Rhea - Saturn's Moon Facts for Kids
(Distance of the Moon from Earth 384,000 km/ 240,000 mi)
Diameter: 1530 km (960 mi)
Day length & time to orbit Saturn: 4.5 days
Note: These numbers have all been rounded.
Rhea is Saturn's second biggest moon, but it's still small.
If you could set Rhea down on the eastern seaboard of the United States – not a good idea! – it would reach from New York City to Miami, Florida.
Giovanni Domenico Cassini discovered Rhea in 1672, but astronomers knew almost nothing about it for over three hundred years.
Cassini discovered four of Saturn's moons in the late seventeenth century. Telescopes got bigger and better. But even in the biggest ground telescopes, the moons are still only dots. The first close-up pictures came in the early 1980s when Voyager 1 visited Saturn. Since 2004 we've learned much more from NASA's Cassini mission.
Saturn's moons didn't have names for two hundred years.
In the late nineteenth century, English astronomer John Herschel named the seven moons that were known in his day. Astronomers had been numbering them, using Roman numerals. Saturn I was the moon closest to the planet. But when new ones were discovered, sometimes they needed renumbering. It could be confusing remembering which moon was which.
John Herschel named Saturn's moons from Greek mythology.
“Saturn” is the Roman name for the Greek god Cronos. He got to be king of the Titans after overthrowing his father Uranus the sky god. So Herschel called the moons after other Titans, including Rhea who was the wife of Cronos. He did this for his own use, but other astronomers started using the names too. By now they're official.
The same side of Rhea always faces Saturn.
Like our own Moon, as Rhea orbits Saturn, the same side always faces the planet. This means that Rhea also turns once on its axis as it travels once around Saturn.
Rhea's day is four and a half Earth days long.
Rhea is farther away from Saturn than the Moon is from Earth, so we might expect it to orbit more slowly than the Moon does. Yet the Moon takes over 27 days to orbit and Rhea takes only four and a half. We need to remember that Saturn is much more massive than Earth, so its gravity pulls more strongly. If Rhea didn't move fast enough, it would fall out of its orbit.
Rhea is a dirty snowball.
Some of the icy moons of the giant planets have rocky centers, but not Rhea. It's made mostly of ice, but about a quarter of it is rock. Instead of a rocky core, the rock and ice are all mixed together.
Rhea has an oxygen-rich atmosphere.
The oxygen sounds exciting, but it's far too thin to breathe. It's an exosphere, not a proper atmosphere. Earth has an exosphere – it's where the atmosphere is so thin that it's merging with outer space.
Rhea is extremely cold.
It's easy to guess that Rhea would be cold. After all, Saturn is twelve times as far away from the Sun as we are. And unlike Earth, Rhea doesn't have an atmosphere that acts like a blanket. Even in direct sunlight, the temperature is -174 oC (-281 oF). Sometimes people say that something is “as hard as a rock.” This is usually just an expression, but on Rhea it's so cold that ice really is as hard as a rock.
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