The Solar System's volcanic powerhouse
Io
Moon of Jupiter · discovered 1610 by Galileo Galilei
Io is the innermost of Jupiter's four large Galilean moons and the most volcanically active body known in the Solar System. Slightly larger than Earth's Moon, it hosts hundreds of active volcanoes, some throwing plumes of sulfur and gas hundreds of kilometres into space.
Io's fire does not come from leftover heat of formation. It comes from being squeezed: an orbital tug-of-war with Jupiter and its sister moons flexes Io's interior day after day, turning gravity into heat and repainting the whole surface in yellow, orange, red and black.
Key facts
- Diameter
- 3,643 km1.05× Earth's Moon
- Mass
- 8.93 × 10²² kg
- Surface gravity
- 1.796 m/s²0.183 g
- Density
- 3,528 kg/m³
- Distance from Jupiter
- 421,700 km≈ 5.9 Jupiter radii
- Orbital period
- 42.5 hoursSame as its rotation (tidally locked)
- Discovered
- 1610Galileo Galilei
- Surface
- Sulfur frosts, silicate lava flows and volcanic calderas
- Atmosphere
- Thin, patchy sulfur dioxide that partly freezes in eclipse
- Surface temperature
- About −130 °C on average; lava above 1,200 °C
- Notable feature
- Loki Patera, a lava lake about 200 km across
- Named after
- Io, a priestess of Hera loved by Zeus in Greek myth
Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.
Surface and geology
More than 400 active volcanic centres have been identified on Io. The largest, Loki Patera, is a depression roughly 200 kilometres across filled with a lava lake whose crust periodically founders and overturns, causing the volcano to brighten dramatically. Pele produces an umbrella-shaped plume up to about 300 kilometres high and surrounds itself with a red ring of sulfur deposits more than 1,000 kilometres wide.
Io's lava is thought to be mostly silicate rock, some of it hotter than typical basalt on Earth today, while sulfur and sulfur dioxide frosts give the surface its vivid colours. Eruptions bury the landscape so quickly that not a single impact crater has been confirmed.
Between the volcanoes rise isolated mountains, not volcanic cones but tilted blocks of crust pushed up as the surface is loaded by fresh lava. The tallest, in Boösaule Montes, stand around 17 kilometres high, taller than Mount Everest.
Interior and composition
Io is the densest of Jupiter's large moons and is made almost entirely of rock, with an iron or iron-sulfide core and a silicate mantle. Unlike its icy neighbours, it holds essentially no water.
Scientists long debated whether a global ocean of magma lies beneath the crust. Gravity measurements from NASA's Juno spacecraft, published in 2024, suggest there is no shallow global magma ocean; instead, each volcano is likely fed by its own magma chamber within a mostly solid mantle.
Orbit and rotation
Io circles Jupiter every 1.77 days and is tidally locked. It is locked in a 1:2:4 orbital resonance, called the Laplace resonance, with Europa and Ganymede: for every orbit of Ganymede, Europa completes two and Io four. These regular tugs keep Io's orbit slightly elliptical.
As Io's distance from Jupiter varies, the giant planet's gravity flexes its solid surface up and down by as much as about 100 metres. The resulting friction generates enormous internal heat, powering Io's volcanoes. Io also ploughs through Jupiter's magnetic field, feeding a doughnut-shaped torus of charged particles and generating electrical currents that create auroral spots on Jupiter itself.
Origin
Io is thought to have formed, like the other Galilean moons, from a disc of gas and dust around the young Jupiter. Close to the planet, conditions were warmer, which may help explain why Io is rocky while the outer moons are rich in ice. Any water Io once had has probably been lost to heat and volcanic activity over billions of years.
Exploration
Io was discovered by Galileo Galilei in 1610, but its true nature only became clear in the space age. Every spacecraft that has passed through the Jupiter system has added to its story, despite the intense radiation close to the planet.
Voyager 1 and 2 (1979)
Voyager 1 revealed the first active volcanism seen beyond Earth when navigation engineer Linda Morabito spotted a plume on the edge of an image. Voyager 2 confirmed that most plumes were still erupting months later.
Galileo (1995–2003)
NASA's Galileo orbiter made close flybys that mapped Io's hot spots, measured lava temperatures and watched volcanoes change from one encounter to the next.
New Horizons (2007)
On its way to Pluto, New Horizons captured a giant plume rising from the volcano Tvashtar, offering a striking time-lapse view of an eruption.
Juno (2016–present)
Juno's close passes in December 2023 and February 2024, at about 1,500 kilometres, delivered the sharpest views of Io in decades and the gravity data that argue against a shallow magma ocean.
The moons of Jupiter
Surprising facts about Io
- Io's volcanism was predicted in a scientific paper published just days before Voyager 1 arrived in March 1979, based on calculations of tidal heating.
- Io's solid surface rises and falls by up to about 100 metres as it orbits Jupiter, far more than Earth's ocean tides.
- Io sheds roughly a tonne of material every second into space, supplying the giant ring of plasma that encircles Jupiter.
- Its sulfur dioxide atmosphere partly collapses into frost when Io passes into Jupiter's shadow and regenerates when sunlight returns.
- Volcanic resurfacing is so rapid that Io has no confirmed impact craters at all, making it one of the youngest surfaces in the Solar System.
Keep exploring
- Europa, moon of Jupiter: an ocean hidden under ice
- Ganymede, moon of Jupiter: the giant moon with a magnetic heart
- Callisto, moon of Jupiter: a battered relic of the early solar system
- Jupiter: the planet guide
- Watch Io orbit Jupiter in the interactive 3D solar system
- All the major moons of the Solar System