A newly discovered lake near Titan's equator suggests the possibility that methane is being leaked into the atmosphere from a source hidden under the surface of Saturn's moon.

The discovery of the 927 square-mile methane lake was a surprise for scientists, who previously believed lakes were only present at Titan's poles. The lake is in a tropical region of Titan characterized by vast dune fields. In addition to the lake, several small ponds are also scattered around the area, with shallow depths of just a few inches, similar to marshes on Earth.

While the region does receive some rain, in the form of methane, this rain quickly evaporates from the sun's energy, and is transported through circulation to the poles, where several large lakes exist. Thus it's suspected that the large tropical lake is the result of excess methane venting up from beneath the surface of the lake.

"This discovery was completely unexpected because lakes are not stable at tropical latitudes," said University of Arizona planetary scientist Caitlin Griffith.

Methane sources beneath Titan's surface leaking into the atmosphere have long been suspected as an explanation for the otherwise vast amounts of methane that make up Titan's atmosphere; methane that is destroyed over time by solar rays.

Hints of subterranean methane were first discovered by the Huygens probe, which landed on the surface of Titan in 2005.

"While rainfall may have carved the downhill drainage features, the flood plain, in which the probe landed, was caused by liquid flowing from a different source of methane. In addition, a puff of methane was detected upon landing, which suggests that the site was damp" explained Griffith.

It was near Huygens' landing site that images from Cassini, the spacecraft which has been orbiting Saturn since 2004, showed a dark region, which was eventually determined to be the methane lake.

How methane has pooled under Titan's surface could be explained by the changing orbit of Titan around Saturn. It is believed Titan's orbit is now less elliptical than it was in the past. This changing orbit would have caused a large gravitational pull on the moon, leading to powerful tides and the release of heat which could have caused the formation of methane beneath the surface.

The findings of the discovery of Titan's methane lake appear in the June 14 issue of the journal Nature.