The largest moon of Saturn and the second-largest moon in the solar system. Titan has a thick, cold, nitrogen-rich atmosphere that supports winds, clouds, and rain. The rain, which consists of droplets of liquid methane and ethane, fills hundreds of lakes and seas in Titan’s polar regions, and appears to carve river channels. Tiny grains of methane ice form long, tall dunes across parts of Titan, and “volcanoes” may spurt frozen water onto the surface. A vast ocean of liquid water may lie far beneath Titan’s crust. The Huygens probe landed on Titan in 2006, with its images showing rounded pebbles on the surface, perhaps smoothed by water. Titan is difficult to study from afar because the atmosphere is topped by “smog” — a layer of hydrocarbons that turn orange when exposed to sunlight. The Cassini spacecraft, which ferried Huygens to Titan, has made by dozens of passes by the big moon, using radar and instruments sensitive to infrared and ultraviolet light to peer through the haze and map much of Titan’s surface.