Rhea

Moon· orbits Saturn

Rhea is Saturn's second-largest moon — a heavily cratered ball of water ice and rock with a tenuous oxygen–carbon-dioxide exosphere.

Open in 3D viewSee Rhea in the live, interactive 3D map

Key facts

Mass

2.31 × 10²¹ kg

0.0314 Moon masses

Radius

764 km

44% of our Moon's radius

Surface gravity

0.264 m/s²

3% of Earth's gravity

Day length

4.5 days

Orbit around Saturn

4.52 Earth days

Science

Physical

Mass: 2.31 × 10²¹ kg · 0.0314 Moon masses; JPL / IAU mean radii; spacecraft-tracked GM where measured
Radius: 764 km · 44% of our Moon's radius; JPL SSD Planetary Satellite Physical Parameters (mean radius)
Density: 1,237 kg/m³ · 1.2× water; Computed from mass and radius
Surface gravity: 0.264 m/s² · 3% of Earth's gravity; Computed from mass and radius
Escape velocity: 635 m/s · 6% of Earth's escape velocity; Computed from mass and radius

Orbit around Saturn

Distance from Saturn: 527,200 km; From JPL orbital elements
Orbital period: 4.5 days; From JPL orbital elements
Eccentricity: 0.001; From JPL orbital elements
Inclination to Saturn's Laplace plane: 0.30 °; From JPL orbital elements · The Laplace plane is the one a moon's orbit precesses about: close in it is the planet's equatorial plane, the plane of the rings, and further out it tilts toward the planet's own orbit.

Rotation

Rotation period: 4.5 days
Tidal locking: Tidally locked to its parent

Temperature

Mean: 76 K (-197 °C)

Composition

Bulk composition: Mostly water ice with about a quarter rock, probably only weakly differentiated.

Radiation & temperature

Insolation: 0.0113 S⊕ · 1% of the sunlight Earth receives; Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
Bond albedo: 0.50 · reflects 50% of incoming sunlight
Equilibrium temperature: 76 K (-197 °C); Computed from albedo + distance from the Sun (airless black body)
Measured mean temperature: 76 K (-197 °C)

Discovery & history

Discovered: December 23, 1672
By: Giovanni Domenico Cassini
Method: Visual discovery (telescopic)
Name: Rhea, the Titaness mother of the gods in Greek myth.

Cassini discovered Rhea, Saturn's second-largest moon, in 1672. A heavily cratered ball of ice and rock, it briefly seemed to possess a faint ring of its own — which would have been the only ring known around a moon, though later observations cast doubt on it.