Moons · Space Codex
Io
Squeezed by Jupiter and tugged by Europa and Ganymede, Io is heated from within until it erupts constantly.
The most volcanic world known
At a glance
Key points
Hundreds of volcanoes resurface it so fast that no impact crater has ever been found.
- Hundreds of active volcanoes; no impact craters
- Loki Patera alone may emit about 15% of Io's heat
- Its volcanic gas feeds a plasma torus around Jupiter
Interior
Io is a rocky world with an iron core, the only Galilean moon without much water. Its orbit is kept slightly oval by the resonance with Europa and Ganymede, so Jupiter's tides flex it by tens of metres each orbit, and that friction becomes heat. Juno and Galileo gravity data published in 2024 rule out a shallow global magma ocean: the heat escapes through a mostly solid mantle laced with molten rock.
Surface
Sulfur and sulfur dioxide frost paint the surface yellow, orange, white and red, while fresh silicate lava cools almost black. Loki Patera, a lava lake about 200 km wide, may release around 15% of the moon's total heat on its own. Plumes from vents such as Pele rise hundreds of kilometres before raining back as rings of red deposit.
Link to Jupiter
Volcanic gas escapes into space and is swept up by Jupiter's magnetic field, forming a doughnut of charged particles, the Io plasma torus, that circles the planet along Io's orbit and powers bright auroral spots in Jupiter's atmosphere.
What we still don't know
Exactly where the tidal heat is deposited in the interior, and how long Io has been this active.
Missions
- 1977–
Jupiter 1979, Saturn 1980; interstellar space since 25 Aug 2012.
- 1989–2003
Galileo
NASACompletedFirst Jupiter orbiter (1995–2003) and first probe into a giant planet's atmosphere.
- 2011–
In Jupiter orbit since Jul 2016 on an extended mission. NASA's status page is inconsistent; perijoves are planned into 2027.
First-hand · our 3D model
How we render it
Io is a sphere wrapped in a global map built from USGS and NASA spacecraft mosaics. At full quality the volcano Pele throws an umbrella-shaped plume, drawn as an additive cone that strengthens when backlit.
It is drawn at about 8.1% of Jupiter's radius, enlarged from its true 2.6% so it stays visible. It orbits almost exactly in Jupiter's equatorial plane, with a 1.76-day period kept in true ratio to its neighbours, and keeps one face towards Jupiter.
Moon sizes and orbits are compressed so a whole system fits on screen, but never reordered: each moon is drawn at 0.72 × (true size ratio)^0.6 of its planet, capped so no large moon is drawn bigger than it really is, and orbit distances are log-compressed into a band that clears the planet's rings. Orbital periods keep their true ratios, every moon is tidally locked, and on lighter devices only the two largest moons of each planet are drawn.
Related entries
Sources
- Park et al. 2024, Nature (Caltech record) (opens in a new tab)
- JPL Solar System Dynamics: satellite physical parameters (opens in a new tab)
- JPL Solar System Dynamics: satellite mean orbital elements (opens in a new tab)
- NASA NSSDC Jovian Satellite Fact Sheet (opens in a new tab)
- NASA Science: Europa facts (opens in a new tab)
- NASA Science: Io (opens in a new tab)
- NASA NTRS: Loki Patera (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.