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Solar System Moons Series

Io

The most volcanic world in the solar system: a sulfur hell kneaded by Jupiter.

Author: Juan Carlos Euyoque Aguilera · October 7, 2026 · CC BY 4.0

Image of Io (NASA/JPL, public domain)
Io in true color (Galileo spacecraft, NASA/JPL, public domain). Not to scale.
3,643.2 km
Diameter
421,700 km
Distance from Jupiter
1.769 days
Orbital period
~130 K
Temperature

Io, the innermost Galilean moon, orbits Jupiter at 421,700 km. The giant's gravity squeezes and stretches it on every 1.769-day orbit, and that internal friction —tidal heating— made it the most volcanically active body in the solar system: over 400 active volcanoes.

Its yellow surface is sulfur and sulfur dioxide thrown up by plumes rising hundreds of kilometers. Voyager 1 saw them in 1979; Galileo measured them up close; Juno keeps watch today.

Measured data [DATO]

DataValueStatusSource
Diameter3,643.2 km[DATO] observadoNASA Planetary Fact Sheets
Mass8.932×10²² kg[DATO] observadoNASA Planetary Fact Sheets
Mean density3,530 kg/m³[DATO] observadoNASA Planetary Fact Sheets
Surface gravity1.796 m/s²[DATO] observadoNASA Planetary Fact Sheets
Orbital period1.769 days[DATO] observadoNASA Planetary Fact Sheets
Rotation periodsynchronous (1.769 days)[DATO] observadoNASA Planetary Fact Sheets
Distance from Jupiter421,700 km[DATO] observadoNASA Planetary Fact Sheets
Temperature~130 K mean (volcanic hotspots: thousands of K)[DATO] observadoNASA Science
AtmosphereTenuous: sulfur dioxide (SO₂)[DATO] observadoNASA Science
Notable featureMost volcanically active body in the solar system[DATO] observadoNASA/JPL

Truths [DATO]

  • [DATO] Io is the most volcanically active body in the solar system: its volcanoes blast plumes up to ~300 km high (Pele), seen by Voyager 1 in 1979.
  • [DATO] The heat does not come from plate tectonics —Io has none— but from tidal heating: Jupiter deforms its interior on every orbit.
  • [DATO] Its yellow-green color is sulfur and sulfur dioxide: the surface is endlessly repaved with fresh volcanic material.
  • [DATO] Material escaping Io feeds Jupiter's plasma torus, a cloud of charged particles around the planet.

Labeled myths [MITO]

[MITO] Io is the hottest place in the solar system.

False: it is the most volcanically active, but its mean temperature is a frigid ~130 K (−143 °C); the extreme heat is confined to volcanic hotspots.

Fuente: NASA Science

[MITO] Io's volcanoes work like Earth's.

False: Earth moves tectonic plates; Io has no plates — its volcanism is powered by the tidal friction Jupiter exerts on its interior.

Fuente: NASA/JPL

Open questions [PREGUNTA]

  • [PREGUNTA] Is there a global magma ocean beneath Io's crust?
  • [PREGUNTA] How is its crust recycled without plate tectonics?
  • [PREGUNTA] How much material does Io lose to space each year?

The chain in brief

  1. [DATA] 1. Kneaded by Jupiter: It orbits Jupiter at 421,700 km, and the giant's gravity stretches and squeezes it on every 1.769-day orbit.
  2. [DATA] 2. The heat comes from tides: Io has no plate tectonics: the internal friction of that kneading — tidal heating — is what heats its interior.
  3. [DATA] 3. Over 400 active volcanoes: It is the most volcanically active body in the solar system: its volcanoes blast plumes up to ~300 km high, seen by Voyager 1 in 1979.
  4. [DATA] 4. A sulfur world that repaves itself: Its yellow color is sulfur and sulfur dioxide: the surface is endlessly repaved with fresh volcanic material, and the material that escapes feeds Jupiter's plasma torus.
  5. [DISPUTED] 5. A magma ocean beneath the crust?: Whether there is a global magma ocean beneath its crust, how that crust recycles without plate tectonics, and how much material Io loses to space each year remain open.