Metis , also known as , is the innermost known moon of Jupiter. It was discovered in 1979 in images taken by ''

Voyager 1 ''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin '' Voyager 2'', ''V ...

'', and was named in 1983 after the first wife of

Zeus Zeus or , , ; grc, Δῐός, ''Diós'', label= genitive Boeotian Aeolic and Laconian grc-dor, Δεύς, Deús ; grc, Δέος, ''Déos'', label= genitive el, Δίας, ''Días'' () is the sky and thunder god in ancient Greek relig ...

Metis Metis or Métis may refer to: Ethnic groups * Métis, recognized Indigenous communities in Canada and America whose distinct culture and language emerged after early intermarriage between First Nations peoples and early European settlers, primar ...

. 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 Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...

, 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 Stephen P. Synnott (born 1946) is an American astronomer and Voyager scientist at JPL, and expert in spacecraft optical navigation techniques. He has discovered several natural satellites of outer Solar System planets such as Metis, Puck, Lariss ...

in images taken by the ''

'' probe and was provisionally designated as . In 1983, it was officially named after the mythological

, a

Titaness In Greek mythology, the Titans ( grc, οἱ Τῑτᾶνες, ''hoi Tītânes'', , ''ho Tītân'') were the pre-Olympian gods. According to the ''Theogony'' of Hesiod, they were the twelve children of the primordial parents Uranus (mythology), ...

who was the first wife of

(the Greek equivalent of

). 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. Although the '' Juno'' orbiter, which arrived at Jupiter in 2016, has a camera called JunoCam, it is almost entirely focused on observations of Jupiter itself. During close observations of Jupiter, it may capture some distant images of the innermost moons Metis and Adrastea.

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 There are 82 known moons of Jupiter, not counting a number of moonlets likely shed from the inner moons. All together, they form a satellite system which is called the Jovian system. The most massive of the moons are the four Galilean moons: ...

. 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 Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measur ...

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 moon In astronomy, an inner moon or inner natural satellite is a natural satellite following a prograde, low-inclination orbit inwards of the large satellites of the parent planet. They are generally thought to have been formed ''in situ'' at the sam ...

s. 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 Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Ea ...

(~ 0.06°) relative to the equator of Jupiter. Due to

tidal locking Tidal locking between a pair of co- orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked b ...

, Metis rotates synchronously with its orbital period (about 7 hours), with its longest axis aligned towards Jupiter. Metis lies inside Jupiter's

synchronous orbit A synchronous orbit is an orbit in which an orbiting body (usually a satellite) has a period equal to the average rotational period of the body being orbited (usually a planet), and in the same direction of rotation as that body. Simplified meanin ...

radius (as does Adrastea), and as a result,

tidal force The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...

s 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. Metis is the fastest-moving of Jupiter's moons. It orbits Jupiter at 31.5 km/s. Jupiter casts a shadow on all of Metis for 68 minutes each Metian day.

Relationship with Jupiter's rings

Metis's orbit lies ~1,000 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 sphere The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sph ...

s due to their low density.

Notes

References

Cited sources

* * * * * (discovery) * (naming the moon) * * * *

External links

Metis Profile
b
NASA's Solar System Exploration
{{DEFAULTSORT:Metis (Moon) Moons of Jupiter 19790304 Discoveries by Stephen P. Synnott Moons with a prograde orbit