Titan is the only planetary satellite with an atmosphere, which comprises nitrogen and hydrocarbons such as methane and ethane. With a diameter of 5,150 kilometres, Titan is larger than the planets Mercury or Pluto.

When the Huygens probe, which is hitching a ride on Cassini, descends to Titan's surface, scientists expect to find liquid hydrocarbon lakes, tarry ponds and icy outcrops of land. "We may see weather systems that invlove methane raining down on to the surface," adds Andrew Coates, a space scientist from University College London.