Observations carried out with the Cassini spacecraft have once again confirmed the theory that Saturn's moon Titan has lakes on its surface, after picturing Ontario Lacus located in the south pole of the body, an accumulation of liquid methane and ethane with a size larger than that of the Lake Ontario in the North American continent.

"This is the first observation that really pins down that Titan has a surface lake filled with liquid," said Robert Brown of the University of Arizona's Lunar and Planetary Laboratory, Tucson, lead author of the study. Titan is the only moon in the solar system known to support a thick atmosphere. It has a size one and a half times that of the Moon and contains organic molecules, precursors to life.

Long before Cassini reached the Saturnian system, astronomers had predicted that Titan was covered by a global ocean of hydrocarbons. Later observations showed that it wasn't actually true, although they did reveal features on the surface similar to those of lakes and seas back here on Earth as well as erosion produced by flowing liquids. The radar techniques used to peer to the surface of the moon on the other hand, could not distinguish between liquid and solid.

"Detection of liquid ethane in Ontario Lacus confirms a long-held idea that lakes and seas filled with methane and ethane exist on Titan," said Larry Soderblom of the U.S. Geological Survey in Flagstaff, Arizona.

By making use of the visual and infrared mapping spectrometer on board the Cassini spacecraft in order to penetrate through Titan's atmosphere, extending as much as 1,000 kilometers above the surface, the research team was now able to image features such as the shoreline of the lake, which formed as the hydrocarbon mixture partially evaporated into the atmosphere.

"It seems to have partially evaporated, and that makes sense because the south pole has just gone through summer. The maximum evaporation will have already occurred or is in the process of occurring. There's still a lot of liquid left in that lake, and we don't think it's going to evaporate much further," Brown said.

Once the summer season is over, the lake is likely to fill up again as methane rains down on the surface much in the same way as water does on Earth. "Liquid ethane-methane mix would be less viscous than water. If riffling your fingers through it you wouldn't feel as much resistance," Brown said about the properties of the liquid content of the lake.