Cassini image of Methone's leading side taken on 20 May 2012
|Discovered by||Cassini Imaging Team |
|Discovery date||June 1, 2004|
|Orbital characteristics |
|Epoch June 20, 2004 (JD 2453177.5)|
|Inclination||0.007°±0.003° (to Saturn's equator)|
Methone was first seen by the Cassini Imaging Team and given the temporary designation S/2004 S 1. Methone is also named Saturn XXXII (32). The Cassini spacecraft 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. 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.
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°.
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. The moons Pallene and Aegaeon are thought to be similarly smooth. Methone has two different sharply defined albedo regions, one distinctly (~13%) darker centered on Methone's leading point. Its brighter area has an albedo of ~0.70. 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. 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, and a similar irradiation anisotropy might be behind Methone's albedo pattern.
Methone's mean radius is 1.45±0.03 km.
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.
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.
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