Liquid seas exist on the surface of just two worlds in the Solar System: Earth and Saturn's moon Titan. Discovered by NASA's Cassini spacecraft about a decade ago, the hydrocarbon seas of Titan are more exotic, of course, as they exist in liquid form at temperatures around -180 degrees Celsius.

Now, after the Cassini spacecraft has made a number of flybys of Titan, scientists assessing light and other radiation emanating from the moon's surface say they have a better handle on exactly what is in one of those seas. And to their surprise, they have found that the second largest lake on Titan, Ligeia Mare, is composed of nearly pure methane.

“We expected to find that Ligeia Mare would be mostly ethane, which is produced in abundance in the atmosphere when sunlight breaks methane molecules apart,” said Alice Le Gall, lead author of the new study. “Instead, this sea is predominantly made of pure methane."

Further analysis of the data, which assessed the transparency of the sea, indicated that this methane extends to a depth of about 160 meters in some locations, down to a sludge layer atop the moon's crust. This combination of sea and seafloor suggests that in Titan's atmosphere, nitrogen and methane react to form organic molecules, which then fall to the surface as a methane rain. The heaviest of these molecules, such as benzene, sink to the bottom of the sea.

Were you to boat upon Titan, then, you might gaze down into a clear, deep sea. Instead of being colored blue like Earth's oceans, however, these clear seas would bear the yellowish and orangish hues of Titan's skies. And might anything live in those oceans? It's a tantalizing possibility to consider.