NASA's Cassini spacecraft captured a series of images, released today, showing a marking, darker than anything else around it. It is remarkably lake-like, with smooth, shore-like boundaries unlike any seen previously on Titan.

"I'd say this is definitely the best candidate we've seen so far for a liquid hydrocarbon lake on Titan," said Dr. Alfred McEwen, imaging team member and a professor at the University of Arizona, Tucson.

The putative lake measures 230 kilometers by 70 kilometers (145 miles long by 45 miles) wide, about the size of Lake Ontario, on the U.S. Canadian border.

The feature lies in Titan's cloudiest region, which is presumably the most likely site of recent methane rainfall. This, coupled with the shore-like smoothness of the feature's perimeter makes it hard for imaging scientists to resist some speculation about what might be filling the lake, if it indeed is one.

Cassini imaging team member Dr. Tony DelGenio of NASA's Goddard Institute for Space Studies in New York, said, "It's possible that some of the storms in this region are strong enough to make methane rain that reaches the surface. Given Titan's cold temperatures, it might take a long time for any liquid methane collecting on the surface to evaporate. So it might not be surprising for a methane-filled lake to persist for a long time."

A computer-enhanced image of the feature and a three-frame movie showing the evolution of nearby clouds can be seen here.

Despite earlier predictions, no definitive evidence for open bodies of liquid has yet been found on Titan. Cassini has not yet been in a favorable position for using the camera to check for glints from possible surface liquids in the south polar region.

"This feature is unique in our exploration of Titan so far," said Dr. Elizabeth Turtle, Cassini imaging team associate at the University of Arizona. "Its perimeter is intriguingly reminiscent of the shorelines of lakes on Earth, which are smoothed by water erosion and deposition."

An alternate explanation is that this feature was once a lake, but has since dried up, leaving behind dark deposits, Turtle said. Yet another possibility is that the lake is simply a broad depression filled by dark, solid hydrocarbons falling from the atmosphere onto Titan's surface.

In this case, the smooth outline might be the result of a process unrelated to rainfall, such as a sinkhole or a volcanic caldera.

Dr. Torrence Johnson, an imaging team member at NASA's Jet Propulsion Laboratory, said, "It reminds me of the lava lakes seen on Jupiter's moon, Io."

"Eventually, as the seasons change over a few years, the convective clouds may migrate northward to lower latitudes," said DelGenio, "If so, it will be interesting to see whether the Cassini cameras record changes in the appearance of the surface as well."

"It is already clear that whatever this lake-like feature turns out to be, it is only one of many puzzles that Titan will throw at us as we continue our reconnaissance of the surface over the next few years," said Dr. Carolyn Porco, imaging team leader at the Space Science Institute in Boulder, Colo.

Thirty-nine more Titan flybys are planned for Cassini's prime mission. In future flybys the science teams will search for opportunities to observe the lake feature again and to look for mirror-like reflections from smooth surfaces elsewhere on Titan. Such reflections would strongly support the presence of liquids.

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