Rhea
What is Rhea?
Moons· Diameter 1,527 km. Saturn's second-largest moon, a bright world about three-quarters ice. Its surface is covered with old craters, and in 2010 the Cassini spacecraft directly detected an extremely thin oxygen atmosphere.
📍 Visible from where you are right now — computed in your browser
Orbit and size
| Diameter | 1,527 km |
|---|---|
| Distance from Saturn | 527,000 km |
| Orbit | 4.52 d |
| Composition | ~75% ice, rest rock |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Parent planet and sibling moons
Rhea's thin oxygen air, the first measured directly beyond Earth
In 2010 the Cassini spacecraft passed 97 km above Rhea and directly detected a very thin atmosphere of oxygen and carbon dioxide. It was the first time a spacecraft had captured molecules of an oxygen atmosphere at a world other than Earth. This atmosphere is more than a trillion times thinner than Earth's, nothing like air that could be breathed.
The oxygen has nothing to do with life. Fast-moving particles carried by Saturn's magnetic field strike Rhea's icy surface, splitting water molecules and knocking oxygen loose. Where the carbon dioxide comes from is still uncertain. It may form when organic material in the ice is broken down by the same bombardment, or it may be carbon dioxide trapped in the ice slowly leaking out.
Europa and Ganymede have oxygen atmospheres made the same way. The lesson of this measurement is that finding oxygen on an icy moon is not, by itself, a sign of life. Radiation splitting ice is enough to produce it.
The moon in depth
Giovanni Cassini discovered Rhea in 1672. At 1,527 km across, it is Saturn's largest moon after Titan. It orbits 527,000 km from the planet once every 4.52 days. Its density is only 1.24 times that of water, which suggests that about three-quarters of its mass is ice and the rest rock. It reflects sunlight well and looks bright grey-white.
Most of Rhea's surface is covered with old craters. On one side there are thin, bright streaks, and close-up images from the Cassini spacecraft showed them to be ice cliffs. Where the ground had cracked, clean ice cliffs were exposed, and from a distance their reflected sunlight looked like bright lines. Similar streaks of ice cliffs were found on the neighbouring moon Dione, suggesting the crusts of both moons cracked in similar ways.
In 2008 scientists reported signs that Rhea might have a faint ring. If true, it would have been the first moon known to have rings, but later Cassini images found nothing. Following Saturn in the orbit view shows Rhea circling outside Dione and inside Titan.
Frequently asked
- How big is Rhea?
- The diameter of Rhea is 1,527 km.
- How far is Rhea from Earth?
- The distance from saturn of Rhea is 527,000 km.
- Which planet does Rhea orbit?
- Rhea orbits Saturn.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.