Io is the most volcanically active world in the Solar System - a yellow-orange moon splashed with sulfur, where hundreds of volcanoes erupt lava and launch plumes hundreds of kilometres high. It is the innermost Galilean moon, and Jupiter’s gravity gives it no rest.
Over 400 active volcanoes make it the most volcanically active body known.
Volcanic plumes rise as high as 500 km in the weak gravity and near-vacuum.
Its surface repaves itself so fast that Io has essentially no impact craters.
Tidal heating: gravity as an oven
Io is caught between Jupiter’s enormous pull and the rhythmic tugs of Europa and Ganymede, which keep its orbit slightly eccentric. As its distance from Jupiter varies, the moon’s solid body flexes by tens of metres every orbit. That constant kneading generates friction heat - enough to melt rock and power all its volcanoes without any sunlight at all.
Physical characteristics
Diameter
3,643 km
Mass
8.930e+22 kg
Surface gravity
1.8 m/s²
Density
3.53 g/cm³
Tidally locked
Yes
Mean temperature
-130 °C
Temperature range
-183 °C to 1600 °C
Geometric albedo
0.63
Atmosphere and composition
Atmosphere
A thin, patchy sulfur-dioxide atmosphere, fed by volcanoes and frost.
Composition
Silicate rock around a molten iron-sulfide core; the surface is coated in sulfur and sulfur-dioxide frost.
Discovery
Discovered by
Galileo Galilei
Year
1610
How
one of the first astronomical telescopes
Missions
Voyager 2NASA · Flyby probe · 1977 · active
Launched 16 days before its twin, Voyager 2 is the only spacecraft ever to visit all four giant planets - Jupiter, Saturn, Uranus and Neptune. It entered interstellar space in 2018 and is still returning data.
Voyager 1NASA · Flyby probe · 1977 · active
The most distant human-made object, now roughly 25 billion km from the Sun. After historic flybys of Jupiter, Saturn and Titan, Voyager 1 became the first spacecraft to enter interstellar space in 2012.
GalileoNASA · Orbiter · 1989–2003 · completed
The first spacecraft to orbit an outer planet, Galileo circled Jupiter from 1995 to 2003 and dropped a probe into its atmosphere. In 2003 it was deliberately flown into Jupiter so it could never crash into - and contaminate - Europa’s possible ocean.
JunoNASA · Orbiter · 2011–2025 · completed
Juno looped over Jupiter’s poles from 2016 to 2025, peering beneath the clouds to map the planet’s interior, gravity and magnetic field. Its extended mission added daring close flybys of Ganymede, Europa and volcanic Io.
What is still uncertain
Volcanic vents reach roughly 1600 °C, while the ordinary surface stays near -130 °C; the upper end of the range applies only at active hot spots.