VASTWARDCelestial Atlas
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Object profile · IO

Io

Volcanic Galilean moon

Jupiter's innermost Galilean moon and the most volcanically active world known.

Atlas data reviewed
Editorial stage
Published
Visibility
Public
False-color Galileo global mosaic of yellow, orange, white and black volcanic terrains on Io.Direct observation
Galileo near-infrared, green and violet mosaic enhanced to reveal subtle surface differences.

Official false-color data mosaic. The colors emphasize compositional differences and are not exactly what human eyes would see.

NASA/JPL/University of Arizona ↗
Vastward explanation

What this world is

Jupiter and the neighboring moons repeatedly flex Io. That tidal kneading turns orbital energy into internal heat and feeds constant volcanism.

Why it looks this way now

Spacecraft directly observe plumes, lava fields and surface changes. The exact distribution of melt and the depth of magma storage remain model-dependent.

02

Where it sits

The innermost of the four large Galilean moons, locked into a 1:2:4 orbital resonance with Europa and Ganymede.

How to read this mapTwo-scale location · Solar System to the Jupiter moons

Swipe sideways to inspect the full map

Two-scale location · Solar System to the Jupiter moonsThe left panel places Jupiter in planetary order; the right places Io among the profiled Jupiter moon orbits.Scale one · Solar SystemJupiterPlanet 5 from the SunSunScale two · Jupiter moon systemJupiterIoProfiled orbit 1 of 4Mean distance · 421,700 km
Blue marks the current moon and orbit

Io is 1 of 4 profiled Jupiter moons by distance, at a mean center distance of about 421,700 km.

Muted lines and dots show system context

The left panel locates Jupiter, dashed lines mark the scale change, and muted orbits and dots represent other profiled moons on the right. This is the innermost profiled moon in the system; Europa is adjacent outside.

The figure uses two linked scales and compares only the same-system moons currently profiled by Vastward; orbit spacing, body sizes and marker positions are compressed for teaching.
03

Read the numbers

Reference values retain their units, context, evidence state and published uncertainty.

Physical measurements

Mean radiusglobal mean
1,821.49kmCalculated measurementThe mean radius is about 1.05 times the radius of Earth's Moon.

Irregular moons have no single true radius; the mean radius is an equal-volume comparison.

Published uncertainty: ± 0.5 km

JPL tabulated uncertainty for the adopted mean radius.

Massglobal mean
8.929649E22kgCalculated measurementMass is not read from a scale. Motion under gravity constrains GM, from which mass is derived.

Calculated from JPL GM with a conventional gravitational constant, then rounded for display.

Approximate value

Derived from JPL's gravitational parameter GM and a conventional gravitational constant, then rounded.

Mean densityglobal mean
3.5276g/cm³Calculated measurementThe densest major moon, closer to rocky planets than to ice-rich outer moons.

This is a whole-body average and cannot by itself prove the composition of every interior layer.

Published uncertainty: ± 0.0029 g/cm³

JPL tabulated uncertainty for the adopted mean density.

Reference gravitysurface
1.7963m/s²Calculated measurementRepresentative surface gravity is about 18.3% of Earth's.

Shape, terrain and local mass distribution make real gravity vary by location.

Approximate value

Calculated from the adopted GM and mean radius. Irregular shape and local terrain can change the actual value.

Temperaturesurface
-130°CScientific modelMost of the surface is extremely cold, while volcanic hotspots break that simple picture.

The value is a representative cold surface reference, not the temperature of an erupting vent.

Approximate value

Representative cold surface value. Active volcanic vents can be dramatically hotter, so it is not a global uniform temperature.

Orbit and rotation

Average distancesystem reference
421,700kmCalculated measurementThis is a representative center-to-center distance between Io and Jupiter.

The real distance changes around an elliptical orbit.

Approximate value

Rounded orbital distance from the moon center to the planet center. The real orbit is not a perfect circle.

Rotation periodrelative to distant stars
1.769Earth daysCalculated measurementTidal locking makes one rotation take nearly the same time as one orbit around the planet.

This is a sidereal rotation, not a local sunrise-to-sunrise solar day.

