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Janus (moon)

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PIA12714 Janus crop.jpg
Janus as imaged by Cassini on 7 April 2010: highest-resolution full-disk image to date
Discovered byAudouin Dollfus
Discovery date15 December 1966
Saturn X
Named after
AdjectivesJanian /ˈniən/[2][3]
Orbital characteristics[4]
Epoch 31 December 2003 (JD 2 453 005.5)
151460±10 km
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]
Albedo0.71±0.02 (geometric) [6]
Temperature76 K

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



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])


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.


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[edit]

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).


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 /ˈdəs/ Ἴδας
Lynceus /ˈlɪnsəs/ Λυγκεύς
Phoibe (of Messenia) /ˈfɔɪb/ 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 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]


In popular culture[edit]

  • 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[edit]


  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)


  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 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. ^ USGS: Janus nomenclature
  18. ^ JPL/NASA: Moon-Made Rings.
  19. ^ JPL/NASA: Creating New Rings.


External links[edit]

Media related to Janus (moon) at Wikimedia Commons

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