Illustration depicting the view from the Titan surface with Saturn in the sky. Titan is the only moon in the Solar System with clouds and a thick atmosphere primarily composed of nitrogen, along with traces of methane and other hydrocarbons. It is also the only celestial body, aside from Earth, where stable bodies of surface liquid have been found. On 14 January 2005, the European Space Agencys (ESA) Huygens probe landed on the surface of Titan, just off the easternmost tip of a bright region called Adiri. The probe photographed hills with rivers running down to a plain. The current understanding is that the hills are made mainly of water ice. Dark organic compounds, created in the upper atmosphere by the Suns ultraviolet radiation, may rain from Titans atmosphere. These compounds are carried down the hills with the methane rain and are deposited on the plains.