11th Largest Moon of Jupiter - Metis (1st Moon outwards from Jupiter)

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.

Metis is the 11th largest moon of Jupiter and 69th largest moon in the Solar System currently known.


Discovery

Metis was discovered in 4 March 1979 by Stephen P. Synnott in images taken by the Voyager 1 probe.

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.

Naming

Metis was provisionally designated as S/1979 J3. In 1983 it was officially named after the mythological Metis, a Titaness who was the first wife of Zeus (the Greek equivalent of Jupiter).

In Greek mythology, Metis was of the Titan generation and, like several primordial figures, an Oceanid, in the sense that Metis was born of Oceanus and Tethys, of an earlier age than Zeus and his siblings. Metis was the first great spouse of Zeus.

By the era of Greek philosophy in the fifth century BCE, Metis had become the goddess of wisdom and deep thought.

Stats

Diameter (mean): 43 km
Semi-major axis: 127,690 km
Orbital Period: 0.295 day

Orbit

Metis orbits Jupiter at a distance of ~128,000 km (1.79 Jupiter radii) within the planet's main ring.

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, 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.

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. 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.

Physical Characteristics

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. Metis' density implies that that moon is composed of water ice with a porosity of 10–15%.

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.