Mean diameter: approximately 3,643 km · Orbits Jupiter.
Io is only slightly wider than Earth’s Moon, yet their surfaces tell very different stories. Jupiter’s gravity and the timed gravitational pulls of neighboring Europa and Ganymede deform Io, producing internal heat.
That tidal heating powers extraordinary volcanic activity. The yellow, orange and dark markings in the Galileo image help distinguish this volcanic world from the ice-dominated moons elsewhere in the countdown.
Sources: NASA moon guide · JPL size data.
Image source: NASA/JPL/University of Arizona; reuse terms. Resized and JPEG-compressed.