 Cassini image of Methone's leading side taken on 20 May 2012. Its unusually smooth surface is likely from the accumulation of dust | |
| Discovery | |
|---|---|
| Discovered by | Cassini Imaging Team[1] |
| Discovery date | June 1, 2004 |
| Designations | |
Designation | Saturn XXXII |
| Pronunciation | /mɛˈθoʊniː/[2] |
Named after | ΜεθώνηMethōnē |
| Adjectives | Methonean/mɛθəˈniːən/[3] |
| Orbital characteristics[4] | |
| Epoch June 20, 2004 (JD2453177.5) | |
| 194700 km[5] | |
| Eccentricity | 0.002[5] |
| 1.009549 d[5] | |
| Inclination | 0.007°±0.003° (to Saturn's equator) |
| Satellite of | Saturn |
| Group | Alkyonides |
| Physical characteristics | |
| Dimensions | 3.88 × 2.58 × 2.42 km (±0.04 × 0.08 × 0.04 km)[6]: 2 |
| 2.90±0.06 km[6]: 2 | |
| Volume | 12.8 km3[a] |
| Mass | (3.92±1.00)×1012 kg[6]: 3 |
Meandensity | 0.307±0.030 g/cm3[6]: 3 |
| 0.008–0.013 mm/s2[6]: 3 | |
| 0.0005 km/s at longest axis to 0.0007 km/s at poles | |
| synchronous | |
| zero | |
Methone/mɛˈθoʊniː/ is a small, egg-shapednatural satellite ofSaturn that orbits out pastSaturn's ring system, between the orbits ofMimas andEnceladus. It was discovered in 2004, though it was not until 2012 that it was imaged in detail by theCassini spacecraft.
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Methone was first discovered by the Cassini Imaging Team[1][8][9] and given the temporary designationS/2004 S 1. Methone is also namedSaturn XXXII (32). TheCassini 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.[10] It was ratified at the IAU General Assembly in 2006.Methone (GreekΜεθώνη) was one of theAlkyonides, the seven beautiful daughters of theGiantAlkyoneus.
Methone's orbit is perturbed by a 14:15mean-motion resonance with the much largerMimas. This causes itsosculating orbital elements to vary with an amplitude of about 20 km (12 mi) in semi-major axis, and 5° inlongitude of its periapsis on a timescale of about 450 days. Its eccentricity also varies, albeit on different timescales, between 0.0011 and 0.0037, and its inclination between about 0.003° and 0.020°.[4]
In May 2012, theCassini spacecraft took its first close-up photographs of Methone, revealing a remarkably smooth, but non-spherical moonlet.[11] The other arc-imbedded moonlets,Pallene andAnthe, are thought to be similar.[12]
Methone's smoothness and excellent ellipsoidal fits suggest that it has developed anequipotential surface, and this may be composed largely of an icy fluff, a material that might be mobile enough to explain the moonlet's lack of craters. This material property causes Methone to take the shape of atriaxial ellipsoid, a type of ellipsoid in which all 3 of its principal axes are of different lengths. These differences reflect the balance betweentidal forces exerted by Saturn andcentrifugal forces from the moonlet's own rotation, as well as the moonlet's own force of gravity.[13]
Methone's longest axis points towards Saturn, and is 1.6x longer than its polar axis. This elongation is caused by tidal forces, whereas the elongation of its intermediate-length axis (1.07x the length of the polar axis) is caused by the centrifugal force of Methone's rotation.[13]
Methone's low-density regolith may respond to impacts in a way that smooths its surface more rapidly than on rigid moonlets such as Janus or Epimetheus. Movement of the regolith may also be facilitated by more "exotic" processes such as electrostatic effects.[13]
Methone has two sharply definedalbedo regions, with albedos of 0.61±0.06 and 0.7±0.03. The darker of these regions is centered on Methone's leading side, reminiscent of the thermal anomalies on the leading hemispheres of Mimas andTethys,[14] and it has been suggested that increased exposure to electrons fromSaturn's magnetosphere is responsible. However, in those examples, the thermal anomalies coincide with a distinct UV/IR coloration, which in the case of Methone is either highly subdued or undetectable.[13] Thus a physical, rather than compositional difference may be responsible. Possibilities include variations in regolith grain size, compaction, or particle microstructure.[13]
Assuming that Methone is inhydrostatic equilibrium, i.e. that its elongated shape simply reflects the balance between thetidal 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.[13][12]
Material blasted off Methone bymicrometeoroid impacts is believed to be the source of theMethone Ring Arc, a faint partial ring around Saturn co-orbital with Methone that was discovered in September 2006.[15][16]
| Geography | |
|---|---|
| Moons | |
| Astronomy | |
| Exploration | |
| Related | |
