Io

Moon· orbits Jupiter

Io is the innermost Galilean moon and the most volcanically active world in the Solar System, repaved by hundreds of erupting volcanoes powered by tidal heating from Jupiter.

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Key facts

Mass

8.93 × 10²² kg

0.015 M⊕

Radius

1,821 km

1.05× wider than our Moon

Surface gravity

1.8 m/s²

18% of Earth's gravity

Day length

42.31 hours

Orbit around Jupiter

1.76 Earth days

Science

Physical

Mass: 8.93 × 10²² kg · 0.015 M⊕; JPL / IAU mean radii; spacecraft-tracked GM where measured
Radius: 1,821 km · 1.05× wider than our Moon; JPL SSD Planetary Satellite Physical Parameters (mean radius)
Density: 3,528 kg/m³ · 3.5× water; Computed from mass and radius
Surface gravity: 1.8 m/s² · 18% of Earth's gravity; Computed from mass and radius
Escape velocity: 2.56 km/s · 23% of Earth's escape velocity; Computed from mass and radius

Orbit around Jupiter

Distance from Jupiter: 421,800 km; From JPL orbital elements
Orbital period: 42.31 hours; From JPL orbital elements
Eccentricity: 0.004; From JPL orbital elements
Inclination to Jupiter's Laplace plane: 0.00 °; From JPL orbital elements · The Laplace plane is the one a moon's orbit precesses about: close in it is the planet's equatorial plane, the plane of the rings, and further out it tilts toward the planet's own orbit.

Rotation

Rotation period: 42.31 hours
Tidal locking: Tidally locked to its parent

Atmosphere

Composition: Tenuous sulfur dioxide (SO₂)
Surface pressure: 1.0 × 10⁻4 Pa

Temperature

Mean: 110 K (-163 °C)
Maximum: 130 K (-143 °C)

Composition

Bulk composition: A silicate-rock body with a partly molten interior and an iron core; its surface is coated in sulfur and sulfur-dioxide frosts, continually resurfaced by volcanism.

Radiation & temperature

Insolation: 0.0354 S⊕ · 4% of the sunlight Earth receives; Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
Bond albedo: 0.52 · reflects 52% of incoming sunlight
Equilibrium temperature: 100 K (-173 °C); Computed from albedo + distance from the Sun (airless black body)
Measured mean temperature: 110 K (-163 °C)
Greenhouse warming: +10 K; Measured temperature minus the airless equilibrium — the greenhouse effect

Discovery & history

Discovered: January 8, 1610
By: Galileo Galilei
Method: Visual discovery (telescopic)
Name: Io, a priestess of Hera loved by Zeus in Greek myth.

One of the four 'Galilean moons' Galileo found around Jupiter in January 1610. Tidal heating makes Io the most volcanically active body in the Solar System.