Titan (moon)

Titan is the largest moon of Saturn and the second largest in the Solar System. It is the only moon that is known to have a dense atmosphere (denser than that of Earth), and is the only known object in the Solar System besides Earth with clear evidence of stable bodies of surface liquid. Titan is one of seven gravitationally rounded moons of Saturn, and is the second-most distant among them. Frequently described as a planet-like moon, Titan is 48.16% larger in diameter than Earth's moon, and 80% more massive. It is the second-largest moon in the Solar System after Jupiter's Ganymede, and is larger than Mercury, and yet Titan is only 40% as massive as Mercury, because Mercury is mainly iron and rock, whereas Titan is mostly ice, which is less dense. Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn and the sixth known planetary satellite (after Earth's moon and the four Galilean moons of Jupiter). Titan orbits Saturn at 20 Saturn radii or 1.2 million km above Saturn's apparent surface. From Titan's surface, Saturn, disregarding its rings, subtends an arc of 5.09 degrees, rendering it more than ten times as large in Titan's sky as the Moon is in Earth's sky. Titan is primarily composed of ice and rocky material, with a rocky core surrounded by various layers of ice, including a crust of ice Ih and a subsurface layer of ammonia-rich liquid water. Much as with Venus before the Space Age, the dense opaque atmosphere prevented understanding of Titan's surface until the Cassini–Huygens mission in 2004 provided new information, including the discovery of liquid hydrocarbon lakes in Titan's polar regions and the discovery of its atmospheric super-rotation. The geologically young surface is generally smooth, with few impact craters,...