Rhea is Saturn’s second-largest moon, an ancient, frigid ice world whose surface is saturated with craters that have been accumulating for billions of years. It offers a glimpse of what an icy body looks like when internal activity shut down early and impacts were left to do all the sculpting.
Cassini detected a whisper of oxygen and CO₂ around it - roughly 5 trillion times thinner than Earth’s air.
Though second-largest of Saturn’s moons, it has under 2% of Titan’s mass.
Its battered surface is one of the most heavily cratered in the Saturn system.
An atmosphere made by radiation
Rhea is far too cold for ice to evaporate, yet Cassini flew through a faint envelope of oxygen and carbon dioxide there in 2010. Charged particles trapped in Saturn’s magnetic field constantly bombard the surface and split water-ice molecules apart, releasing oxygen - an atmosphere manufactured by radiation chemistry, trillions of times thinner than Earth’s air.
Physical characteristics
Diameter
1,528 km
Mass
2.310e+21 kg
Surface gravity
0.26 m/s²
Density
1.24 g/cm³
Tidally locked
Yes
Mean temperature
-195 °C
Temperature range
-220 °C to -174 °C
Atmosphere and composition
Atmosphere
An extremely thin exosphere of oxygen and carbon dioxide, detected by Cassini in 2010.
Composition
Roughly three-quarters water ice and one-quarter rock, only weakly differentiated.
Discovery
Discovered by
Giovanni Cassini
Year
1672
Missions
Voyager 1NASA · Flyby probe · 1977 · active
The most distant human-made object, now roughly 25 billion km from the Sun. After historic flybys of Jupiter, Saturn and Titan, Voyager 1 became the first spacecraft to enter interstellar space in 2012.
Cassini orbited Saturn for 13 years, delivering ESA’s Huygens probe to the surface of Titan in January 2005 - the first landing in the outer Solar System. It ended with a deliberate plunge into Saturn in 2017 to protect the potentially habitable moons Enceladus and Titan.