Io
What is Io?
Moons· Diameter 3,643 km. The most volcanically active world we know — tidal kneading by Jupiter keeps 400+ sulfur volcanoes erupting.
📍 Visible from where you are right now — computed in your browser
bearing · altitude · rise time · best time tonight
The orbit of Io at this moment: https://nowsky.org/solar/orbit?fw=moon:jupiter:io&sp=0.012 — replayed live, no login.
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
Frequently asked
- How big is Io?
- 3,643 km.
- Which planet does Io orbit?
- Jupiter.
- In one line, what is Io?
- Moons — The most volcanically active world we know — tidal kneading by Jupiter keeps 400+ sulfur volcanoes erupting.
Loki, the lava lake that warms a whole moon
Loki Patera, Io's largest volcano, is a lava lake 200 km across that alone produces a tenth of all the heat Io radiates. Its surface is a crust of cooled rock, and a wave in which that crust founders on one side to expose fresh lava sweeps around the lake roughly every 500 days — a cycle large enough to be tracked as Loki brightens and dims in ground-based telescopes.
Io's eruptions exceed 1,300 °C, hotter than Earth's basaltic lavas, and are thought to resemble the magma oceans of the early Earth. In 2023–2024 the Juno spacecraft came within 1,500 km and photographed a new lava lake and volcanic plumes near the south pole; whether Io's interior is a wholly molten magma ocean or only partly melted is still debated. Of the moons Galileo saw four centuries ago, Io is the living world we have seen closest.
The moon in depth
Io is the most volcanic world we know: 400-plus active volcanoes venting sulfur, their plumes rising 300 km in the feeble gravity. Its mottled yellow-orange-black surface earned it the nickname 'pizza moon'.
The heat comes from a tug-of-war: Jupiter's grip versus the 1:2:4 resonance with Europa and Ganymede kneads Io endlessly, generating tens of times Earth's total geothermal output from tidal flexing alone — volcanoes fueled by orbital mechanics, not radioactivity.
In 1979 navigation engineer Linda Morabito spotted a strange umbrella shape on a Voyager 1 frame — the first active volcano ever seen beyond Earth. In the Orbit view, Io is the moon hugging Jupiter closest.
Io also powers the space around Jupiter: volcanic sulfur ions ride the magnetic field into a donut-shaped plasma torus, and a flux tube carrying millions of amperes links moon to planet, stamping Io's own 'footprint aurora' on Jupiter's poles. Nowhere else does a moon sign its name in a planet's auroras.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.