Saturn is a gas giant composed mostly of hydrogen and helium, with the lowest density of any planet in our Solar System, at about two-thirds that of water. Its rings are made of countless individually orbiting particles that are primarily water ice, most of them smaller than a house, and its inner rings may be the flattest structures in our Solar System, with a diameter about 30 million times longer than their two-story height. Saturn's largest moon, Titan, is the only known extraterrestrial world to host rain, rivers, stable lakes and seas, where methane plays the role of water on Earth. Another moon, Enceladus, has a liquid water ocean beneath its frozen water shell, and bright plumes of water ice and water vapor erupt from its volcanically active south pole. Saturn has a mass equal to 95 Earths, a day of 10 hours and 39 minutes, a year of 29.4 Earth years, 53 known moons, a diameter 9.4 times that of Earth, and a mean temperature of -178°c.