Object profile · CALLISTO
Callisto
Ancient cratered Galilean moon
A vast, ancient impact archive whose quiet exterior may conceal a deep salty layer.
- Atlas data reviewed
- Editorial stage
- Published
- Visibility
- Public
Direct observationOfficial data mosaic. Color and geometry were processed for a coherent globe; it is not a single exposure.
NASA/JPL/DLR ↗What this world is
Callisto has not erased most ancient craters, so its surface works like an old page of Solar System impact history.
Why it looks this way now
Magnetic measurements are consistent with a conducting salty layer below the ice, but the ocean interpretation remains indirect.
Where it sits
The outermost Galilean moon, beyond the strong 1:2:4 resonance of Io, Europa and Ganymede.
Swipe sideways to inspect the full map
Callisto is 4 of 4 profiled Jupiter moons by distance, at a mean center distance of about 1,882,700 km.
The left panel locates Jupiter, dashed lines mark the scale change, and muted orbits and dots represent other profiled moons on the right. Ganymede is the adjacent profiled moon inside; it is the outermost profiled moon in the system.
Read the numbers
Reference values retain their units, context, evidence state and published uncertainty.
Physical measurements
- Mean radiusglobal mean
- 2,410.3kmCalculated measurementThe mean radius is about 1.39 times the radius of Earth's Moon.
Irregular moons have no single true radius; the mean radius is an equal-volume comparison.
Published uncertainty: ± 1.5 km
JPL tabulated uncertainty for the adopted mean radius.
- Massglobal mean
- 1.075661E23kgCalculated 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
- 1.8340g/cm³Calculated measurementA roughly even whole-body mixture of rock and ice is compatible with the low bulk density.
This is a whole-body average and cannot by itself prove the composition of every interior layer.
Published uncertainty: ± 0.0034 g/cm³
JPL tabulated uncertainty for the adopted mean density.
- Reference gravitysurface
- 1.2358m/s²Calculated measurementRepresentative surface gravity is about 12.6% 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
- -139°CScientific modelThe surface is cold enough for water to remain hard ice over geologic time.
A representative surface value cannot describe every sunlit and shadowed location.
Approximate value
Representative surface reference. Day, night and latitude vary substantially.
Orbit and rotation
- Average distancesystem reference
- 1,882,700kmCalculated measurementThis is a representative center-to-center distance between Callisto 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
- 16.689Earth 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
- 16.689Earth 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.
Atmosphere and inside
Atmosphere
Callisto has an extremely thin carbon-dioxide and oxygen environment, far below a substantial atmosphere.
- Carbon dioxideRelative abundance described; exact fraction not listed
- Molecular oxygenRelative abundance described; exact fraction not listed
Surface and interior
Callisto may be only partly differentiated, with a possible conductive ocean below an ancient ice-rich crust.
- 1Partly differentiated rock-ice interior
Gravity data allow an interior where rock and ice are only partly separated.
Scientific model - 2Possible salty liquid layer
An induced magnetic signature is consistent with an electrically conducting salty layer, not direct sampling of an ocean.
Scientific model - 3Ancient cratered ice-rich crust
Direct images reveal one of the most heavily cratered surfaces in the Solar System.
Direct observation
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.
- 01Direct observation
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.
- 02Calculated measurement
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.
- 03Direct observation
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.
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.
Missions and instruments
Galileo
Galileo mapped Callisto, measured its gravity and magnetic response, and tested models of a conductive subsurface layer.
Official mission ↗- 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.
Official source trail
Links below are the exact records used for this profile. Access dates are retained with the content.
- 01
NASA Science · official-page
Callisto Facts
- Source updated
- No source update date published
- Access checked
- 2026-07-31
- 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
- 03
NASA Science Photojournal · image
Global Callisto in Color
- Source updated
- No source update date published
- Access checked
- 2026-07-31
- 04
NASA Science · official-page
Galileo mission
- Source updated
- No source update date published
- Access checked
- 2026-07-31
Continue exploring
Atlas is a starting point. Use these relationships to move into explanation, experiment and mission thinking.