Io
What is Io?
Moons· Diameter 3,643 km. The most volcanically active world known. The pull of Jupiter and its neighbouring moons kneads Io like dough, and the heat keeps more than 400 volcanoes active.
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Orbit and size
| Diameter | 3,643 km |
|---|---|
| Parent | Jupiter |
| Volcanoes | 400+ active volcanoes |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Parent planet and sibling moons
Loki, the lava lake that gives off a tenth of Io's heat
Loki Patera, Io's largest volcano, is a lake of lava about 200 km across. This one lake gives off roughly a tenth of all the heat Io radiates. Its surface is a crust of cooled, hardened rock. Every 400 to 600 days that crust starts sinking from one side, exposing fresh, hot lava. The cycle is large enough that telescopes on Earth can see Loki brighten and fade.
Io's lava is hotter than 1,300 °C, hotter than the lava usually seen at Earth's volcanoes. For a long time scientists wondered whether Io's interior was one big ocean of melted rock. In 2023 and 2024 the Juno spacecraft came within 1,500 km of Io and measured its gravity. A study published in December 2024 concluded that Io's mantle is mostly solid and only partly melted.
Io's volcanoes are named after gods of fire, volcanoes and the Sun from many cultures, such as Pele, the Hawaiian volcano goddess, Prometheus from Greek myth and Loki from Norse myth. The volcano Pele shoots a plume 300 km high and has left a red ring more than 1,000 km across around its vent.
The moon in depth
Io is the most volcanically active place in the solar system. It has more than 400 active volcanoes, and because its gravity is weak, material they throw out rises as high as 300 km. Sulfur compounds from the eruptions stain the surface in patches of yellow, orange and black, which earned Io the nickname 'pizza moon'.
Io's heat comes from a tug-of-war with Jupiter. Io orbits in step with Europa and Ganymede, and their regular tugs keep its orbit from becoming a perfect circle. So each time Io moves closer to Jupiter and away again, Jupiter's huge gravity stretches and squeezes it. Just as a wire gets hot when bent back and forth, this flexing heats Io's interior. The heat released is greater than all the heat escaping from inside Earth.
Io's volcanoes were first seen in 1979 in pictures from Voyager 1. Linda Morabito, an engineer working on the spacecraft's navigation, noticed a strange umbrella-shaped cloud at the moon's edge. It was the first active volcano ever confirmed beyond Earth. In the orbit view, Io is the large moon circling closest to Jupiter.
Io also shapes the space around Jupiter. Sulfur and oxygen from its volcanoes become electrically charged particles that Jupiter's magnetic field sweeps up into a doughnut-shaped ring along Io's orbit. A powerful electric current also flows between Io and Jupiter, leaving a bright spot in Jupiter's polar auroras that moves along with Io.
Frequently asked
- How big is Io?
- The diameter of Io is 3,643 km.
- Which planet does Io orbit?
- Io orbits Jupiter.
- In one line, what is Io?
- Moons — The most volcanically active world known.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.