Titan is remarkable because it is the only known moon in the solar system that has a substantial atmosphere—largely nitrogen with a minor amount of methane and a rich variety of other hydrocarbons. Its surface is completely hidden from view (except at infrared and radio wavelengths) by a dense, hazy atmosphere.
The diameter of Titan is 3,200 mi (5,150 km), and it is the second-largest satellite in the solar system after Jupiter's Ganymede. Titan is larger than the planet Mercury.
Titan's surface temperature is about –280°F (–175°C), and its surface pressure is about 50% greater than the surface pressure of Earth. In 1990, radio telescope data showed that Titan reflects and scatters radio waves, suggesting that the satellite has a solid surface, possibly with small hydrocarbon lakes or ponds.
Infrared images of Titan taken in late 1999 by the W. M. Keck II telescope in Hawaii also revealed features that could be frozen land masses separated by frigid hydrocarbon seas and lakes. Other features might be highlands, and one dark area appeared to be a large impact crater or basin.