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Moon of Saturn
Rhea
Saturn's second largest moon, a battered ball of ice with a whisper of air
Key facts
- Diameter
- About 949 miles / 1,528 km (mean radius about 764 km)
- Distance from Saturn
- About 327,000 miles / 527,000 km
- One orbit
- About 4.5 Earth days
- One day (rotation)
- Same as its orbit, about 4.5 Earth days (tidally locked)
- Discovered
- 23 December 1672 by Giovanni Domenico Cassini
- Named after
- Rhea, the Titan mother of the Olympian gods in Greek mythology
- Surface
- Heavily cratered water ice with bright icy cliffs
- Made of
- About three quarters water ice and one quarter rock
- Surface temperature
- About -174 °C in sunlight, down to about -220 °C in shade
- Atmosphere
- An extremely thin exosphere of oxygen and carbon dioxide, found by Cassini in 2010
- Visited by
- Voyager 1 (1980), Voyager 2 (1981) and Cassini (2004 to 2017, five targeted flybys)
About Rhea
Rhea is a moon of Saturn. It is Saturn's second biggest moon. Rhea is made mostly of ice. There is some rock inside too. It looks like a dirty snowball. Rhea is very, very cold. It is covered in craters. Craters are big round dents. Space rocks made them long ago. Some craters are bigger than a town. Rhea has bright wispy lines. They are icy cliffs on its surface. A man called Cassini found Rhea. He used a telescope long ago. A spacecraft named Cassini flew past too. It took close-up pictures of Rhea. Rhea circles Saturn every four and a half days.
Rhea is the second largest moon of Saturn, after giant Titan. It is a sphere of ice and rock about 1,528 kilometres across. Scientists think it is roughly three quarters ice and one quarter rock. That makes Rhea a bit like a huge, frozen, dirty snowball.
Rhea orbits Saturn at a distance of about 527,000 kilometres. One orbit takes about four and a half Earth days. Saturn's gravity keeps the same side of Rhea facing the planet. This means Rhea takes exactly one orbit to rotate once on its axis.
Rhea's surface is one of the most cratered places in the solar system. Some of its craters are more than 40 kilometres wide. Bright wispy streaks cross the surface, which turned out to be icy cliffs. Rhea is bitterly cold, at about minus 174 degrees Celsius in the sunshine.
In the shade it drops to about minus 220 degrees Celsius. The astronomer Giovanni Cassini discovered Rhea with a telescope in 1672. Much later, a spacecraft named after him studied Rhea up close. In 2010, Cassini found a very thin atmosphere of oxygen and carbon dioxide.
It is far too thin to breathe, but it was a surprise.
Rhea is the second largest of Saturn's moons, with a mean radius of about 764 km (a diameter of roughly 1,528 km), less than a third the radius of Titan. It orbits Saturn at an average distance of about 527,000 km, completing one orbit in about 4.5 Earth days, and it is tidally locked, so its rotation period matches its orbital period. Giovanni Domenico Cassini discovered it on 23 December 1672. It is named after the Titan Rhea of Greek mythology, mother of the first generation of Olympian gods, in keeping with the convention of naming Saturn's moons after Titans.
Rhea's mean density is about 1.24 times that of liquid water (JPL gives 1.2372 g/cm³), which implies a composition of roughly three quarters water ice and one quarter rock. NASA describes it as a frozen dirty snowball. Its surface is very bright, reflecting most of the sunlight that falls on it, and bitterly cold: about -174 °C in sunlit areas and down to about -220 °C in shadow.
The surface is heavily cratered and appears ancient. It has two broad terrain types: regions dominated by craters larger than about 40 km across, and areas with only smaller craters, suggesting some later resurfacing. Notable impact basins include Tirawa and the bright, relatively young crater Inktomi. Bright wispy streaks seen by Voyager were later revealed by Cassini to be ice cliffs formed by fractures, several hundred metres high, similar to the features on Dione.
Voyager 1 and Voyager 2 returned the first images of Rhea in 1980 and 1981. The Cassini spacecraft, which orbited Saturn from 2004 to 2017, made five targeted flybys, including a pass just 69 km above the surface in January 2011. In 2010 Cassini detected an extremely tenuous exosphere of oxygen and carbon dioxide, with surface oxygen densities about five trillion times lower than Earth's; the oxygen is thought to be produced as charged particles from Saturn's magnetosphere break apart surface ice.
One common misstatement deserves correction. In 2008 NASA announced that Rhea might have a faint ring system, and some references still describe it as the first moon with rings. Later targeted Cassini observations of the proposed ring plane found no evidence of ring material, so Rhea is not currently considered to have rings.
Did you know?
- Rhea has a whisper of air: an exosphere of oxygen and carbon dioxide, but its oxygen is about five trillion times thinner than the air on Earth.
- In 2011 the Cassini spacecraft skimmed just 69 km above Rhea's surface, closer than many aeroplanes fly above the Earth.
- Scientists once thought Rhea had rings of its own, but when Cassini looked carefully it found no ring material at all.
Neighbours around Saturn
Questions children ask
How big is Rhea?
Rhea is about 1,528 km across, which makes it Saturn's second largest moon. Only Titan is bigger, and Rhea's radius is less than a third of Titan's. Rhea is still much smaller than our own Moon.
What is Rhea made of?
Rhea is mostly water ice, with some rock mixed in. Its density is only about 1.24 times that of water, so scientists think it is roughly three quarters ice and one quarter rock, a bit like a frozen dirty snowball.
Does Rhea have rings?
Probably not. In 2008 scientists thought Cassini had found signs of faint rings around Rhea, which would have been a first for any moon. When Cassini looked directly at the area where the rings should be, it found nothing, so the idea has been dropped.
Who discovered Rhea and how?
The Italian-French astronomer Giovanni Domenico Cassini discovered Rhea on 23 December 1672 using a telescope. Over 300 years later, the spacecraft named after him flew past Rhea five times and photographed its surface in detail.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- Which is Saturn's only moon larger than Rhea?
- What is Rhea mostly made of?
- About how long does Rhea take to orbit Saturn once?
Titan
Iapetus
The Moon