Methone (moon)

Methone
Cassini image of Methone's leading side taken on 20 May 2012
Discovery
Discovered by Cassini Imaging Team [1]
Discovery date June 1, 2004
Designations
Pronunciation /mɪˈθoʊniː/ mi-THOH-nee
Adjectives Methonean
Orbital characteristics [2]
Epoch June 20, 2004 (JD 2453177.5)
194440±20 km
Eccentricity 0.0001
1.009573975 d[3]
Inclination 0.007°±0.003° (to Saturn's equator)
Satellite of Saturn
Physical characteristics
Dimensions (3.88±0.04)×(2.58±0.08)×(2.42±0.04) km[4]
Mean radius
 1.45±0.03 km[4]
Mean density
 0.31+0.05
−0.03 g/cm3[4]
synchronous
zero

    Methone is a very small natural satellite of Saturn orbiting between the orbits of Mimas and Enceladus. It was discovered in 2004, and in 2012 was imaged more closely by the Cassini orbiter probe.

    History

    Discovery image of Methone on 1 June 2004[5]

    Methone was first seen by the Cassini Imaging Team[1][6][7] and given the temporary designation S/2004 S 1. Methone is also named Saturn XXXII (32). The Cassini spacecraft has made two visits to Methone and its closest approach was made on May 20, 2012 with a minimum distance of 1,900 km (1,181 mi) from it.

    The name Methone was approved by the IAU Working Group on Planetary System Nomenclature on January 21, 2005.[8] It was ratified at the IAU General Assembly in 2006. Methone (Greek Μεθώνη) was one of the Alkyonides, the seven beautiful daughters of the Giant Alkyoneus.

    Orbit

    Methone's orbit is visibly affected by a perturbing 14:15 mean-longitude resonance with the much larger Mimas. This causes its osculating orbital elements to vary with an amplitude of about 20 km (12 mi) in semi-major axis, and 5° in longitude of its periapsis on a timescale of about 450 days. Its eccentricity also varies on different timescales between 0.0011 and 0.0037, and its inclination between about 0.003° and 0.020°.[2]

    Physical characteristics

    In May 2012, the Cassini spacecraft obtained its first close-up photographs of Methone, revealing an egg-shaped moonlet with a remarkably smooth surface, with no visible craters.[9] The moons Pallene and Aegaeon are thought to be similarly smooth.[10] Methone has two different sharply defined albedo regions, one distinctly (~13%) darker centered on Methone's leading point.[4] It brighter area has an albedo of ~0.70.[4] UV and IR spectra gave no indication of a color difference between the two regions, suggesting that a physical rather than compositional difference may be responsible.[4] Increased exposure to electrons from Saturn's magnetosphere has been proposed to be responsible for thermal anomalies on the leading hemispheres of Mimas and Tethys,[11] and a similar irradiation anisotropy might be behind Methone's albedo pattern.[4]

    Methone's mean radius is 1.45±0.03 km.[4]

    Assuming that Methone is in hydrostatic equilibrium, i.e. that its elongated shape simply reflects the balance between the tidal force exerted by Saturn and Methone's gravity, its density can be estimated: 0.31+0.05
    −0.03     g/cm3    , among the lowest density values obtained or inferred for a Solar System body. This indicates that Methone is composed of icy fluff, material that might be mobile enough to explain the lack of craters.[4][10]

    Relationship with Saturn's rings

    Material blasted off Methone by micrometeoroid impacts is believed to the source of the Methone Ring Arc, a faint partial ring around Saturn co-orbital with Methone that was discovered in September 2006.[12][13]

    References

    Bibliography

    • "Cassini Imaging Science Team". Cassini Imaging Central Laboratory for Operations. Retrieved 2012-01-01.
    • Battersby, S. (2013-05-17). "Saturn's egg moon Methone is made of fluff". www.newscientist.com. New Scientist. Retrieved 2013-05-21.
    • Green, Daniel W. E. (August 16, 2004). "S/2004 S 1 and S/2004 S 2" (discovery). IAU Circular. 8389. Retrieved 2012-01-01.
    • Green, Daniel W. E. (January 21, 2005). "S/2004 S 1 and S/2004 S 2" (naming the moon). IAU Circular. 8471. Retrieved 2012-01-01.
    • Hedman, M. M.; Murray, C. D.; Cooper, N. J.; Tiscareno, M. S.; Beurle, K.; Evans, M. W.; Burns, J. A. (2008-11-25). "Three tenuous rings/arcs for three tiny moons". Icarus. 199 (2): 378–386. Bibcode:2009Icar..199..378H. doi:10.1016/j.icarus.2008.11.001. ISSN 0019-1035.
    • "PIA06105: Cassini Uncovers New Moon". Photojournal. JPL/NASA. August 16, 2004. Retrieved 2012-01-01.
    • Lakdawalla, E. (2012-05-21). "Methone, an egg in Saturn orbit?". www.planetary.org. The Planetary Society. Retrieved 2012-05-23.
    • Porco, C. C.; Baker, E.; Barbara, J.; Beurle, K.; Brahic, A.; Burns, J. A.; Charnoz, S.; Cooper, N.; Dawson, D. D.; Del Genio, A. D.; Denk, T.; Dones, L.; Dyudina, U.; Evans, M. W.; Giese, B.; Grazier, K.; Helfenstein, P.; Ingersoll, A. P.; Jacobson, R. A.; Johnson, T. V.; McEwen, A.; Murray, C. D.; Neukum, G.; Owen, W. M.; Perry, J.; Roatsch, T.; Spitale, J.; Squyres, S.; Thomas, P.; Tiscareno, M. (February 25, 2005). "Cassini Imaging Science: Initial Results on Saturn's Rings and Small Satellites". Science. 307 (5713): 1226–1236. Bibcode:2005Sci...307.1226P. doi:10.1126/science.1108056. PMID 15731439.
    • Schenk, P.; Hamilton, D. P.; Johnson, R. E.; McKinnon, W. B.; Paranicas, C.; Schmidt, J.; Showalter, M. R. (January 2011). "Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites". Icarus. 211 (1): 740–757. Bibcode:2011Icar..211..740S. doi:10.1016/j.icarus.2010.08.016.
    • Carolyn Porco, et al. (2008-09-05). "More Ring Arcs for Saturn". Cassini Imaging Central Laboratory for Operations. Archived from the original on 2008-10-10. Retrieved 2008-09-05.
    • Spitale, J. N.; Jacobson, R. A.; Porco, C. C.; Owen, W. M., Jr. (2006). "The orbits of Saturn's small satellites derived from combined historic and Cassini imaging observations" (PDF). The Astronomical Journal. 132 (2): 692–710. Bibcode:2006AJ....132..692S. doi:10.1086/505206.
    • Thomas, P. C. (July 2010). "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission" (PDF). Icarus. 208 (1): 395–401. Bibcode:2010Icar..208..395T. doi:10.1016/j.icarus.2010.01.025.
    • Thomas, P. C.; Burns, J. A.; Tiscareno, M. S.; Hedman, M. M.; et al. (2013). "Saturn's Mysterious Arc-Embedded Moons: Recycled Fluff?" (PDF). 44th Lunar and Planetary Science Conference. p. 1598. Retrieved 2013-05-21.
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