Io

Io is Jupiter’s innermost large moon and the most volcanically active world known, with eruptions powered largely by tidal heating.

Tidal heating: Internal heating produced when repeated gravitational flexing dissipates energy as heat. Jupiter and neighboring moons continually flex Io, supplying energy for its volcanism.
Loki Patera: A large volcanic depression on Io known for powerful, recurring thermal brightenings. Its changing heat output makes it one of Io’s most closely monitored volcanic features.
Voyager 1: A NASA spacecraft launched in 1977 that explored the outer Solar System and entered interstellar space. Its 1979 encounter revealed active volcanism on Io, overturning expectations of a geologically quiet moon.
Io plasma torus: A ring-shaped cloud of electrically charged particles orbiting Jupiter along Io’s path. Ionized gas from Io accumulates here, making the moon a major source of plasma.
Europa: A moon of Jupiter whose icy crust covers a likely global subsurface ocean. Europa shares tidal heating with Io but uses much of that energy beneath an ice shell.
Orbital resonance: A configuration in which orbiting bodies exert regular gravitational influences because their orbital periods have a simple ratio. The Laplace resonance sustains Io’s orbital eccentricity and strengthens tidal heating.
Pele (volcano): An active volcanic center on Io associated with a vast, persistent sulfur dioxide plume. Pele’s plume and dark deposits exemplify Io’s largest-scale volcanic activity.
Galileo (spacecraft): A NASA spacecraft that orbited Jupiter from 1995 to 2003 and studied its moons and atmosphere. Galileo repeatedly observed Io’s eruptions, surface changes, and interaction with Jupiter’s environment.
Io flux tube: A magnetic structure linking Io to Jupiter and carrying electrical currents between them. The moon’s motion through Jupiter’s magnetic field drives currents along this connection.
Enceladus: A small icy moon of Saturn that vents water-rich material from fractures near its south pole. Enceladus channels internal heat into icy plumes rather than Io’s silicate and sulfur volcanism.

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