Scientists have deciphered the enigma behind one of the the dark, lake-like features observed at the surface of Saturn’s moon, Titan. Before the Cassini mission, scientists believed Titan to be covered by oceans of methane, ethane and other light hydrocarbons, but with the help of an instrument aboard the spaceship, they positively identified liquid hydrocarbons, as well as ethane, in one of the lakes, making Titan the only body in our Solar System to have liquid on its surface, alongside Earth.

The instrument aboard Cassini made the discovery based on the way the lake-like features absorb and reflect infrared light. “This is the first observation that really pins down that Titan has a surface lake filled with liquid,” said Bob Brown, from the University of Arizona, Tucson, team leader of Cassini’s visual and mapping instrument.

The lake is situated in Titan’s south polar region and the observations were made during a close flyby in December 2007. Known as Ontarius Lacus, the lake covers an area of 7,800 square miles (20,000 square kilometers), larger than the North America’s Lake Ontario, and extends 150 miles (235 kilometers), NASA scientists reported.

Titan is Saturn’s largest moon and the only moon known to have such a dense atmosphere, which consists of 95 percent nitrogen and five percent methane. The ethane and hydrocarbons also identified in the atmosphere are products from atmospheric chemistry caused by the breakdown of methane by sunlight, NASA explained. The hydrocarbons sometimes react even further and form aerosol particles.

The fact that the atmosphere is so dense makes the identification of the materials on the surface a very difficult one. In order to establish the composition of the Ontarius Lacus, Cassini managed to peer through the atmosphere that extends over 600 miles above the surface, by using a technique that removes the interference from the atmospheric hydrocarbons. This is how liquid ethane was identified.

“Detection of liquid ethane confirms a long-held idea that lakes and seas filled with methane and ethane exist on Titan,” explained Larry Soderblom, Cassini interdisciplinary scientists with the U.S. geological Survey in Flagstaff, Ariz. “The fact that we could detect the ethane spectral signatures of the lake even when it was so dimply illuminated, and at a slanted viewing path through Titan’s atmosphere, raises expectations for exciting future lake discoveries by our instrument.”

The liquid cycle on Titan is different from that of Earth based on an obvious difference: instead of water, Titan has methane. The Cassini observations have led scientists to believe that there is no water ice, ammonia, ammonia hydrate or carbon dioxide in Ontarius Lacus.

The data returned from Cassini’s visual and infrared mapping spectrometer revealed that the lake is actually evaporating, although enough liquid still remains. But scientists say the phenomenon is normal, considering that the south pole has just gone through summer.

The images revealed that the lake has a smooth surface, not troubled by any wave as scientists would have expected, it’s ringed by a dark beach, a bright shoreline, and a shelf, which became exposed upon evaporation.

Whether Titan is able or will ever be able to support life, the question is still hard to answer. But what scientists can tell us for now is that its atmosphere is very similar to Earth’s atmosphere in its early days, which suggests that if the right chemical reactions start taking place, we could have the same conditions on Titan as we did on Earth when the first forms of life appeared.