Janus (moon)
- Saturn X redirects here. For the spurious moon reported in 1905, see Themis (moon)
 Janus, as imaged by Cassini before Saturn in 2006 | |
| Discovery | |
|---|---|
| Discovered by | Audouin Dollfus |
| Discovery date | December 15, 1966 |
| Designations | |
| Adjectives | Janian |
| Orbital characteristics[1] | |
| Epoch December 31, 2003 (JD 2 453 005.5) | |
| 151 460 ± 10 km | |
| Eccentricity | 0.0068 |
| 0.694 660 342 d | |
| Inclination | 0.163 ± 0.004° to Saturn's equator |
| Satellite of | Saturn |
| Physical characteristics | |
| Dimensions | 193×173×137 km [2] |
| 89.4 ± 3.0 [2] | |
| Mass | 1.912 ± 0.005 ×1018 kg [1] |
Mean density | 0.64 ± 0.06 g/cm³[2] |
| ~0.0137 m/s2 | |
| synchronous | |
| zero | |
| Albedo | 0.71 ± 0.02 (geometric)[3] |
Janus (Template:Pron-en,[4] or as in Latin Ianus) 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' 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 figure out 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.[5] 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.[6] On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus.[7]
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] Voyager 1 confirmed this in 1980.[9] (See below 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.[10]) Janus was observed by Dan Pascu on February 19, 1980 (S/1980 S 1,[11]) and then by John W. Fountain, Stephen M. Larson, Harold J. Reitsema and Bradford A. Smith on the 23rd (S/1980 S 2.[12])
All of these people thus share, to various degrees, the title of discoverer of Janus.
Name
Janus is named after Janus, the two-faced Roman god. Although the name was informally proposed soon after the initial 1966 discovery,[6] it was not officially given this name until 1983.[13] Epimetheus received its name at the same time.
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' 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.
Ring
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.[14] Its source is particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.[15]
Gallery
-
Janus as imaged by Cassini on 2008-02-20. -
Janus and Prometheus lie above and below Saturn's rings in this Cassini image from 2006-04-29. -
Janus as viewed by Voyager 2 on 1981-08-25.
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See also
References
- ^ a b Spitale, J. N.; et al. (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.
{{cite journal}}: Explicit use of et al. in:|author=(help) - ^ a b c Porco, C. C.; et al. (2006). "Physical Characteristics and Possible Accretionary Origins for Saturn's Small Satellites" (PDF). Bulletin of the American Astronomical Society. 37: 768.
{{cite journal}}: Explicit use of et al. in:|author=(help) - ^ Verbiscer, A.; French, R.; Showalter, M.; and Helfenstein, P.; Enceladus: Cosmic Graffiti Artist Caught in the Act, Science, Vol. 315, No. 5813 (February 9, 2007), p. 815 (supporting online material, table S1)
- ^ In US dictionary transcription, Template:USdict.
- ^ IAUC 1987: Probable New Satellite of Saturn January 3, 1967 (discovery)
- ^ a b IAUC 1995: Saturn X (Janus) February 1, 1967 (naming the moon)
- ^ IAUC 1991: Possible New Satellite of Saturn January 6, 1967
- ^ Fountain, J. W.; and Larson, S. M.; Saturn's ring and nearby faint satellites, Icarus, Vol. 36 (October 1978), pp. 92–106
- ^ "Saturn: Moons: Janus". Solar System Exploration: Planets. NASA. Retrieved February 28, 2009.
- ^ IAUC 3417: New Ring and Satellites of Saturn October 25, 1979
- ^ IAUC 3454: Saturn February 25, 1980
- ^ IAUC 3456: 1980 S 2 February 29, 1980
- ^ 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)
- ^ NASA Planetary Photojournal, PIA08328: Moon-Made Rings
- ^ Cassini-Huygens press release, NASA Finds Saturn's Moons May Be Creating New Rings, October 11, 2006
External links
- Janus Profile by NASA's Solar System Exploration
- The Planetary Society: Janus
- 'Solar System Dynamics' by Murray and Dermott The standard text on the subject, describes the orbits in detail.
- Quicktime illustration of co-orbital motion from Murray and Dermott
- Cassini image of Janus and Epimetheus near the time of their orbital swap.