It's been raining liquid methane on Titan. That's according to an analysis of just-released images revealing a possible new lake in the south polar region of Saturn's largest moon.

Measuring about 3,200 miles (5,150 kilometers) across, Titan is larger than the planet Mercury and about 40 percent the diameter of Earth. It is the only moon in the solar system with a dense, planet-like atmosphere (10 times denser than Earth's).

The possible new lake showed up in images taken in 2005 by the Cassini spacecraft's Imaging Science Subsystem (ISS). Cassini's photos taken in 2004 of that same region didn't show such a dark spot.

Scientists think the most plausible explanation for the sudden appearance is a lake filled by recent rainfall.

This conclusion, detailed in the Jan. 29 issue of the journal Geophysical Research Letters, confirms the thinking of various Cassini scientists who have suspected the presence of such lakes on Titan, as well as methane rain.

Methane deluge

The new analysis, by Elizabeth Turtle, Cassini imaging team associate at the Johns Hopkins University Applied Physics Lab in Laurel, Md., and her colleagues, provides more evidence for the link between methane rain and the springing up of a lake on the surface of Titan.

Turtle's team noted a possible source for the rainfall — a team using the Keck Observatory spotted a large outburst of clouds over Titan's south pole in October 2004. Cassini images also show cloud systems above the south polar region during that intervening year.

"The clouds, when we look at them over a period of a few hours, behave like convective clouds, the same way you have thunderstorms on Earth," Turtle told SPACE.com. "If you watch them over a few hours, they kind of billow upwards the same way thunderheads do on Earth."

She added that the dark features "could be the result of a downpour from this big system of clouds that was seen in October."

Titan's northern lakes

Turtle's team also examined more recent Cassini images taken in 2008 of Titan's northern latitudes, sections of which had not been imaged with the ISS until now. Comparisons of the south-pole images with those of the north pole confirmed greater stores of liquid methane in the northern hemisphere compared with the southern hemisphere, Turtle said.

Those liquid stores at the north pole could grow as summer arrives at the northern hemisphere and storm clouds dump liquid hydrocarbons onto the area.

Some of the known north polar lakes are large. If full, Kraken Mare, with an area of 154,000 square miles (400,000 square kilometers) would be almost five times the size of North America's Lake Superior. Together, the lake areas in the northern region observed with Cassini's ISS cover more than 197,000 square miles (510,000 square kilometers), an area that is nearly 40 percent larger than Earth's Caspian Sea.

Underground reservoirs

One remaining question is whether such lakes could resupply Titan's atmosphere with hydrocarbons over geological time scales. Over time, chemical reactions in Titan's atmosphere destroy the hydrocarbons.

"Our new map provides more coverage of Titan's poles, but even if all of the features we see there were filled with liquid methane, there's still not enough to sustain the atmosphere for more than 10 million years," Turtle said.

Combined with previous analyses, the new observations suggest that underground methane reservoirs must exist.

Funding for the Cassini program is set to end Sept. 30, 2010, though scientists say the spacecraft could continue to return valuable data and images for years to come. Next month, Cassini mission officials are expected to present their case to NASA headquarters for a seven-year extension and associated funding.

• Video: Parachuting onto Titan
• Images: Cassini's Latest Discoveries
• Cassini Special Report