NASA scientists have announced that at least one of the giant lakes previously spied on Saturn's moon Titan contains liquid hydrocarbons - making it the "only body our solar system beyond Earth known to have liquid on its surface", as the agency puts it.

The lakes were identified by the Cassini spacecraft, and although scientists couldn't be sure they were liquid, "their dark appearance in radar indicates smoothness and their other properties point to the presence of liquids".

Investigators further suspected these features would be filled with methane, ethane and other light hydrocarbons, but Titan's hazy atmosphere - 95 per cent nitrogen and five per cent ethane, methane and other hydrocarbons - made confirmation difficult.

Back in December last year, though, Cassini used its visual and mapping instrument to cut through the atmospheric hydrocarbon smog and probe "chemically different materials based on the way they absorb and reflect infrared light" in the lake dubbed Ontario Lacus, lying in Titan's south polar region. The feature is roughly 7,800 square miles in area, "slightly larger than North America's Lake Ontario", NASA notes.

The resulting data showed that the lake contains liquid hydrocarbons, and certainly ethane. Larry Soderblom, a Cassini interdisciplinary scientist with the US Geological Survey, said: "Detection of liquid ethane confirms a long-held idea that lakes and seas filled with methane and ethane exist on Titan. 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."

NASA explains that the ethane is present in "a liquid solution with methane, other hydrocarbons and nitrogen". It adds: "At Titan's surface temperatures, approximately -300°F (-184°C), these substances can exist as both liquid and gas. Titan shows overwhelming evidence of evaporation, rain, and fluid-carved channels draining into what, in this case, is a liquid hydrocarbon lake."

Cassinin's obervations "also suggest the lake is evaporating", since it's "ringed by a dark beach, where the black lake merges with the bright shoreline". The spacecraft "also observed a shelf and beach being exposed as the lake evaporates".

Soderblom concluded: "During the next few years, the vast array of lakes and seas on Titan's north pole mapped with Cassini's radar instrument will emerge from polar darkness into sunlight, giving the infrared instrument rich opportunities to watch for seasonal changes of Titan's lakes." ®

Related stories

• Robot nuclear windjammer to sail patio-gas oceans of Titan (15 October 2009)
• Boffin: Titan moon largely made of LPG, not cheese (7 April 2009)
• Boffins plan Titanic balloon trip (12 November 2008)
• John Glenn blasts Moonbase-to-Mars NASA roadmap (1 August 2008)
• Boffins reveal yet more of Titan's secrets (4 June 2007)
• Cassini spies titanic lake on Titan (14 March 2007)
• Mile-high mountains pop up on Titan (13 December 2006)
• Cassini spots ethane clouds on Titan (18 September 2006)
• Could there be lakes on Titan? (26 July 2006)
• Boffins map surface of Titan by accident (25 July 2006)
• Cassini finds land on Titan (20 July 2006)
• Cassini radar discovers space Arabia (5 May 2006)
• Earth may have 'infected' Titan with life (20 March 2006)
• Boffins mull life on Titan (25 July 2005)
• Is that a lake on Titan? (29 June 2005)
• Say what you like about Saturn: its moons really have atmosphere (17 March 2005)
• Titan: rains of methane, mountains of ice (22 January 2005)
• ESA shows off Titanic views (18 January 2005)