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	/mᵻˈθoʊniː/ mi-THOH-nee
Adjectives	Methonean
Orbital characteristics [2]
Epoch June 20, 2004 (JD 2453177.5)
Semi-major axis	194440±20 km
Eccentricity	0.0001
Orbital period	1.009573975 d[3]
Inclination	0.007°±0.003° (to Saturn's equator)
Satellite of	Saturn
Physical characteristics
Mean radius	1.6±0.06 km [4]
Mean density	0.31+0.05 −0.03 g/cm^3 [5]
Rotation period	synchronous
Axial tilt	zero

Methone is a very small natural satellite of Saturn orbiting between the orbits of Mimas and Enceladus.

Discovery and naming

Discovery image of Methone on June 1, 2004^[6]

Methone was first seen by the Cassini Imaging Team^[1][7][8] 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.^[9] 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]

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.^[10] The moons Pallene and Aegaeon are thought to be similarly smooth.^[11] There is a sharply defined distinctly (~13%) darker albedo region centered on Methone's leading point. 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.^[5] 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,^[12] and a similar irradiation anisotropy might be behind Methone's albedo pattern.^[5]

Methone's mean radius is 1.6 km and its mass estimated to be 9×10¹² kg.^[13]

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, an estimate of its density can be calculated. The result is 0.31 g/cm³, among the lowest density values obtained or inferred for a Solar System body. This suggests Methone is composed of icy fluff, material that might be mobile enough to explain the lack of craters.^[5][11]

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.^[14][15]

References

1. Cassini Imaging Team.
2. Spitale Jacobson et al. 2006.
3. NASA Celestia Archived March 9, 2005, at the Wayback Machine.
4. Thomas 2010.
5. Thomas 2013.
6. JPL/NASA: Cassini Uncovers New Moon.
7. IAUC 8389.
8. Porco Baker et al. 2005.
9. IAUC 8471.
10. Lakdawalla, E. 2012.
11. Battersby, 2013.
12. Schenk Hamilton et al. 2011, pp. 745–46.
13. Methone: Facts & Figures
14. Porco et al., 2008.
15. Hedman et al., 2009.

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". New Scientist web site. New Scientist. Retrieved 2013-05-21. External link in |work= (help)
• 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?". The Planetary Society web site. The Planetary Society. Retrieved 2012-05-23. External link in |work= (help)
• 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. 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.
• Porco C. C., et al. (2008-09-05). "More Ring Arcs for Saturn". Cassini Imaging Central Laboratory for Operations web site. Retrieved 2008-09-05. External link in |work= (help)
• 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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• IAU Working Group for Planetary System Nomenclature