Rhea is Saturn’s second-largest moon. Only Titan is larger, while Rhea has a diameter of about 1,528 kilometres and ranks among the largest moons in the Solar System.
Giovanni Domenico Cassini discovered Rhea on December 23, 1672. Its bright, heavily cratered surface is dominated by water ice, and measurements indicate that the moon is composed roughly of three-quarters ice and one-quarter rock. Rhea’s low density reflects that icy interior rather than the denser rocky composition of Earth’s Moon.
Rhea is tidally locked to Saturn, meaning it rotates once for every orbit and always keeps the same hemisphere facing the planet. It completes an orbit in about 4.5 Earth days. Enceladus is sometimes chosen by mistake because it is one of Saturn’s most famous moons, especially for its water-rich plumes, but it is much smaller. Iapetus and Dione are also smaller than Rhea.