Metis (moon)

Not to be confused with 9 Metis.

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	127974 km
Apoapsis	128026 km
Mean orbit radius	128000 km (1.792 RJ)
Eccentricity	0.0002
Orbital period	0.294780 d (7 h, 4.5 min)
Average orbital speed	31.501 km/s
Inclination	0.06° (to Jupiter's equator)
Satellite of	Jupiter
Physical characteristics
Dimensions	60 × 40 × 34 km³
Mean radius	21.5±2.0 km
Volume	≈ 42700 km3
Mass	3.6×1016 kg
Mean density	0.86 g/cm³ (assumed)
Surface gravity	0.005 m/s2 (0.0005 g)
Escape velocity	0.012 km/s
Rotation period	synchronous
Axial tilt	zero
Albedo	0.061±0.003
Temperature	≈ 123 K

Metis as taken from Galileo spacecraft

Metis (/ˈmiːtᵻs/; 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 its surface to be imaged.

Metis is tidally locked to Jupiter, and its shape is strongly asymmetrical, with one of the diameters being almost twice as large as the smallest one. It is also one of the two moons known to orbit Jupiter in less than the length of Jupiter's day, the other being Adrastea. 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. In 1983 it was officially named after the mythological Metis, a Titaness who was the first wife of Zeus (the Greek equivalent of Jupiter). 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.

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. Therefore, a very rough estimate of its surface area could be placed between 5,800 and 11,600 square kilometers (approx. 8,700). 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), its mass can be estimated as ~3.6×10¹⁶ kg. This density would imply that it is composed of water ice with a porosity of 10–15%.

The surface of Metis 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 its interior.

Orbit and rotation

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 Jupiter's main ring. Metis's orbit has very small eccentricity (~0.0002) and inclination (~ 0.06°) relative to the equator of Jupiter.

Due to tidal locking, Metis rotates synchronously with its orbital period, with its longest axis aligned towards Jupiter.

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

Because Metis orbits very close to Jupiter, Jupiter appears as a gigantic sphere about 67.9° in diameter from Metis, the largest angular diameter as viewed from any of Jupiter's moons. For the same reason only 31% of Jupiter's surface is visible from Metis at any one time, the most limited view of Jupiter from any of its moons.

Metis is the fastest-moving of Jupiter's moons. It orbits Jupiter at 31.5 km/s.

Relationship with Jupiter's rings

Metis's orbit lies ~1000 km within the main ring of Jupiter. It orbits within a ~500 km wide "gap" or "notch" in the ring. 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. 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.

See also

• Inner satellite
• Rings of Jupiter

References

Cited sources

• (discovery)
• (naming the moon)

External links

• Metis Profile by NASA's Solar System Exploration