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Janus (moon)

Saturn X redirects here. For the spurious moon reported in 1905, see Themis (moon)
Janus as imaged by Cassini on 2010-04-07: highest-resolution full-disk image to date
Discovered by Audouin Dollfus
Discovery date December 15, 1966
Pronunciation /ˈnəs/ JAY-nəs
Named after
Adjectives Janian
Orbital characteristics[1]
Epoch December 31, 2003 (JD 2 453 005.5)
151460±10Lua error: Unmatched close-bracket at pattern character 67.
Eccentricity 0.0068
0.694660342Lua error: Unmatched close-bracket at pattern character 67.
Inclination 0.163°±0.004°Lua error: Unmatched close-bracket at pattern character 67. to Saturn's equator
Satellite of Saturn
Physical characteristics
Dimensions 203 × 185 × 152.6 km [2]
Mean radius
89.5±1.4Lua error: Unmatched close-bracket at pattern character 67. [2]
Volume ≈ 3000000Lua error: Unmatched close-bracket at pattern character 67.
Mass (1.8975±0.0006)×1018Lua error: Unmatched close-bracket at pattern character 67.[2]
Mean density
0.63±0.03Lua error: Unmatched close-bracket at pattern character 67.[2]
0.011–0.017 m/s2[2]
Albedo 0.71±0.02 (geometric) [3]

Janus is an inner satellite of Saturn. It is also known as Saturn X (Roman numeral X = 10). It is named after the mythological Janus.

Discovery and orbit

The following is a summary. For more detailed information about Janus and Epimetheus's unusual shared orbit, see Epimetheus.

Janus occupies practically the same orbit as the moon Epimetheus. This caused some confusion for astronomers, who assumed that there was only one body in that orbit, and for a long time struggled to understand what was going on. It was eventually realized that they were trying to reconcile observations of two distinct objects as a single object.

The discovery of Janus is attributed to its first observer: Audouin Dollfus, on December 15, 1966.[4] The new object was given the temporary designation S/1966 S 2. Previously, Jean Texereau had photographed Janus on October 29, 1966 without realising it; Dollfus named it at the same occasion.[5] On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus.[6]

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.[7] Voyager 1 confirmed this in 1980.[8] (See co-orbital moon for a more detailed description of their unique arrangement.)

Janus was observed on subsequent occasions and given different provisional designations. It was observed by the Pioneer 11 probe when it passed near Saturn on September 1, 1979: three energetic particle detectors observed its "shadow" (S/1979 S 2.[9]) Janus was observed by Dan Pascu on February 19, 1980 (S/1980 S 1,[10]) and then by John W. Fountain, Stephen M. Larson, Harold J. Reitsema and Bradford A. Smith on the 23rd (S/1980 S 2.[11])

All of these people thus share, to various degrees, the title of discoverer of Janus.


Janus is named after Janus, the two-faced Roman god. Although the name was informally proposed soon after the initial 1966 discovery,[5] it was not officially given this name until 1983,[a] when Epimetheus also received its name.

According to the OED, the adjectival form of the moon's name is Janian.

Physical characteristics

Janus is extensively cratered with several craters larger than 30 km but few linear features. The Janian surface appears to be older than Prometheus's but younger than Pandora's. From its very low density and relatively high albedo, it seems likely that Janus is a very porous and icy rubble pile. The moon is also highly non-spherical.

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.[12] Its source is particles blasted off their surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.[13]

Along with Ephimetheus, Janus maintains the sharp edge of the A Ring by a 7:6 orbital resonance.[14]


See also



  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)



External links

16x16px Media related to Janus (moon) at Wikimedia Commons

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