Titan is the sixth and largest of Saturn's planet-like moons, with a radius a little larger than that of the planet Mercury, but with less than half of its mass.
It's remarkable not only for its size, but for the fact it is the only moon in the Solar System to have a dense atmosphere that supports weather systems and liquid pooling on its surface.
Known to astronomers since its discovery by Dutch astronomer Christiaan Huygens in 1655, its dense covering of orange fog has made it difficult to study. Only with the depositing of a probe from the Cassini–Huygens research mission in early 2005 did researchers get some sense of what Titan's surface was like.
Today it's understood that Titan has an atmosphere that's a little denser than Earth's, consisting mostly of nitrogen, with clouds of methane and a fog of various organic particles. Its surface has rough and smooth patches, seemingly refreshed by volcanic forces in the past, with active features like electrically charged sand dunes and deep lakes of cold liquid hydrocarbons.
These pools of liquid hydrocarbon have inspired speculation that complex organic - and even biological - chemistry might be possible on Titan.
NASA is planning missions to this odd little world in coming years in the hopes of learning more about its atmosphere, weather systems, and potentially even a unique biosphere.