Titan

... is the largest moon of Saturn. Discovered in 1655 by the Dutch astronomer Christiaan Huygens, it was the sixth known planetary satellite (after Earth's Moon and the four Galilean moons of Jupiter). As well as being the only moon known to have a dense atmosphere, it's the only object in space, other than Earth, where clear evidence of stable bodies of surface liquid has been found.

The atmosphere of Titan is composed largely of nitrogen; minor components lead to the formation of methane and ethane clouds, and nitrogen–rich organic smog. The climate – including wind and rain – creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas (probably of liquid methane and ethane), and deltas, and is dominated by seasonal weather patterns as on Earth. With its liquids (both on and under the surface) and robust nitrogen atmosphere, Titan's methane cycle is analogous to Earth's water cycle, at the much lower temperature of about 94 K (–179.2 °C).

Titan is the sixth nearest moon to Saturn. Often described as a planet–like moon, it's 50% larger than Earth's Moon, and 80% more massive. It's the second–largest moon in the Solar System, after Jupiter's Ganymede. (Ganymede has only a very thin atmosphere, composed mainly of oxygen.) Titan is larger than the planet Mercury, but only 40% as massive.

Titan is primarily composed of water ice and rocky material. Much as with Venus before the Space Age, the dense opaque atmosphere prevented understanding of Titan's surface until new information from the Cassini–Huygens mission in 2004, including the discovery of liquid hydrocarbon lakes in Titan's polar regions. The geologically young surface is generally smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found.

(The above is a slightly edited version of the introduction to Wikipedia's Titan page.)