Metis (moon)
Metis, the innermost known moon of Jupiter, appeared in history as nothing more than a tiny dot. On the 4th of March 1979, astronomer Stephen P. Synnott spotted it in photographs captured by the Voyager 1 spacecraft. It was so small, so faint, that for years it resisted real scrutiny. What exactly was this speck orbiting closer to Jupiter than any other known moon? What is it made of? Why does it appear brighter on one side than the other? And what role, if any, does it play in shaping the enormous ring system that encircles the giant planet? The answers, when they arrived, turned out to be stranger and more consequential than its modest size suggests.
Stephen P. Synnott identified Metis from Voyager 1 images in 1979, giving the moon a provisional designation before it had a proper name. Four years later, in 1983, it was officially named after Metis, a Titaness from Greek mythology. That figure carried a particular resonance: Metis was the first wife of Zeus, the Greek prototype for the Roman god Jupiter, and the mother of Athena. The choice connected the moon to the same mythological lineage as the planet it circles.
Voyager 1's photographs showed Metis only as a dot. That was the full extent of human knowledge about it for roughly two decades. The Galileo spacecraft changed that. Between early 1996 and September 2003, Galileo made additional observations and imaged almost all of the moon's surface. By 1998, the mission had also begun placing constraints on Metis's composition. A particularly detailed image was captured on the 4th of January 2000. The transition from a featureless point of light to a mapped, analyzed world happened within a single spacecraft generation.
Metis has an irregular shape, and its dimensions are strikingly uneven. The largest diameter is almost twice the size of the smallest. That asymmetry makes it the second smallest of Jupiter's four inner satellites.
Its surface is heavily cratered, dark, and reddish in color. One of the more unexpected findings involves the contrast between its two hemispheres. The leading hemisphere, the side that faces forward as Metis travels along its orbit, is 1.3 times brighter than the trailing hemisphere. Scientists think this difference comes from the physics of impact. The leading face absorbs hits at higher velocity and higher frequency than the trailing face. Those collisions excavate material from the interior, material thought to be ice, leaving a brighter surface behind.
The mass of Metis is not known precisely, but estimates put it at roughly 6.4 times ten to the power of 16 kilograms. That figure assumes the mean density exceeds 1.5 grams per cubic centimeter, which is itself the minimum needed to ensure surface gravity always pulls objects inward regardless of where they stand.
Metis orbits Jupiter at a distance of roughly 128,000 kilometers, or about 1.79 Jupiter radii. One complete trip around the planet takes approximately 7 hours. Metis is tidally locked, meaning it rotates once per orbit, keeping its longest axis permanently pointed toward Jupiter. The planet casts a shadow across all of Metis for 68 minutes out of every Metian day.
The orbit is nearly circular: eccentricity of roughly 0.0002 and inclination relative to Jupiter's equator of about 0.06 degrees. Those are vanishingly small departures from a perfect, flat circle.
Metis is one of only two moons known to complete an orbit of Jupiter in less time than it takes Jupiter itself to rotate once. The other is Adrastea. Both orbit inside Jupiter's synchronous orbit radius, the threshold at which orbital speed matches the planet's rotation. That placement has a long-term consequence. Tidal forces are slowly pulling Metis inward, and its orbit is decaying.
Tidal forces do more than drag Metis closer to Jupiter over time. They raise a deeper question about the moon's survival. If Metis's density resembles that of Amalthea, another of Jupiter's inner moons, then Metis orbits within what is known as the fluid Roche limit. That is the distance inside which tidal forces are strong enough to pull apart a body held together only by its own gravity, as if it were a liquid.
Yet Metis has not broken apart. The explanation lies in a distinction between two kinds of structural integrity. A fluid body and a rigid body respond to tidal stress differently. Scientists conclude that while Metis may sit within its fluid Roche limit, it must lie outside its rigid Roche limit. Its internal strength, its rockiness, is holding it together against forces that would shred a looser aggregate. Surviving inside a zone that should destroy it makes Metis one of the more structurally interesting small bodies in the solar system.
Metis orbits roughly 1,000 kilometers inside the main ring of Jupiter. Within that ring, it travels through a gap or notch approximately 500 kilometers wide. The gap is clearly related to the moon's presence, though the exact mechanism connecting the two has not been established.
What is better understood is the moon's role as a source of ring material. Metis supplies a significant portion of the dust that makes up Jupiter's main ring. The material comes from meteorite impacts on the moon's surface. Because Metis and the other small inner satellites have low densities, the outer edges of their Roche spheres, the zones within which their gravity can hold onto material, lie close to their surfaces. Ejecta from impacts can therefore escape into space relatively easily, and that escaped debris feeds the ring.
The New Horizons spacecraft photographed Metis orbiting at the edge of Jupiter's main ring on the 24th of February 2007, adding another observational milestone to a moon that was once just a dot in a Voyager photograph. Its orbit continues to shrink, and the question of how long Metis will remain a coherent body remains open.
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Common questions
Who discovered Metis and when?
Metis was discovered in 1979 by Stephen P. Synnott using images taken by the Voyager 1 spacecraft. It received a provisional designation at the time and was not officially named until 1983.
Why is Metis named after a figure from Greek mythology?
In Greek mythology, Metis was a Titaness and the first wife of Zeus, who served as the Greek prototype for the Roman god Jupiter. She was also the mother of Athena. The name connects the moon to the same mythological tradition as the planet it orbits.
How long does it take Metis to orbit Jupiter?
Metis completes one orbit of Jupiter in approximately 7 hours. That is faster than Jupiter itself rotates, making Metis one of only two known moons to orbit the planet in less time than a Jovian day. The other is Adrastea.
Why is one side of Metis brighter than the other?
The leading hemisphere of Metis is about 1.3 times brighter than the trailing hemisphere. Scientists believe this is because the leading face experiences more frequent and higher-velocity meteorite impacts, which excavate bright material, thought to be ice, from the interior.
Is Metis in danger of being torn apart by Jupiter's gravity?
Metis may orbit within its fluid Roche limit, the distance inside which tidal forces could disintegrate a body held together only by self-gravity. Because it has not broken apart, scientists conclude it must lie outside its rigid Roche limit, meaning its internal solid strength is holding it together.
What role does Metis play in Jupiter's ring system?
Metis is a major contributor of dust to Jupiter's main ring. Meteorite impacts on its surface eject material that easily escapes into space, because the moon's low density means its gravitational sphere of influence barely extends beyond its surface. That escaped debris feeds the ring. Metis orbits within a roughly 500-kilometer-wide gap in the ring, though the exact connection between the moon and the gap remains unexplained.
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3 references cited across the entry
- 1bookA Practical Dictionary of the English LanguageNoah Webster — 188
- 2webThebe - NASA Science2017-11-30