Hugo's Solar System
Moon of Jupiter
Io
Jupiter's volcano moon, where hundreds of fiery mountains never stop erupting
Key facts
- Diameter
- About 2,260 miles / 3,640 km (mean radius 1,821.49 km), slightly larger than our Moon
- Distance from Jupiter
- About 262,000 miles / 422,000 km on average
- One orbit
- About 1.77 Earth days
- One day (rotation)
- About 1.77 Earth days, the same as its orbit, so one face always points at Jupiter
- Discovered
- 8 January 1610 by Galileo Galilei
- Named after
- Io, a priestess of Hera loved by Zeus in Greek myth
- Surface
- Rocky, with over 400 active volcanoes, lava lakes and sulphur frosts
- Surface temperature
- About -163 °C on average; hot spots above 1,000 °C
- Atmosphere
- Very thin, about 90 per cent sulphur dioxide
- Visited by
- Pioneer 10 and 11, Voyager 1 and 2, Galileo, Cassini, New Horizons, Juno
- Parent planet's known moons
- Jupiter has 115 known moons as of August 2026
About Io
Io is a moon of Jupiter. It is a little bigger than our Moon. Io is covered in volcanoes. Hundreds of them are erupting right now. They shoot hot lava high into the sky. Some lava lakes glow red and orange. Io is yellow, orange and black. It looks a bit like a pizza. Jupiter's pull squeezes Io all the time. That squeezing makes Io hot inside. Io goes round Jupiter in under two days. A spacecraft called Voyager 1 saw a volcano erupt. That was a long time ago. Io is the most fiery moon we know.
Io is the closest of Jupiter's four big moons. It is a rocky sphere about 3,640 km across, slightly larger than our Moon. Io orbits Jupiter at a distance of about 422,000 km. One orbit takes just 1.77 days.
Io rotates once in the same time, so it always keeps the same face towards Jupiter. Io is the most volcanically active world in the solar system. It has more than 400 active volcanoes. Some blast plumes of gas and dust up to 500 km high.
There are lakes of molten rock, and one called Loki Patera is about 202 km wide. Why is a small moon so hot inside? Io is caught in a tug-of-war between Jupiter's gravity and the pulls of its neighbours, Europa and Ganymede. Their tugs stretch and squash Io, and the rubbing inside heats it like a bent paperclip.
Io's thin atmosphere is mostly sulphur dioxide, which paints the surface yellow, orange and black. Voyager 1 spotted the first erupting volcano in 1979. Later, Galileo, New Horizons and Juno studied Io too. Juno flew within about 1,500 km of the surface in December 2023 and again in February 2024.
Io is the innermost of the four Galilean moons, discovered by Galileo Galilei on 8 January 1610 (Simon Marius made a rival claim and later supplied the name, following Kepler's suggestion that Jupiter's moons be named after Zeus's lovers). It is named after a priestess of Hera in Greek myth. JPL gives Io a mean radius of 1,821.49 km, a diameter of about 3,640 km, and a density of 3.53 g/cm³, so it is a rocky, iron-cored body slightly larger than our Moon and about a quarter the diameter of Earth. It orbits Jupiter at a semi-major axis of about 422,000 km with a period of about 1.77 days, and rotates synchronously.
Io is the most volcanically active body in the solar system, with over 400 active volcanoes and plumes that can reach 500 km above the surface. The energy source is tidal heating: Io is locked in a 4:2:1 orbital resonance with Europa and Ganymede, which keeps its orbit slightly eccentric (about 0.004). Jupiter's gravity therefore flexes Io continuously and friction inside melts the mantle. Loki Patera, a lava lake about 202 km across, alone contributes roughly a quarter of Io's global heat output. Mean surface temperature is about 110 K (-163 °C), but hot spots reach 1,300 K and occasionally 1,600 K. A common misconception is that Io's colours come from sulphur alone; sulphur and sulphur dioxide frosts paint the surface, but the lavas themselves are silicate rock. The thin atmosphere is about 90 per cent sulphur dioxide, and material escaping from Io feeds a plasma torus around Jupiter.
Pioneer 10 and 11 passed through the system in the 1970s. Voyager 1 discovered active volcanism in March 1979, in an image taken on 8 March, three days after its closest approach; it was the first seen beyond Earth. Galileo, in orbit around Jupiter until 2003, made the most detailed study, and Ulysses, Cassini and New Horizons (28 February 2007) observed Io from a distance. Juno flew within about 1,500 km on 30 December 2023 and 3 February 2024. Its final approved mission extension ran through September 2025. ESA's JUICE and NASA's Europa Clipper are expected to observe Io from afar in the coming decade.
Did you know?
- Io's volcanic plumes can climb about 500 km high, more than fifty times the height of Mount Everest.
- Voyager 1 spotted an erupting volcano on Io in March 1979, the first ever seen beyond Earth.
- Io's surface is repainted so fast by lava and ash that almost no impact craters survive on it.
Neighbours around Jupiter
Questions children ask
Why does Io have so many volcanoes?
Jupiter's enormous gravity pulls on Io, while the moons Europa and Ganymede tug it from the other side. Io is stretched and squeezed on every orbit, and the rubbing inside heats it up until rock melts and bursts out as volcanoes.
Is Io hot or cold?
Both. Most of the surface is bitterly cold, about -163 °C, because Io is so far from the Sun. But around the volcanoes and lava lakes the ground can be hotter than 1,000 °C, hot enough to glow.
Why is Io yellow and orange?
The volcanoes puff out sulphur and sulphur dioxide gas. It freezes onto the ground as yellow, orange, white and black frosts and dust. That is why Io is sometimes said to look like a pizza.
Could astronauts ever land on Io?
Not safely with today's technology. Io sits inside Jupiter's fierce radiation belts, has almost no air, and its surface is constantly changing. Spacecraft such as Juno only make quick flybys rather than staying close.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- What makes Io the most volcanically active world in the solar system?
- Which spacecraft first saw a volcano erupting on Io?
- How long does Io take to orbit Jupiter?
Europa
The Moon
Enceladus