The largest moon of Saturn (only slightly smaller than Ganymede) was a major objective of the Voyager 1 encounter in November 1980 because Earth-based measurements showed that Titan is the only satellite with a prominent atmosphere. The Voyager images showed that a dense cover of clouds or haze completely obscured Titan’s surface. Titan’s atmosphere was found to be 1.6 times as dense as Earth’s atmosphere at sea level and is composed mostly of nitrogen. Many scientists believe Titan’s atmosphere may be similar to the conditions that existed on Earth, possibly allowing the development of primitive lifeforms.
Voyager 1 image (Press Release P-24067C).