Moons · Space Codex
Rhea
A cold, ancient ice world with a whisper of oxygen and carbon dioxide around it, the first oxygen exosphere found directly at another world.
Saturn's second-largest moon
At a glance
Key points
- Saturn's second-largest moon
- Has a tenuous oxygen and CO₂ exosphere
- Cratered plains about 4 billion years old
Surface
Rhea's bright, heavily cratered terrain is among the oldest in the Saturn system. Like Dione, its trailing side carries wispy lines that Cassini resolved in 2006 as fractures and canyons with cliffs several hundred metres high.
A thin breath of air
Charged particles trapped in Saturn's magnetic field strike Rhea's ice and split water molecules, freeing oxygen. The resulting exosphere is about 5 trillion times less dense than Earth's air at the surface.
What we still don't know
Whether Rhea's interior is a uniform mix of ice and rock, as a Cassini gravity measurement suggests, or partly separated into layers.
Missions
- 1977–
Jupiter 1979, Saturn 1980; interstellar space since 25 Aug 2012.
- 1997–2017
Cassini–Huygens
NASA / ESA / ASICompletedOrbited Saturn 2004–2017 and ended by diving into the planet.
First-hand · our 3D model
How we render it
Rhea is a sphere wrapped in a global map built from USGS and NASA spacecraft mosaics.
It is drawn at about 5.3% of Saturn's radius, enlarged from its true 1.3% so it stays visible. It orbits in Saturn's equatorial frame, inclined 0.3°, with a 4.52-day period kept in true ratio to its neighbours, and keeps one face towards Saturn.
Moon sizes and orbits are compressed so a whole system fits on screen, but never reordered: each moon is drawn at 0.72 × (true size ratio)^0.6 of its planet, capped so no large moon is drawn bigger than it really is, and orbit distances are log-compressed into a band that clears the planet's rings. Orbital periods keep their true ratios, every moon is tidally locked, and on lighter devices only the two largest moons of each planet are drawn.
Related entries
Sources
- JPL Solar System Dynamics: satellite physical parameters (opens in a new tab)
- JPL Solar System Dynamics: satellite mean orbital elements (opens in a new tab)
- NASA NSSDC Saturnian Satellite Fact Sheet (opens in a new tab)
- NASA Science: Rhea (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.