Metis (moon)
Innermost known moon of Jupiter, discovered by Voyager 1 in 1979.
Metis, designated Jupiter XVI, holds the title of Jupiter’s innermost known moon. First spotted as a mere dot in 1979 by the Voyager 1 probe, it received its official name four years later, honoring the Titaness Metis—the first wife of Zeus and mother of Athena. This name was granted as a special exception to the usual IAU rule that prograde Jovian moons must end in "a." For years, little was known about it until the Galileo spacecraft arrived, imaging nearly its entire surface between 1996 and 2003 and offering the first real clues about its composition.
Physically, Metis is a study in asymmetry. It is tidally locked to Jupiter, and its shape is strongly lopsided—its longest dimension nearly double its shortest, measuring roughly 60 by 40 by 34 kilometers. This makes it the second smallest of Jupiter’s four inner satellites. Its exact mass and composition remain unknown, but its density is likely above 1.5 grams per cubic centimeter, which would put its mass at around 6.2 × 10¹⁶ kilograms or more. The surface is dark and reddish, and while individual craters are too small to resolve, its irregular outline hints at a violent history of collisions. One hemisphere is notably brighter than the other—the leading side is 1.3 times more reflective, probably because faster, more frequent impacts there excavate brighter material, likely ice, from below.
Orbit and rotation are tightly linked. Metis circles Jupiter at a distance of about 128,000 kilometers (1.79 Jupiter radii), well inside the planet’s main ring. Its orbit is nearly circular and almost perfectly aligned with Jupiter’s equator, with an eccentricity of roughly 0.0002 and an inclination of about 0.06 degrees. Because it is tidally locked, it rotates once every seven hours, keeping its longest axis pointed toward Jupiter. Each Metian day, Jupiter’s shadow covers the moon for 68 minutes. Like its companion Adrastea, Metis orbits inside Jupiter’s synchronous orbit radius, meaning tidal forces are slowly dragging it inward. If its density matches that of Amalthea, it lies within the fluid Roche limit—yet it hasn’t broken apart, so it must be outside the rigid Roche limit.
Metis’s relationship with Jupiter’s rings is intimate. Its orbit sits about 1,000 kilometers inside the main ring, within a 500-kilometer-wide gap or notch whose origin remains unclear but is clearly linked to the moon.
Quick Facts
- Pronounced
- ˈ · m · iː · t · ɪ · s
- Adjective
- Metidian, Metidean m · ɛ · ˈ · t · ɪ · d · i · ə · n
- Named After
- Μήτις Mētis
- Discoverer
- S. Synnott
- Discovered
- 4 March 1979
- Orbit Ref
- Evans Porco et al. · 2002Burns Simonelli et al. · 2004
- Eccentricity
- 0.0002
- Inclination
- 0.06° (to Jupiter's equator)
- Satellite Of
- Jupiter
Facts from the source article.
Lore & Background
Metis was discovered in 1979 by Stephen P. Synnott in images taken by the Voyager 1 probe and was provisionally designated 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 prototype for the Roman god Jupiter) and mother of Athena. The name was approved as part of an exemption from the IAU convention, which dictates that prograde moons of Jupiter must receive names ending in 'a'. The photographs taken by Voyager 1 showed Metis only as a dot, so knowledge about it was very limited until the arrival of the Galileo spacecraft, which imaged almost all of its surface and put constraints on its composition by 1998.
Metis is tidally locked to Jupiter, with its longest axis aligned toward Jupiter. Its shape is strongly asymmetrical, with the largest diameter almost twice the smallest. It orbits within the main ring of Jupiter and is thought to be a major contributor of ring material. The surface is dark and appears reddish in color; while image resolution is not enough for individual craters to be identified, its irregular shape is indicative of a heavy collisional history. There is a substantial asymmetry between the leading and trailing hemispheres: the leading hemisphere is 1.3 times brighter than the trailing one, probably caused by higher velocity and frequency of impacts on the leading hemisphere, which excavates bright material (presumably ice) from its interior.
Metis orbits Jupiter at a distance of ~128,000 km within Jupiter's main ring, with very small eccentricity (~0.0002) and inclination (~0.06°) relative to Jupiter's equator. Due to tidal locking, it rotates synchronously with its orbital period of about 7 hours. Jupiter casts a shadow on all of Metis for 68 minutes each Metian day. Metis lies inside Jupiter's synchronous orbit radius (as does Adrastea), and tidal forces slowly cause its orbit to decay. If its density is similar to Amalthea's, its orbit lies within the fluid Roche limit; however, because it has not broken up, it must lie outside its rigid Roche limit.
Reader's Guide
Metis holds significance as the innermost known moon of Jupiter and one of only two moons known to orbit Jupiter in less than the length of Jupiter's day, the other being Adrastea. Its orbit lies within Jupiter's main ring, and it is thought to be a major contributor of ring material, primarily dust ejected from its surface by meteorite impacts. The material is easily lost into space because the satellite's surface lies fairly close to the edge of its Roche sphere due to its low density. Metis orbits within a ~500 km wide gap in the main ring, which is clearly related to the moon, though the origin of this connection has not been established. Its discovery by Voyager 1 in 1979 and subsequent imaging by the Galileo spacecraft between early 1996 and September 2003 allowed its surface to be imaged and constraints on its composition to be made. The moon's irregular shape and dark, reddish surface, along with a leading/trailing hemisphere brightness asymmetry, point to a heavy collisional history. Its orbit is slowly decaying due to tidal forces, and while its density is not known, it is likely higher than 1.5 g/cm³, with an estimated mass of ~6.2×10¹⁶ kg or higher.
Did You Know?
- Metis was discovered in 1979 in images taken by Voyager 1 and was provisionally designated S/1979 J 3.
- The leading hemisphere of Metis is 1.3 times brighter than the trailing hemisphere, likely due to impacts excavating bright material (presumably ice) from its interior.
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