Metis — Jupiter's innermost moon

Physical characteristics

Metis is small and markedly non-spherical. Observations indicate approximate dimensions of about 60 × 40 × 34 km, making it a noticeably elongated and irregular body rather than a rounded world. Because of its small size it does not have sufficient gravity to pull itself into a sphere, a fact often described by calling such bodies non-spherical.

• Estimated size: roughly 60 × 40 × 34 km across.
• Estimated mass: order of 10^16–10^17 kg (estimates depend on assumed density).
• Rotation: tidally locked to Jupiter, showing the same face toward the planet.
• Surface: dark, reddish in appearance and heavily cratered.

Surface, composition and interior

Direct measurements of Metis’s composition are limited. Its low reflectivity and reddish tint resemble the other inner satellites of Jupiter, and its density must be inferred indirectly. By analogy with Amalthea — another inner moon whose bulk density (~0.86 g/cm³) suggests a porous mixture of water ice and darker material — Metis is often assumed to have a low density and significant porosity. Using such analogies produces rough mass estimates on the order of 7 × 10^16 kg, but those figures are model-dependent. High-resolution images show a surface peppered with impact craters and coated by dark material; a representation of these features can be seen in some processed images and mission stills (surface images).

Orbit, rings and dynamical role

Metis orbits very close to Jupiter, inside the planet’s main ring. Its proximity to Jupiter means it lies well within the planet’s strong gravitational and radiation environment. Small moons in this region are important for ring dynamics: impacts by micrometeoroids on their surfaces can eject dust that becomes part of the ring, and tidal forces and orbital resonances shape ring structure. Metis, together with nearby inner satellites such as Adrastea and Amalthea, is therefore thought to be a source and sculptor of material in Jupiter’s inner ring system.

Discovery, name and notable distinctions

Metis was discovered by Stephen P. Synnott from images taken during the Voyager 1 flyby in 1979 and was later named after the Titaness associated with wisdom and counsel. It is notable primarily for its location: no other confirmed Jovian moon orbits closer to the planet. As an inner, irregular satellite it contrasts with the larger, spherical Galilean moons (Io, Europa, Ganymede and Callisto) and with more distant, capture-origin irregular satellites.

Key points

• Voyager 1 provided the discovery images that revealed Metis.
• Its naming follows Jovian mythological conventions; see Titaness Metis and her association with Zeus.
• Metis is small, non-spherical, dark and reddish in hue.
• Comparisons with Amalthea inform plausible density and porosity but do not replace direct measurement.
• Images and surface studies are limited; enhanced pictures and analyses are referenced in archived mission material (surface images).

Because much of what is known about Metis comes from brief spacecraft encounters and telescopic study, several aspects of its interior and exact composition remain uncertain. Future missions or targeted observations could refine estimates of its mass, porosity and the processes by which it supplies dust to Jupiter’s rings.