Why is Io so volcanic?

Because of tides. Io's orbit is kept slightly oval by an orbital resonance with Europa and Ganymede, so Jupiter's gravity constantly squeezes and stretches it. The friction from this flexing heats Io's interior enough to drive continuous eruptions.

How many volcanoes does Io have?

Around 400 active volcanoes have been identified, more than on any other known world. Some, such as Loki Patera, are vast lava lakes; others produce plumes hundreds of kilometres high.

Its surface is coated with sulphur and sulphur compounds from volcanic eruptions. Sulphur dioxide frost forms white patches, some sulphur deposits are red or orange, and fresh lava is dark.

Could people visit Io?

Not safely with current technology. Io lies within Jupiter's intense radiation belts, where an unprotected person would receive a lethal dose within a day, and the surface is about −130°C away from the lava.

Can I explore Io in the app?

Yes. You can land on Io's lava plains near an active plume and explore them in interactive 3D, with Jupiter filling the sky above.

The most volcanic world known, painted in sulphur under a sky filled by Jupiter

Io's lava plains

3D landscape · Io, moon of Jupiter

Land here and explore →

Io is the innermost of the four large moons of Jupiter discovered by Galileo Galilei in 1610, and at about 3,640 km across it is slightly larger than our own Moon. There the resemblance ends. Io has hundreds of active volcanoes, lava lakes, fountains of lava and plumes that rise hundreds of kilometres into space, and its surface is repainted so quickly that no impact craters have ever been found on it. In this app you can land on Io's lava plains and explore them in interactive 3D, near an active plume with Jupiter overhead.

Io's activity is driven not by leftover heat from its formation, as on Earth, but by tides. Its orbit is locked in a rhythm with its neighbours Europa and Ganymede, which keeps it slightly oval, so Jupiter's enormous gravity constantly flexes the moon and heats its interior. The result is a world unlike any other in the Solar System.

Key facts

Diameter: ≈ 3,640 kmSlightly larger than Earth's Moon
Surface gravity: ≈ 1.8 m/s² (0.18 g)A little more than on our Moon
Surface temperature: ≈ −130°C on averageLava at active vents can exceed 1,200°C
Orbit around Jupiter: ≈ 1.8 daysAbout 422,000 km from the planet, similar to the Moon's distance from Earth
Active volcanoes: About 400More than any other known body
Tallest plumes: Up to ≈ 400 km highPele and Tvashtar are among the largest
Atmosphere: Very thin sulphur dioxideAround a billionth of Earth's surface pressure
Jupiter in the sky: ≈ 19–20° acrossNearly 40 times the width of the full Moon from Earth
Water: Essentially noneIo is rock and sulphur, unlike the icy moons further out

Heated by tides

Io, Europa and Ganymede are locked in an orbital resonance: for every orbit Ganymede makes around Jupiter, Europa makes two and Io makes four. The regular tugs between the moons stop Io's orbit from becoming perfectly circular. As Io moves slightly closer to Jupiter and then slightly further away, the planet's gravity stretches and relaxes it, and its solid surface rises and falls by as much as about 100 metres.

This constant flexing produces friction and heat inside Io, much as bending a paperclip back and forth makes it warm. The heat flowing out of Io is many times greater, for its size, than the heat escaping from Earth. Remarkably, Io's volcanism was predicted in a scientific paper published in March 1979, just days before Voyager 1 flew past and confirmed it.

Volcanoes, lava lakes and plumes

Io has around 400 active volcanoes. Many are paterae, irregular volcanic depressions rather than tall cones, often floored with dark, fresh lava. Its lavas are thought to be mostly silicate rock similar to Earth's basalts, and some eruptions appear to be hotter than any seen on Earth today, possibly resembling the very hot lavas that erupted early in our planet's history.

Loki Patera is the most powerful volcano on Io: a depression about 200 km across, thought to hold a vast lava lake whose cooled crust periodically founders and sinks, renewing the surface in a wave that sweeps across the lake. It brightens and fades on a cycle of roughly 400–500 days and can be watched from telescopes on Earth.

Io's plumes are its most dramatic sight. Gas and fine particles of sulphur and sulphur dioxide shoot from vents and arc high above the surface before falling back, forming umbrella shapes. Pele's plume reaches roughly 300–400 km and leaves a huge red ring of sulphur deposits around it, over 1,000 km wide. Tvashtar's plume, photographed by New Horizons in 2007, rose about 330 km.

Mountains without volcanoes

Io's tallest mountains are not volcanoes. Peaks such as Boösaule Montes, around 17 km high, are blocks of crust pushed up as the surface is buried by eruptions and compressed from above.

