Partially lit moon Io.

Io


Volcanic activity on this Jovian moon is so abundant, it resurfaces the face of Io with new deposits faster than comets, and asteroids can pepper it with large craters.

Facts About Io

Jupiter's moon Io is the most volcanically active world in the solar system, with hundreds of volcanoes, some erupting lava fountains dozens of miles (or kilometers) high.

Io is caught in a tug-of-war between Jupiter's massive gravity and the smaller but precisely timed pulls from two neighboring moons that orbit farther from Jupiter—Europa and Ganymede. Io’s volcanoes are at times so powerful that they are seen with large telescopes on Earth, and Io even has lakes of molten silicate lava on its surface. Only slightly larger than Earth’s Moon, Io is about one-quarter the diameter of Earth itself.

Get the Facts
A rust-colored sphere is shown against a black background. The left half is concealed in shadow, with only a very dim outline visible. The right half is fairly well-lighted, with the surface smooth in some areas and in others covered with splotches and peaks of light tan, or spots and dimples of dark orange or dark grayish brown.
Jupiter's turbulent moon, Io, captured during a close approach by NASA's Juno spacecraft on Dec. 30, 2023.
NASA/JPL-Caltech/SwRI/MSSS

Io by the Numbers

How big is Io? How far is it from the Sun?

Use this tool to compare Io to Earth, and other worlds.

Compare and Analyze
An image of an orangish-brown moon showing craters and other surface detail; the right hemisphere is lit with sunlight reflected from Jupiter.
Jupiter's moon Io, its night side illuminated by reflected sunlight from Jupiter, or "Jupitershine."
NASA/JPL-Caltech/SwRI/MSSS | Image processing by Emma Wälimäki © CC BY

Exploring Io

Most of what we know about Io comes from the Galileo spacecraft, which made a detailed study of the Jupiter system.

It was Voyager 1 that spotted the first signs of the little moon’s powerful volcanism in 1979. Since 2003, three spacecraft – Ulysses, Cassini, New Horizons, and Juno – have made additional close observations of Io. Juno is currently orbiting Jupiter, but its prime mission is focused on the giant planet, not the moons.

Learn More
Artist's rendition of NASA's Galileo at Jupiter.
An illustration of NASA's Galileo spacecraft at Jupiter and one its moons.
NASA

Io Multimedia

Images of Io and more.

Go to Multimedia
A rust-colored sphere is shown against a black background. The left half is concealed in shadow, with only a very dim outline visible. The right half is fairly well-lighted, with the surface smooth in some areas and in others covered with splotches and peaks of light tan, or spots and dimples of dark orange or dark grayish brown.
Jupiter's turbulent moon, Io, was photographed during a close approach by NASA's Juno spacecraft on Dec. 30, 2023.
NASA/JPL-Caltech/SwRI/MSSS

Learn More

  • Orange and yellow moon Io is seen against the darkness of space.

    NASA Photojournal: Io

    Images of Io gathered by NASA spacecraft.

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