Illustration of Titan. Titan is the largest moon of Saturn, the second largest in the Solar System (after Ganymede of Jupiter). It was discovered by Christiaan Huygens in 1655. Titan is the only moon in the Solar System known to have clouds and a thick, dense atmosphere primarily composed of nitrogen, with traces of methane and other hydrocarbons. The surface temperatures of Titan are extremely cold, with an average temperature of around -179 degrees Celsius. NASAs Cassini spacecraft made numerous gravity measurements of Titan, revealing that the moon is hiding an underground ocean of liquid water, likely mixed with salts and ammonia. This discovery suggests that conditions suitable for life may exist on Titan, although any life there would likely be very different from life on Earth.