Metis (moon)

Metis

Image of Metis was taken by Galileo's solid state imaging system between November 1996 and June 1997.
Discovery
Discovered by	S. Synnott
Discovery date	March 4, 1979
Orbital characteristics
Periapsis	127,974 km[1]
Apoapsis	128,026 km[1]
Mean orbit radius	128,000 km (1.792 RJ)[2][3]
Eccentricity	0.0002[2][3]
Orbital period	0.294780 d (7 h 4.5 min)[2][3]
Average orbital speed	31.501 km/s[1]
Inclination	0.06° (to Jupiter's equator)[2][3]
Satellite of	Jupiter
Proper orbital elements
Physical characteristics
Dimensions	60×40×34 km³[4]
Mean radius	21.5 ± 2.0 km[4]
Volume	~42,700 km³
Mass	3.6 ×1016 kg[1]
Mean density	0.86 g/cm³ (assumed)
Equatorial surface gravity	0.005 m/s² (0.0005 g)[1]
Escape velocity	0.012 km/s[1]
Rotation period	synchronous
Axial tilt	zero[4]
Albedo	0.061 ± 0.003[5]
Temperature	~123 K

Metis as taken from Galileo spacecraft

Metis ( /ˈmiːtɨs/ mee-təs; Greek: Μήτις), also known as Jupiter XVI, is the innermost moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis. Additional observations made between early 1996 and September 2003 by the Galileo spacecraft allowed the surface of the moon to be imaged.

Metis is tidally locked to Jupiter, which led^{[citation needed]} to a high asymmetry in the shape of the moon, with one of the diameters being almost twice as large as the smallest one. It is also one of the three moons in the Solar System known to orbit its planet in less than the length of that planet's day, the other two being Jupiter's Adrastea and Mars's Phobos. It orbits within the main ring of Jupiter, and is thought to be a major contributor of material to the rings.

Discovery and observations

Metis was discovered in 1979 by Stephen P. Synnott in images taken by the Voyager 1 probe and was provisionally designated as S/1979 J 3.^[6][7] In 1983 it was officially named after the mythological Metis, a Titaness who was the first wife of Zeus (the Greek equivalent of Jupiter).^[8] The photographs taken by Voyager 1 showed Metis only as a dot, and hence knowledge about Metis was very limited until the arrival of the Galileo spacecraft. Galileo imaged almost all of the surface of Metis and put constraints on its composition by 1998.^[4]

Physical characteristics

Metis has an irregular shape and measures 60×40×34 km across, which makes it the second smallest of the four inner satellites of Jupiter.^[4] The bulk composition and mass of Metis are not known, but assuming that its mean density is like that of Amalthea (~0.86 g/cm³),^[9] its mass can be estimated as ~3.6×10¹⁶ kg. Metis' density implies that that moon is composed of water ice with a porosity of 10–15%, and Adrastea may be similar.^[9]

The Metidian surface is heavily cratered, dark, and appears to be reddish in color. There is a substantial asymmetry between leading and trailing hemispheres: the leading hemisphere is 1.3-times brighter than the trailing one. The asymmetry is probably caused by the higher velocity and frequency of impacts on the leading hemisphere, which excavate a bright material (presumably ice) from the interior of the moon.^[5]

Orbit

Metis is the innermost of Jupiter's four small inner moons. It orbits Jupiter at a distance of ~128,000 km (1.79 Jupiter radii) within the planet's main ring. Its orbit has very small eccentricity (~0.0002) and inclination (~ 0.06°) relative to the equator of Jupiter.^[2][3]

Due to tidal locking, Metis rotates synchronously with its orbital period, with its longest axis aligned towards Jupiter.^[3][4]

Metis lies inside Jupiter's synchronous orbit radius (as does Adrastea), and as a result, tidal forces slowly cause its orbit to decay, and the moon will eventually impact Jupiter. If its density is similar to Amalthea's, Metis' orbit lies within the fluid Roche limit; however, since it has not broken up, it must lie outside its rigid Roche limit.^[3]

