Io size and scale context

Io is Jupiter's third-largest moon and the most volcanically active world in the Solar System. Tidal flexing from Jupiter, Europa, and Ganymede keeps its interior hot enough to renew the surface with lava flows and sulfur-rich deposits. Read its scale context, comparisons, and journey links on the canonical object page.

MoonRocky moon·3,470 km

IoGalilean moon·3,643 km

CallistoGalilean moon·4,821 km

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Moons

Moon

The Moon is Earth's natural satellite and our clearest visual anchor on planetary scale. It is still drifting away from Earth by about 3.8 centimeters each year, a slow tidal migration that has been reshaping the Earth-Moon system for billions of years.

Scale fact3,470 kmdiameter

Object class: Rocky moon
Mass: ~0.012 Earth masses
Host: Earth
Estimated age: ~4.5 billion years
Composition: Silicate rock

Moons

Io

Scale fact3,643 kmdiameter

Object class: Galilean moon
Mass: ~0.015 Earth masses
Host: Jupiter
Estimated age: ~4.5 billion years
Composition: Silicate rock with sulfur-rich surface deposits

Moons

Callisto

Callisto is Jupiter's second-largest moon and the outermost of the four Galilean moons. Its ancient, heavily cratered surface records a quieter history than Io, Europa, and Ganymede, while Galileo spacecraft data suggest a possible salty ocean deep below the ice.

Scale fact4,821 kmdiameter

Object class: Galilean moon
Mass: ~0.018 Earth masses
Host: Jupiter
Estimated age: ~4.5 billion years
Composition: Rock and water ice