for National Geographic News
Methane rains on Saturn's moon Titan may have created a new lake about four times the size of Yellowstone National Park, scientists say.
The new lake covers about 13,000 square miles (34,000 square kilometers). It's part of a system of lakelike features around Titan's south pole.
Scientists have been studying what appear to be methane lakes near both of Titan's poles since the craft arrived in the Saturnian system in 2004. The work suggests the large, frigid moon also has methane rain.
The new lake could simply be a shallow marsh, the scientists admit, but data suggest the rainstorm that created it might have been torrential enough to form something deeper.
"Models of thunderstorms on Titan have shown that you can get tens of centimeters [several inches] of rain from a single storm," said Elizabeth Turtle, a planetary scientist from the Johns Hopkins University Applied Physics Lab, who announced the find yesterday.
Team member Tony DelGenio of NASA's Goddard Institute for Space Studies called the find "the most direct evidence yet for the idea that there is liquid methane on [Titan's] surface."
Pictures taken by NASA's Cassini spacecraft in 2005 showed a large lakelike body that wasn't there a year earlier.
The pictures also show that over that year, giant thunderhead-like clouds had formed in the area where the lakelike feature appeared.
"We see clouds that behave very much as clouds on Earth, and we see evidence of flooding on the surface, just as a lot of people in the [U.S.] Midwest saw last year," Turtle said.
Cassini scientists hadn't noticed the new lake before, DelGenio added, because they only went looking for such features when they realized the area had seen heavy rains.
The scientists are excited about what the discovery reveals about Titan's climate.
When Cassini arrived in 2004, it was summer in Titan's southern hemisphere and winter in the north. Storm clouds appeared mostly near the south pole.
Now it's getting close to the spring equinox and there are no longer any southern storms.
"But more have popped up at mid-latitudes and occasionally at low latitudes," DelGenio said.
"The longer we stay, the more we get to see the seasons progress," he said. "A few years down the line, in 2015, 2016, and 2017—if we last that long—we may be able to see the northern summer."
Cassini completed its planned four-year mission in 2008 and is now operating on an extended mission slated to last until 2010. But the craft is healthy, and mission managers are hoping for funding to further extend its working life.
Ralph Lorenz, a colleague of Turtle's at the Johns Hopkins lab who was not directly involved in the study, says that studies like these can help us understand climate change on Earth.
"Titan's atmosphere can hold a lot of [methane] moisture," he said in an email, "and consequently has violent rainstorms separated by long droughts.
"It may be that global warming on Earth will take us slightly in a Titan-like direction, with heavier downpours separated by longer droughts."
SOURCES AND RELATED WEB SITES