Nasa scientists said today they had discovered the first direct evidence that the surface of Saturn's largest moon, Titan, has lakes of liquid hydrocarbons - the only object other than Earth to have standing liquids on its surface.

But don't expect Titan to become a holiday resort any time soon: it is 1.2bn kilometres away, has temperatures falling to -139C and features the occasional downpour of methane rain.

Titan is the second largest moon in the solar system after Jupiter's Ganymede. It has a thick atmosphere and is larger than the planet Mercury.

Nasa's research used data from the Cassini-Huygens probe - a joint venture by Nasa, the European Space Agency and the Italian Space Agency - which arrived there in 2004. More than 40 flybys of Titan by the probe established that the moon did not have large global oceans of methane, ethane and other light hydrocarbons as scientists had previously suspected. Instead, Cassini sent back images of hundreds of dark lake-like features, but it was not clear whether these were liquid or dark solid material.

The researchers looked closely at a 20,000 square-mile dark region called Ontario Lacus near Titan's south pole that was observed by Cassini in December last year. The team trained an instrument on the lake that can chemically identify material based on the way it absorbs and reflects infrared light. They confirmed that Ontario Lacus was liquid ethane.

The results were published in today's edition of Nature. "This is the first observation that really pins down that Titan has a surface lake filled with liquid," said Dr Bob Brown of the University of Arizona, Tucson, who was involved in the research.

Due to Titan's distance from the sun and an anti-greenhouse effect caused by Titan's orange-hazy atmosphere, no liquid water can exist on its frigid surface. The moon's atmosphere is 95% nitrogen, with the rest methane, but ethane and other hydrocarbons can form from the breakdown of methane by sunlight.

"Detection of liquid ethane confirms a long-held idea that lakes and seas filled with methane and ethane exist on Titan," said Dr Larry Soderblom from the US Geological Survey in Flagstaff, Arizona. "The fact we could detect the ethane spectral signatures of the lake even when it was so dimly illuminated, and at a slanted viewing path through Titan's atmosphere, raises expectations for exciting future lake discoveries by our instrument."

Cassini has also observed a dark beach fringing the foot-shaped lake. A shelf and more of the beach are being exposed as Ontario Lacus evaporates.