233773635"