Janus (moon)

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Janus

Janus as imaged by Cassini on 2010-04-07: highest-resolution full-disk image to date
Discovery
Discovered by Audouin Dollfus
Discovery date December 15, 1966
Designations
Pronunciation /ˈdʒeɪnəs/ JAY-nəs
Named after Janus
Adjective Janian
Orbital characteristics [1]
Epoch December 31, 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
Physical characteristics
Dimensions 203 × 185 × 152.6 km [2]
Mean radius 89.5±1.4 km [2]
Volume ≈ 3000000 km3
Mass (1.8975±0.0006)×1018 kg [2]
Mean density 0.63±0.03 g/cm³ [2]
Equatorial surface gravity 0.011–0.017 m/s² [2]
Rotation period synchronous
Axial tilt zero
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.

    Name

    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,[lower-alpha 1] 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]

    Gallery

    • Janus as viewed by Voyager 2 on 1981-08-25.

    • Janus and Prometheus lie above and below Saturn's rings in this Cassini image from 2006-04-29.

    • Janus in front of Saturn as imaged by Cassini on 2006-09-25

    • Janus as imaged by Cassini on 2008-02-20.

    • Crescent Janus (2008-06-30)

    See also

    References

    Notes

    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)

    Citations

    1. ↑ Spitale Jacobson et al. 2006.
    2. ↑ 2.0 2.1 2.2 2.3 2.4 Thomas 2010.
    3. ↑ Verbiscer French et al. 2007.
    4. ↑ IAUC 1987.
    5. ↑ 5.0 5.1 IAUC 1995.
    6. ↑ IAUC 1991.
    7. ↑ Fountain & Larson 1978.
    8. ↑ Solar System, NASA: Janus.
    9. ↑ IAUC 3417.
    10. ↑ IAUC 3454.
    11. ↑ IAUC 3456.
    12. ↑ JPL/NASA: Moon-Made Rings.
    13. ↑ JPL/NASA: Creating New Rings.
    14. ↑ Lakdawalla 2007.

    Sources

    External links

    Media related to Janus (moon) at Wikimedia Commons

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    Generally listed in increasing distance from Saturn. Temporary names in italics.
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