Galilean moon · moon of Jupiter
Io
The most volcanically active world in the Solar System, a sulfur-stained moon of Jupiter kneaded by relentless tidal forces.
- Distance from Earth today
- 820.2 million km5.483 AU · light: 45 min 36 s
- Distance from the Sun today
- 795.1 million km5.315 AU
- As of
- 2 November 2026position from NASA/JPL orbital elements
Where it is today
Top view of the plane of Earth’s orbit, compressed distance scale. Open on the big map with the time slider →
Facts
- Diameter
- 3,643 km
- Mass
- 89,296 × 10¹⁸ kg
- Density
- 3,528 kg/m³
- Mean distance from the planet
- 421,800 km
- Orbital period
- 42.3 h
- Orbital eccentricity
- 0.0040
- Orbital inclination
- 0.0°
Source of the figures: JPL SSD – Planetary Satellites (public domain).
Discovery: 1610 – Galileo Galilei
Description
Io is the innermost of the four large moons Galileo discovered in 1610. Gravitational tugs from Jupiter, combined with an orbital resonance with Europa and Ganymede, flex its interior constantly and generate enough heat to power hundreds of active volcanoes. Its surface, painted yellow, orange, red and white by sulfur compounds, is continually repaved by lava, so impact craters are almost absent. Active volcanism there was first seen in Voyager 1 images in 1979; Galileo later monitored the eruptions for years, New Horizons caught a giant plume in 2007, and Juno made close flybys in 2023 and 2024.