Moon of Jupiter · Updated October 2026
Io.
Io is the innermost of Jupiter's four Galilean moons and the most volcanically active body in the solar system. With a diameter of 3,643 km, slightly larger than Earth's Moon, Io has hundreds of active volcanoes, plumes that rise hundreds of kilometres above the surface, and lava lakes. The relentless activity is driven by tidal heating from Jupiter and the orbital resonance with Europa and Ganymede.
View Jupiter system in 3D →Key facts
Io completes one full orbit of Jupiter in just 1.77 days, more than nine laps for every one of Callisto's. Speed the viewer's clock up and you can watch Io lap a sluggish Callisto again and again.
About Io
Io's yellow, orange, and red surface comes from sulphur and sulphur dioxide compounds erupted by its volcanoes. Loki Patera is a lava lake about 200 km long, rimmed with molten lava and dotted with islands, which NASA's Juno imaged in close flybys in 2023 and 2024. Io has almost no impact craters because its surface is constantly resurfaced by lava. Tidal flexing from Jupiter heats the interior to hundreds of degrees; Io is in a 1:2:4 mean-motion resonance with Europa and Ganymede that maintains its eccentric orbit.
How to view Io in 3D
Io orbits Jupiter in real time inside the interactive viewer. Open the parent body to see the orbital geometry, or use the object browser to fly directly to the moon and observe its rotation, surface, and orbit.
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Sources & methodology
Numbers cross-referenced with the sources below; updated October 2026.