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Object profile · RHEA

Rhea

Ancient icy second-largest moon of Saturn

Saturn's second-largest moon, preserving an ancient crater archive and an oxygen-carbon-dioxide exosphere trillions of times thinner than Earth's air.

Atlas data reviewed
Editorial stage
Published
Visibility
Public
Cassini grayscale full-disk mosaic of Rhea showing an ancient surface packed with impact craters.Direct observation
Twenty-one Cassini clear-filter images assembled into a high-resolution global view of Rhea.

Official grayscale data mosaic assembled from multiple frames; it is not a natural-color single exposure.

NASA/JPL/Space Science Institute ↗
Vastward explanation

What this world is

Rhea looks like an old icy Moon because weak tidal heating has not erased many craters. Cassini also caught individual oxygen and carbon-dioxide molecules around it, but that does not make breathable air.

Why it looks this way now

Cassini directly mapped cratered plains, fracture scarps and an ultra-thin exosphere. Interior measurements favor a broadly mixed ice-rock body rather than a sharply separated rocky core.

02

Where it sits

A cold outer member of Saturn's mid-sized moon family, orbiting beyond Dione and inside Titan.

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 Rhea among the profiled Saturn moon orbits.Scale one · Solar SystemSaturnPlanet 6 from the SunSunScale two · Saturn moon systemSaturnRheaProfiled orbit 5 of 9Mean distance · 527,200 km
Blue marks the current moon and orbit

Rhea is 5 of 9 profiled Saturn moons by distance, at a mean center distance of about 527,200 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. Dione is the adjacent profiled moon inside; Titan 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
763.5kmCalculated measurementThe mean radius is about 0.440 times the radius of Earth's Moon.

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

Published uncertainty: ± 0.6 km

JPL tabulated uncertainty for the adopted mean radius.

Massglobal mean
2.306485E21kgCalculated 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
1.2372g/cm³Calculated measurementThe low density is consistent with roughly three parts ice to one part rock by mass, not a hollow body.

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
0.264082m/s²Calculated measurementRepresentative surface gravity is about 2.69% 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
-174°CDirect observationSunlit and shaded surfaces can differ by about 46 °C even though both remain far below water's freezing point.

The displayed value is the approximate sunlit upper reference; deep shade can fall near -220 °C.

Approximate value

NASA reports roughly -174 °C in sunlit areas and down to -220 °C in shaded areas. The displayed value is the warmer reference, not a global mean.

Orbit and rotation

Average distancesystem reference
527,200kmCalculated measurementThis is a representative center-to-center distance between Rhea 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
4.517503Earth 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
4.517503Earth 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

Cassini directly detected an oxygen-and-carbon-dioxide exosphere. Near the surface it is estimated to be about five trillion times less dense than Earth's air.

Molecular oxygenRelative abundance described; exact fraction not listed
Carbon dioxideRelative abundance described; exact fraction not listed

Surface and interior

Rhea is best described as an ice-rich rock mixture beneath an old fractured surface, with its deep arrangement inferred rather than photographed.

  1. 1
    Modelled mixed ice-rock interior

    Cassini gravity and moment-of-inertia constraints favor a largely homogeneous mixture over a strongly separated rocky core.

    Scientific model
  2. 2
    Ancient cratered ice crust

    Direct images show heavily cratered plains, younger-looking patches and bright fracture scarps.

    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 Rhea, constrained its interior mixing and directly detected oxygen and carbon dioxide in its exosphere.

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

    Rhea

    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

    Rhea Full Moon

    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 ↗
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 Rhea 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