Sulphur colours

Io is often compared to a pizza because of its patchwork colours. Much of the surface is yellow, coloured by sulphur and sulphur compounds. Whitish areas are frost of sulphur dioxide, which condenses from the plumes and the thin atmosphere. Red and orange deposits, like the ring around Pele, are thought to be short-chain forms of sulphur that slowly change colour over time, while the darkest patches are fresh silicate lava.

Because eruptions are constant, Io's surface keeps changing. Comparing images taken years apart, scientists have seen new lava flows, fresh plume deposits and whole regions change colour. Io is thought to resurface itself, on average, by about a centimetre a year, enough to bury any impact crater quickly.

Thin air and intense radiation

Io's atmosphere is mostly sulphur dioxide and extremely thin, around a billionth of the pressure at Earth's surface. It is patchy and constantly supplied by volcanoes and by frost turning to gas in sunlight. When Io passes into Jupiter's shadow, which happens every orbit, the temperature drops and much of the atmosphere freezes onto the surface, then returns when sunlight comes back.

Io orbits deep within Jupiter's magnetic field, in one of the harshest radiation environments in the Solar System. Material escaping from Io is ionised and spread around Jupiter in a doughnut of charged particles called the Io plasma torus, and electric currents flow between Io and Jupiter strongly enough to create bright spots in Jupiter's auroras. An unprotected person on Io's surface would receive a lethal dose of radiation within a day, and spacecraft that pass close must be shielded and limit their exposure.

Jupiter overhead

Like the Moon with Earth, Io is tidally locked to Jupiter, so one hemisphere always faces the planet. From there Jupiter hangs fixed in the black sky, about 19–20° across, nearly 40 times as wide as the full Moon appears from Earth. You could cover the full Moon with a fingertip held at arm's length, but you would need both hands to cover Jupiter from Io.

Over Io's 42-hour orbit, Jupiter goes through phases as the Sun moves around the sky, and once every orbit the Sun passes behind the planet, causing an eclipse lasting around two hours. Europa, Ganymede and Callisto would appear as small discs or bright points, and the Sun would look about a fifth of the size it does from Earth.

From Voyager to Juno

Voyager 1 flew past Jupiter in March 1979, and while studying its images the navigation engineer Linda Morabito noticed a plume rising from Io's edge: the first active volcano seen beyond Earth. Voyager 2 followed a few months later. NASA's Galileo spacecraft, which orbited Jupiter from 1995 to 2003, made several close flybys of Io, watching eruptions come and go and measuring very hot lava temperatures.

New Horizons, on its way to Pluto, imaged Io in 2007, capturing the Tvashtar plume. NASA's Juno spacecraft, orbiting Jupiter since 2016, made its closest passes of Io on 30 December 2023 and 3 February 2024, coming within about 1,500 km of the surface and returning the most detailed images of Io's volcanoes in more than two decades, including views of a lava lake with an island in the middle.

Surprising facts about Io's lava plains

Io's volcanism was predicted in print just days before Voyager 1 arrived and confirmed it in 1979.
Jupiter's tides make Io's solid rocky surface rise and fall by up to about 100 metres.
No impact craters have been found on Io: eruptions bury them too quickly.
Io's thin atmosphere partly freezes onto the ground every time the moon passes into Jupiter's shadow.
Io's tallest mountains, up to about 17 km high, are taller than Mount Everest but are not volcanoes.
From Io's Jupiter-facing side, the planet appears nearly 40 times wider than the full Moon does from Earth.

Frequently asked questions

Why is Io so volcanic?: Because of tides. Io's orbit is kept slightly oval by an orbital resonance with Europa and Ganymede, so Jupiter's gravity constantly squeezes and stretches it. The friction from this flexing heats Io's interior enough to drive continuous eruptions.
How many volcanoes does Io have?: Around 400 active volcanoes have been identified, more than on any other known world. Some, such as Loki Patera, are vast lava lakes; others produce plumes hundreds of kilometres high.
Why is Io yellow?: Its surface is coated with sulphur and sulphur compounds from volcanic eruptions. Sulphur dioxide frost forms white patches, some sulphur deposits are red or orange, and fresh lava is dark.
Could people visit Io?: Not safely with current technology. Io lies within Jupiter's intense radiation belts, where an unprotected person would receive a lethal dose within a day, and the surface is about −130°C away from the lava.
Can I explore Io in the app?: Yes. You can land on Io's lava plains near an active plume and explore them in interactive 3D, with Jupiter filling the sky above.

Keep exploring

The landscapes in Ember’s Universe are procedural 3D scenes modelled on real places. The landforms, plants and light follow the real thing, but the details are generated, not mapped.

Sources