Titan

Titan is Saturn’s largest moon and the second-largest in the Solar System — larger in diameter than Mercury. It is the only moon known to have a substantial, dense atmosphere, composed mainly of nitrogen (94%) with methane (5.65%) and trace hydrocarbons. Surface pressure is 1.45 bar — 45% greater than Earth’s.

Titan is the only world besides Earth where stable bodies of liquid are known to exist on the surface. Its lakes and seas — Kraken Mare, Ligeia Mare, Punga Mare and hundreds of smaller lakes — are filled with liquid methane and ethane at 94 K. A full “hydrological” cycle operates: methane clouds rain onto highlands, carve river channels, and pool into seas.

Beneath the icy crust Titan likely hosts a subsurface water-ocean layer tens of kilometers thick. The Cassini-Huygens mission transformed our understanding: Cassini made 127 flybys between 2004-2017, and on 14 January 2005 its piggybacked European probe, Huygens, became the first (and only) spacecraft to land on an outer-solar-system moon. NASA’s Dragonfly — an 8-rotor drone — is scheduled to launch in 2028 and arrive at Titan in 2034 to fly between multiple scientifically diverse sites.