IntroductionJupiter'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. Explore Io ›

Ten Things to Know About Io

10 Need-to-Know Things About Io

Global image of Io (true color)


Small, Not Tiny

Io is only slightly larger than Earth’s moon and about one-quarter the diameter of Earth itself.


Out There

Io orbits the planet Jupiter, which itself orbits the Sun at a distance of 484 million miles (778 million kilometers).


Gravitationally Tamed

Tidal Lock: Over 1.8 Earth days, Io rotates once on its axis and completes one orbit of Jupiter, causing the same side of Io to always face Jupiter.



The moon Io is the most volcanically active world in the solar system. Io even has lakes of molten silicate lava on its surface.


Only Wisps

Io’s very thin atmosphere is primarily sulfur dioxide, which on Earth is sometimes used to preserve dried food.


Moon Without Moons

Io has no known moons of its own, but it is possible for moons to have moons.



Io has no known rings, but it does create a gaseous torus of material along its orbit around Jupiter.


Exploring Io

Spacecraft have studied Io on flybys (Voyager and Cassini) or orbiting Jupiter (Galileo). Most recently, New Horizons observed Io en route to Pluto.


Ingredients for life?

Io almost certainly could not support life as we know it. But that’s not to say it couldn’t harbor some form of life as we don’t know it.


One Cool Fact

Io’s volcanoes are at times so powerful that they are seen with large telescopes on Earth.

Jupiter and Io

Pop Culture

Pop Culture

We’re still getting to know Io, though it’s powerful volcanoes have captured imaginations since their discovery decades ago. Io plays a memorable role in the sequel to 2001: A Space Odyssey2010—in which astronauts make a dangerous spacewalk above Io’s volcanoes to board an abandoned spacecraft.



NASA SpacePlace: Io
NASA Planetary Photojournal: Io

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