Relationship with Jupiter's rings

Metis' orbit lies ~1000 km within the main ring of Jupiter. It orbits within a ~500 km wide "gap" or "notch" in the ring.^[3][10] The gap is clearly somehow related to the moon but the origin of this connection has not been established. Metis supplies a significant part of the main ring’s dust.^[11] This material appears to consist primarily of material that is ejected from the surfaces of Jupiter's four small inner satellites by meteorite impacts. It is easy for the impact ejecta to be lost from the satellites into space because the satellites' surfaces lie fairly close to the edge of their Roche spheres due to their low density.^[3]

See also

• Inner satellite
• Rings of Jupiter

References

1. ^ ^{a b c d e f} Calculated on the basis of other parameters
2. ^ ^{a b c d e} Evans, M.W.; Porco, C.C.; Hamilton, D.P. (2002). "The Orbits of Metis and Adrastea: The Origin and Significance of their Inclinations". Bulletin of the American Astronomical Society 34: 883. Bibcode 2002DPS....34.2403E. 
3. ^ ^{a b c d e f g h i} Burns, J.A.; Simonelli, D.P.; Showalter, M.R.; Hamilton, D.P.; Porco, C.C.; Esposito, L.W.; Throop, H. (2004). "Jupiter’s Ring-Moon System". In Bagenal, F.; Dowling, T. E.; McKinnon, W. B.. Jupiter: The planet, Satellites and Magnetosphere. Cambridge University Press. http://www.astro.umd.edu/~hamilton/research/preprints/BurSimSho03.pdf. 
4. ^ ^{a b c d e f} Thomas, P.C.; Burns, J.A.; Rossier, L.; et al. (1998). "The Small Inner Satellites of Jupiter". ICARUS 135 (1): 360–371. Bibcode 1998Icar..135..360T. doi:10.1006/icar.1998.5976. 
5. ^ ^{a b} Simonelli, D.P.; Rossiery, L.; Thomas, P.C.; et al. (2000). "Leading/Trailing Albedo Asymmetries of Thebe, Amalthea, and Metis". ICARUS 147 (2): 353–365. Bibcode 2000Icar..147..353S. doi:10.1006/icar.2000.6474. 
6. ^ Brian G. Marsden (1980-08-26). "Satellites of Jupiter". IAU Circulars 3507. http://www.cbat.eps.harvard.edu/iauc/03500/03507.html. (discovery)
7. ^ Synnott, S.P. (1981). "1979J3: Discovery of a Previously Unknown Satellite of Jupiter". Science 212 (4501): 1392. Bibcode 1981Sci...212.1392S. doi:10.1126/science.212.4501.1392. ISSN 0036-8075. PMID 17746259. http://links.jstor.org/sici?sici=0036-8075%2819810619%293212:4501%3c1392:1DOAPU%3e2.0.CO%3b2-X&origin=ads. 
8. ^ Brian G. Marsden (1983-09-30). "Satellites of Jupiter and Saturn". IAU Circulars 3872. http://www.cbat.eps.harvard.edu/iauc/03800/03872.html. (naming the moon)
9. ^ ^{a b} Anderson, J.D.; Johnson, T.V.; Shubert, G.; et al. (2005). "Amalthea’s Density Is Less Than That of Water". Science 308 (5726): 1291–1293. Bibcode 2005Sci...308.1291A. doi:10.1126/science.1110422. PMID 15919987. 
10. ^ Ockert-Bel, M.E.; Burns, J.A.; Daubar, I.J.; et al. (1999). "The Structure of Jupiter’s Ring System as Revealed by the Galileo Imaging Experiment". ICARUS 138 (2): 188–213. Bibcode 1999Icar..138..188O. doi:10.1006/icar.1998.6072. 
11. ^ Burns, J.A.; Showalter, M.R.; Hamilton, D.P.; et al. (1999). "The Formation of Jupiter's Faint Rings". Science 284 (5417): 1146–1150. Bibcode 1999Sci...284.1146B. doi:10.1126/science.284.5417.1146. PMID 10325220. 

External links

• Metis Profile by NASA's Solar System Exploration