Approximate value

Rounded synchronous rotation period. The same hemisphere normally faces the primary planet.

Orbital periodrelative to distant stars
1.769Earth daysCalculated measurementThis reports the time for one orbit, not a claim that the path is perfectly circular.

A rounded sidereal period is used for the beginner-facing display.

Approximate value

Rounded sidereal orbital period.

04

Atmosphere and inside

Atmosphere

Io has a thin, patchy sulfur-dioxide atmosphere supplied by volcanoes and surface frost.

Sulfur dioxideRelative abundance described; exact fraction not listed

Surface and interior

Io is a rocky differentiated moon with a metal-rich core, heated mantle and continuously renewed volcanic crust.

  1. 1
    Iron-rich core

    Gravity and magnetic measurements support a dense metallic center.

    Scientific model
  2. 2
    Tidally heated silicate mantle

    Repeated flexing dissipates energy as heat and supports widespread melting.

    Scientific model
  3. 3
    Sulfur-coated volcanic crust

    Direct images show lava flows, calderas and sulfur compounds repeatedly resurfacing the moon.

    Direct observation
05

How we know

Images constrain size, orbital and radio tracking constrain GM, and spectra and thermal emission constrain materials. Every number retains its method and limitation.

  1. 01

    Spacecraft imaging and shape reconstruction

    Repeated views reveal the limb, terrain, crater record and the shape used to estimate size.

    Where this method stops

    Lighting, viewing angle and incomplete coverage can hide topography. A mosaic is not a single untouched photograph.

    Direct observation
  2. 02

    Radio tracking and orbital dynamics

    Engineers measure spacecraft motion and moon orbits, solve for GM, and then derive mass and gravity.

    Where this method stops

    The result depends on trajectory coverage and a dynamical model. Very small moons leave weaker gravitational signatures.

    Calculated measurement
  3. 03

    Spectroscopy and thermal sensing

    The spectrum and thermal glow constrain surface materials, gases and representative temperature.

    Where this method stops

    A spectrum samples the visible surface or atmosphere. It does not directly photograph deep interior layers.

    Direct observation
06

Evidence key

Direct observation
An instrument or sample recorded the phenomenon, with processing still disclosed.
Calculated measurement
Observed motion or signal is converted into a physical quantity using equations and reference constants.
Scientific model
A tested interpretation that fits observations but is not directly imaged or sampled.
Vastward explanation
Original beginner-facing synthesis, traceable to the source records but not itself a measurement.
Artist visualization
An interpretive image, not observational evidence.
07

Missions and instruments

Completed

Galileo

Galileo mapped Io repeatedly, observed eruptions and measured gravity, composition and magnetic interactions.

Official mission ↗
Instruments
  • Solid-State Imaging cameraImages the limb, terrain, color differences and time-dependent surface changes.
  • Spacecraft radio scienceMeasures Doppler and range changes that constrain trajectory, gravity and mass.
  • Near-Infrared Mapping SpectrometerSeparates light by wavelength to constrain composition, gases and temperature.
08

Official source trail

Links below are the exact records used for this profile. Access dates are retained with the content.

  1. 01

    NASA Science · official-page

    Io Facts

    Source updated
    No source update date published
    Access checked
    2026-07-31
    Open official source ↗
  2. 02

    NASA Jet Propulsion Laboratory, Solar System Dynamics · dataset

    Planetary Satellite Physical Parameters

    Source updated
    No source update date published
    Access checked
    2026-07-31
    Open official source ↗
  3. 03

    NASA Science Photojournal · image

    Global Image of Io, False Color

    Source updated
    No source update date published
    Access checked
    2026-07-31
    Open official source ↗
  4. 04

    NASA Science · official-page

    Galileo mission

    Source updated
    No source update date published
    Access checked
    2026-07-31
    Open official source ↗
09

Continue exploring

Atlas is a starting point. Use these relationships to move into explanation, experiment and mission thinking.

Available now

Understand moon systems

Place Io back inside Jupiter's moon system and connect orbit, tides and resonance.

academy
Relationship reserved

Compare the major moons

Compare size, density, gravity and orbit without treating similar colors as identical composition.

lab