Saturn's ancient frozen giant
Rhea
Moon of Saturn · discovered 1672 by Giovanni Cassini
Rhea is Saturn's second-largest moon, about 1,530 kilometres across, and a world of ice and rock covered in ancient craters. It looks much like a larger, colder cousin of Dione, complete with bright wispy cliffs on its trailing side.
Yet Rhea holds surprises. In 2010 Cassini sniffed out a faint exosphere of oxygen and carbon dioxide around it, and for a while scientists even wondered whether it might have rings of its own.
Key facts
- Diameter
- 1,528 km44% of Earth's Moon
- Mass
- 2.31 × 10²¹ kg
- Surface gravity
- 0.264 m/s²0.027 g
- Density
- 1,236 kg/m³
- Distance from Saturn
- 527,108 km≈ 8.7 Saturn radii
- Orbital period
- 4.52 daysSame as its rotation (tidally locked)
- Discovered
- 1672Giovanni Cassini
- Surface
- Ancient, heavily cratered water ice
- Surface temperature
- About −174 °C to −220 °C
- Atmosphere
- Very tenuous exosphere of O₂ and CO₂
- Notable feature
- Bright ray crater Inktomi
- Named after
- Rhea, a Titaness and mother of Zeus
Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.
Surface and geology
Rhea's leading hemisphere is densely cratered, with few smooth areas, making it one of the oldest-looking surfaces among Saturn's moons. Large basins include Tirawa, about 360 kilometres wide, and the even larger, more degraded Mamaldi.
The trailing hemisphere carries bright, wispy markings similar to Dione's. Cassini showed that they too are fractures and ice cliffs, suggesting that Rhea's crust was once stretched by tectonic forces.
One of the freshest features is Inktomi, a crater about 48 kilometres wide surrounded by bright rays of ejected ice that splash across the darker terrain. Its shape earned it the nickname 'the Splat'.
Interior and composition
Rhea is made of roughly three parts ice to one part rock. Cassini gravity measurements suggest it is not fully separated into a distinct core and mantle, though the data have been interpreted in different ways and the details remain uncertain.
Its exosphere is extraordinarily thin, containing oxygen and carbon dioxide produced as charged particles from Saturn's magnetosphere strike the icy surface. Cassini detected it by flying through it in 2010, making Rhea the first body beyond Earth where an oxygen exosphere was directly sampled by a spacecraft.
Orbit and rotation
Rhea orbits Saturn in about four and a half days, beyond Dione and well inside the orbit of Titan. Like most large moons, it is tidally locked, so one hemisphere always leads and the other always trails, which helps explain their different appearances.
Origin
Rhea probably formed from the disc of gas and ice that surrounded young Saturn. Its heavily cratered surface records the intense bombardment of the early Solar System, though some researchers have argued that Saturn's inner moons, possibly including Rhea, could be younger than they look.
Exploration
Giovanni Domenico Cassini discovered Rhea in December 1672, the second Saturnian moon he found. Spacecraft have since visited it several times.
Voyager 1 (1980)
Voyager 1 returned the first detailed images of Rhea's cratered surface and wispy trailing side.
Cassini (2004–2017)
Cassini made several close flybys, the nearest in 2005 at roughly 500 km, discovered the oxygen and carbon dioxide exosphere and mapped Rhea's gravity field.
The moons of Saturn
| Moon | Diameter | Distance | Orbit |
|---|---|---|---|
| Mimas | 396 km | 185,539 km | 22.6 hours |
| Enceladus | 504 km | 237,948 km | 32.9 hours |
| Tethys | 1,062 km | 294,619 km | 45.3 hours |
| Dione | 1,123 km | 377,396 km | 2.74 days |
| Rhea | 1,528 km | 527,108 km | 4.52 days |
| Titan | 5,150 km | 1,221,870 km | 15.9 days |
| Iapetus | 1,469 km | 3,560,820 km | 79.3 days |
Surprising facts about Rhea
- In 2008 Cassini data hinted that Rhea might be surrounded by a thin ring of debris, which would have made it the first moon known to have rings; later searches found no sign of them.
- Rhea's oxygen exosphere is roughly five trillion times less dense than Earth's atmosphere at sea level.
- Rhea is the second-largest moon of Saturn, yet Titan is nearly 60 times more massive.
- The bright rays around the crater Inktomi show it is one of the youngest large impact scars on the moon.