Wikijunior:Solar System/Saturn/Rhea

How big is Rhea?

Rhea is 1530 km in diameter (the distance from one side to the other through the center). Rhea is relatively large when compared with the moons of other planets in the Solar System.

What is its surface like?

The surface of Rhea is heavily cratered and has bright wispy markings on its surface. The surface can be divided into two categories. The first area contains craters which are more than 40 km wide, while the second area contains craters smaller than this.

The leading hemisphere is heavily cratered and is the same brightness throughout. As on Callisto, the craters are eroded. On the trailing hemisphere there are bright swaths on a dark background and few visible craters. These bright swaths may be material from ice volcanoes early in Rhea's history when it was still liquid inside. However, recent observations of Dione, which has the same bright streaks, shows that these are in fact ice cliffs. Therefore, it can be assumed that the bright streaks on the Rhean surface are also ice cliffs. When Cassini does its flyby of Rhea on November 25, 2005, we will know for sure.

How long is a day on Rhea?

One day on Rhea is equal to four and a half days on Earth.

How long is its orbit around Saturn?

One orbit around Saturn also takes four and a half days. Rhea has synchronous rotation, meaning that it takes the same amount of time for it to complete one orbit as it does to complete one rotation on its axis. This also means that the same side of Rhea always faces Saturn.

What is it made of?

One third of Rhea's mass is rock, found at its core. The rest is water ice.

How much would Rhea's gravity pull on me?

If you stood on Rhea's surface, you would weigh less than you would on Pluto — which is about 6% of what you do on Earth.

Who is it named after?

Rhea is named after a Titaness in Greek mythology. The name was suggested by astromoner William Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope, in which he suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used. Before that, it, Tethys, Dione, and Iapetus were called Sidera Lodoicea ("the stars of Louis") to honour king Louis XIV.

How was it discovered?

Rhea was discovered by Giovanni Cassini on December 23, 1672.