Shifting lakes seen on Saturn's moon - Scientists at NASA and APL observe dynamic lakes and clouds on Titan using Cassini-Huygens

Recent images from the Cassini-Huygens spacecraft have shown that Titan, Saturn's largest moon, has large, dynamic hydrocarbon lakes that are filled and modified by rainfall.

The detailed maps from Cassini's Imaging Science Subsystem (ISS) revealed that the seas are much more extensive than previously thought and that they are currently being transformed by cycles of evaporation and precipitation, much like on Earth.

Titan has fascinated astronomers for years. "It's been hypothesized for decades, since Voyager studied Titan's atmosphere, that there would be liquid on the surface," Elizabeth Turtle, a member of Cassini's imaging team at the Hopkins Applied Physics Laboratory, said. The two Voyager spacecrafts conducted flyby studies of Saturn and Titan in the early 1980s. The Voyager missions were the first to detect methane in Titan's atmosphere.

When Cassini approached Titan's south pole in July of 2004, during the southern hemisphere's summer, scientists saw atmospheric clouds and smooth, dark land features they believed to be lakes and seas. An enormous cloudburst was also observed that October.

Newly analyzed images from June 2005 showed that the most prominent dark features had remained the same over the course of a year, but there were also new, smaller dark features - lakes newly formed from precipitation.

Cassini's Visible and Infrared Mapping Spectrometer (VIMS) detected liquid ethane at the surface of the south pole. There also appeared to be channels like riverbeds carved onto Titan's surface, evidence of running liquid at work on the landscape.

Cassini scientists were recently able to process the spacecraft's images and see all these lines of evidence coming together to suggest that the dark features on Titan are seas and lakes created by running liquid and precipitation.

Fifty-five different lakes have been documented in the southern hemisphere. One of the largest is Kraken Mare, at 400,000 square kilometers. By comparison, Lake Superior has an area of 128,000 square kilometers.

So far, the lakes have only been seen in the polar regions. It is thought that the tropics do not receive enough precipitation to form lakes. Cassini's mission has been extended to 2010 to study Titan's northern hemisphere, which is now moving into its summer.

So far, the northern hemisphere shows more extensive lakes than its southern counterpart. This could be a result of seasonal changes or of inherent differences in the poles' geology.

Titan's weather system is believed to be very similar to Earth's. On Earth, liquid water is evaporated, precipitates and carves features like the Grand Canyon into the bedrock.

"Methane on Titan plays exactly the same role [as water on Earth]," Turtle said. "The bedrock on Titan is water ice. That's what's being eroded by the methane rains. The features on the surface suggest Titan's atmosphere behaves like the atmosphere in arid areas on Earth. There's not a lot of precipitation and when there is, it's in these huge deluges." Storms on Titan may last for days.

Titan is Saturn's largest moon, and second in size only to Jupiter's Ganymede in the solar system. Titan's radius is about 40 percent that of Earth's, making it slightly larger than Mercury.

Discovered in the 17th century by Dutch astronomer Christiaan Huygens, it is the only moon in our solar system with a substantial atmosphere and is so far the only body other than Earth to exhibit evidence of surface liquid.

The Cassini spacecraft was launched in 1997 and has been exploring Saturn since 2004. Its funding has been extended through 2010, and scientists hope to extend its mission to 2017, when Cassini will be able to study Titan's northern hemisphere during its summer solstice.

During the transition, scientists are hoping to observe temporary lakes forming near the equator, and large, convective cloud systems that are filling the lakebeds in the north.