Titan

Titan is Saturn's largest moon and the second largest in the solar system, with a mean radius of about 2,575 km. Only Jupiter's Ganymede is bigger, by roughly 2 per cent, and Titan is larger than the planet Mercury, though far less massive. It orbits Saturn at about 1.2 million km every 15 days and 22 hours, and is tidally locked, so one hemisphere always faces the planet. Christiaan Huygens discovered it on 25 March 1655; the name Titan was proposed by John Herschel in 1847, after the Titans of Greek mythology.

Titan is the only moon known to have a substantial atmosphere. It is about 95 per cent nitrogen and about 5 per cent methane, extends nearly 600 km above the surface, and produces a surface pressure roughly 60 per cent higher than Earth's at sea level. Sunlight breaking down methane creates a thick orange photochemical haze, which is why the surface is invisible in ordinary photographs. The surface temperature is about minus 179 degrees Celsius. At that temperature methane and ethane are liquids, and Titan has clouds, rain, rivers, lakes and seas of them, concentrated near the poles; it is the only place besides Earth with stable liquid on its surface. Equatorial regions are dominated by dunes of organic particles. Beneath a crust of water ice, gravity measurements point to a liquid water ocean some 55 to 80 km down.

Pioneer 11 passed through the Saturn system on 1 September 1979, and Voyager 1 made the first close flyby of Titan in 1980, finding only the haze. Cassini arrived at Saturn on 1 July 2004 and completed more than 100 targeted Titan flybys before ending its mission in September 2017, mapping the surface by radar and infrared. ESA's Huygens probe, carried by Cassini, descended by parachute for about two and a half hours and landed on 14 January 2005, transmitting from the surface for more than an hour. It remains the only landing on a moon other than our own. NASA's Dragonfly, a nuclear-powered rotorcraft, is scheduled to launch no earlier than July 2028 and reach Titan in 2034 for a mission of about three years, sampling the surface to study prebiotic chemistry. A common misconception is that Titan's seas are water; they are liquid hydrocarbons, and any water there is frozen as hard as rock.