← ALL CELESTIAL BODIES

Credit: NASA/JPL-Caltech/Space Science Institute Public domain — NASA Media Usage Guidelines

MOON

Rhea

Saturn's second-largest moon — a heavily cratered ice world with a tenuous oxygen exosphere.

Orbits Saturn

Second-largest moon of Saturn; very heavily cratered.
Tenuous oxygen + COu2082 atmosphere detected by Cassini in 2010.
Composition ~75% water ice, ~25% rock.

Physical Properties

Mean radius764 km

Mass2.306518e21 kg

Density1.236 g/cm³

Surface gravity0.264 m/s²

Rotation period108.42 h

Bond albedo0.949

Mean temp76 K

Atmosphere Composition

Oxygen (Ou2082) tenuous
Carbon Dioxide tenuous

Orbit

OrbitsSaturn

Semi-major axis527,108 km

Eccentricity0.001

Inclination0.345°

Orbital period4.518 d

Sources & Further Reading

Numerical values (radius, mass, orbital elements, temperatures) are drawn from NASA NSSDC Planetary Fact Sheets, JPL Horizons, and the JPL Small-Body Database. Last refreshed: 2026-04-18 18:19:24.

Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System. It is composed of approximately three parts water ice to one part rock, heavily cratered over 4 billion years. In 2010 the Cassini spacecraft detected a very thin but permanent oxygen and carbon-dioxide atmosphere — the first time molecular oxygen had been detected in the atmosphere of a world other than Earth or Mars.