Io is the innermost large moon of Jupiter and the most volcanically active world known, its surface painted yellow, orange and white by sulphur.
At a glance
| radius | 1,820 km (0.286 × Earth) |
|---|---|
| distance from its planet | 422,000 km |
| one orbit | 1.77 days |
| reflects | 63% of the sunlight it gets |
Read off the piece’s own catalogue, the same values it draws with.
About Io
Io is a rocky moon a little larger than our own, and it is never still. Hundreds of volcanoes erupt across it, some throwing plumes of sulphur and sulphur dioxide high above the surface, and its lava lakes and fresh flows constantly repave it, so that hardly any impact craters survive. The colours of its surface come from sulphur compounds frozen at different temperatures.
The heat comes from tides. Io is locked in a rhythm of orbits with Europa and Ganymede that keeps its path slightly elongated, so Jupiter’s gravity squeezes and relaxes it over every orbit, flexing the interior and melting rock. This tidal heating was predicted shortly before the Voyager images showed the volcanic plumes, and later missions including Galileo and Juno watched the eruptions change.
Gas escaping from Io feeds a ring of charged particles around Jupiter, the Io plasma torus, and its motion through Jupiter’s magnetic field drives electric currents that leave a glowing footprint in Jupiter’s aurora.
What you see in the piece
Io is lit only by the sunlight reflected from its bright sulphur surface. Fly close to Jupiter and it is the innermost of the four large moons, circling quickly; from another star it cannot be seen.
Questions
Why does Io have so many volcanoes?
Tidal heating. Io’s orbital resonance with Europa and Ganymede keeps its orbit slightly eccentric, so Jupiter’s gravity constantly flexes its interior, generating enough heat to melt rock.
How long does Io take to orbit Jupiter?
Io goes round Jupiter once every 1.77 days, at a distance of 422,000 km.