The lake, called Ontario Lacus, covers about 7,800 square miles at Titan's south pole, and is filled mostly with liquid methane and ethane, say researchers at the University of Arizona's Lunar and Planetary Laboratory in Tucson, Ariz.

"This is the first observation that really pins down that Titan has a surface lake filled with liquid," said lead researcher Robert Brown.

The finding, reported Wednesday by SPACE.com and detailed in the July 31 issue of the journal Nature, confirms the long-held idea that Saturn's moon, which is 150 percent bigger than Earth's moon, contains such gases, said Larry Soderblom, a researcher with the U.S. Geological Survey in Flagstaff, Ariz.

Titan is the only solar system moon known to support a planet-like atmosphere, Space.com noted.

The scientists used the infrared abilities of the spacecraft Cassini to see through Titan's atmosphere. Their measurements showed the lake's shoreline, a beach-like area and the lake's interior.

"It seems to have partially evaporated, and that makes sense because the south pole has just gone through summer," Brown told SPACE.com.