Rhea
What is Rhea?
Moons· Diameter 1,527 km. Saturn's second-largest moon — a bright, three-quarters-ice world of ancient craters and pale ice cliffs, once suspected of having rings of its own. Find it circling inside Titan's orbit in the orbit view.
📍 Visible from where you are right now — computed in your browser
bearing · altitude · rise time · best time tonight
The orbit of Rhea at this moment: https://nowsky.org/solar/orbit?fw=moon:saturn:rhea&sp=0.012 — replayed live, no login.
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
Frequently asked
- How big is Rhea?
- 1,527 km.
- How far is Rhea from Earth?
- 527,000 km.
- Which planet does Rhea orbit?
- Saturn.
The oxygen atmosphere Cassini measured
In 2010 Cassini passed 97 km from Rhea and directly detected an extremely thin atmosphere of oxygen and carbon dioxide — the first direct measurement of an oxygen atmosphere beyond Earth. It has nothing to do with life: particles carried by Saturn's magnetic field strike the surface ice, split water molecules, and knock oxygen loose. The density is about a trillionth of Earth's atmosphere.
Where the carbon dioxide comes from is still uncertain — organic material trapped in the ice may be broken down by the same bombardment, or carbon dioxide within the ice may be slowly leaking out. Europa and Ganymede have oxygen atmospheres produced the same way, and the lesson of the measurement is that oxygen on an icy moon is a signature of radiation, not of life.
The moon in depth
Rhea, discovered by Giovanni Cassini in 1672, is Saturn's second-largest moon at 1,527 km across. It orbits 527,000 km from the planet every 4.52 days, and its density — only 1.24 times that of water — implies that about three-quarters of its mass is ice. Its high reflectivity gives it a bright grey-white look.
Most of the surface is ancient cratered terrain. Bright streaks photographed by the Cassini spacecraft turned out to be ice cliffs catching sunlight. In 2008 scientists reported hints of a faint ring of particles around Rhea, which would have made it the first moon with rings, but later Cassini imaging found nothing and the idea was retired. In its place came a surprise: an extremely thin exosphere of oxygen and carbon dioxide.
Follow Saturn in the orbit view to see Rhea circling outside Dione and inside Titan — a good vantage point for comparing the tightly packed procession of icy moons that makes Saturn's system so distinctive.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.