Titan is larger than the planet Mercury. It is also the second largest moon in the Solar System, and the only moon with a dense atmosphere. Just as on Earth, the main gas is nitrogen, but Titan is much colder. Its surface is in deep freeze at minus 180 degrees Celsius. If it became warmer, Titan might be able to support life.
Until the arrival of the NASA-ESA Cassini-Huygens spacecraft in 2004, little was known about Titan. Its surface is hidden beneath a thick blanket of orange cloud.
On 14 January 2005, ESA's Huygens probe made an historic landing on Titan. During its 2½ hour descent, Huygens was blown eastward by high level winds of more than 100 km per hour. The winds reversed direction at a height of 7 km, carrying Huygens gently westward during the final minutes of the descent. Huygens found an icy landscape marked by dark channels snaking towards a smooth ‘shoreline’.
Titan’s atmosphere is about 5 per cent methane. The action of sunlight on the methane creates the thick orange haze. (On Earth, methane gas is burnt in boilers and cookers.) Methane rainstorms sometimes fill the dry channels, creating temporary rivers that wear away the surface and carry loose material onto nearby plains.
During more than 70 flybys of Titan, Cassini’s radar and infrared instruments have mapped much of the surface. They have found many strange features, including huge dunes of hydrocarbon sand, mountain ranges, methane lakes and possible ice volcanoes. (Hydrocarbons are made of hydrogen and carbon. Examples on Earth are coal, soot and tar.)