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Object profile · IAPETUS

Iapetus

Two-tone outer major moon of Saturn

A yin-yang moon whose charcoal-dark and snow-bright hemispheres are crossed by a mountain ridge around the equator.

Atlas data reviewed
Editorial stage
Published
Visibility
Public
Cassini paired global views of Iapetus, showing a charcoal-dark leading hemisphere and snow-bright trailing terrain.Direct observation
Cassini global comparison combines a multi-filter color composite and a 60-frame mosaic to expose Iapetus's brightness dichotomy.

Official multi-frame, multi-filter data product. Infrared and ultraviolet inputs mean the displayed colors are not a single natural-color exposure.

NASA/JPL-Caltech/Space Science Institute ↗
Vastward explanation

What this world is

Iapetus began with a small brightness difference. Dark ground absorbed more sunlight, warmed more strongly and lost surface ice as vapor; that feedback helped dark areas grow darker and bright cold areas grow brighter.

Why it looks this way now

Cassini directly mapped the global brightness divide and a mountain chain reaching about 10 kilometres high along the equator. Thermal segregation is the leading explanation for amplifying the color divide; the ridge's origin remains unresolved.

02

Where it sits

Saturn's third-largest moon, orbiting far beyond Titan on a distant, inclined path that takes about 79 Earth days.

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

Swipe sideways to inspect the full map

Two-scale location · Solar System to the Saturn moonsThe left panel places Saturn in planetary order; the right places Iapetus among the profiled Saturn moon orbits.Scale one · Solar SystemSaturnPlanet 6 from the SunSunScale two · Saturn moon systemSaturnIapetusProfiled orbit 8 of 9Mean distance · 3,561,000 km
Blue marks the current moon and orbit

Iapetus is 8 of 9 profiled Saturn moons by distance, at a mean center distance of about 3,561,000 km.

Muted lines and dots show system context

The left panel locates Saturn, dashed lines mark the scale change, and muted orbits and dots represent other profiled moons on the right. Hyperion is the adjacent profiled moon inside; Phoebe 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
734.3kmCalculated measurementThe mean radius is about 0.423 times the radius of Earth's Moon.

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

Published uncertainty: ± 2.8 km

JPL tabulated uncertainty for the adopted mean radius.

Massglobal mean
1.805659E21kgCalculated 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
1.0887g/cm³Calculated measurementOnly slightly denser than water, consistent with an ice-rich body mixed with a smaller rock fraction.

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

Published uncertainty: ± 0.0125 g/cm³

JPL tabulated uncertainty for the adopted mean density.

Reference gravitysurface
0.223509m/s²Calculated measurementRepresentative surface gravity is about 2.28% 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.

Temperatureverified station extreme
-143°CDirect observationDark equatorial terrain can become about 30 K warmer than bright ice, enough to move water vapor over geological time.

The displayed value is a measured dark-terrain noon high; bright regions and night are substantially colder.

Approximate value

Cassini CIRS measured dark equatorial terrain near local noon approaching 130 K (-143 °C). Bright terrain peaked closer to 100 K (-173 °C); the displayed value is not a global mean.

Orbit and rotation

Average distancesystem reference
3,561,000kmCalculated measurementThis is a representative center-to-center distance between Iapetus and Saturn.

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
79.3215Earth 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
79.3215Earth 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

Iapetus has no persistent atmosphere. Surface ice can slowly sublime and migrate without building a lasting gas envelope.

Surface and interior

Iapetus is an ice-rich mixed body beneath an ancient two-tone crust. The visible ridge is real; its formation story is still a model choice.

  1. 1
    Ice-rich mixed interior

    Bulk density supports an ice-dominated body containing rock, but available measurements do not uniquely reveal a differentiated core.

    Scientific model
  2. 2
    Two-tone cratered crust and equatorial ridge

    Cassini directly observed dark Cassini Regio, bright icy terrain, large impact basins and a mountain chain tracing the equator.

    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

Cassini-Huygens

Cassini mapped both hemispheres, measured thermal contrast and resolved the equatorial ridge during distant and close flybys.

Official mission ↗
Instruments
  • Imaging Science SubsystemImages the limb, terrain, color differences and time-dependent surface changes.
  • Spacecraft radio scienceMeasures Doppler and range changes that constrain trajectory, gravity and mass.
  • Visual and 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

    Iapetus

    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 View of Iapetus' Dichotomy, PIA11690

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

    NASA Science · official-page

    Cassini mission

    Source updated
    No source update date published
    Access checked
    2026-07-31
    Open official source ↗
  5. 05

    NASA Science · official-page

    Iapetus Temperature Map

    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 Iapetus back inside Saturn'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