A dark patch on the surface of Titan, moon of Saturn, might be a lake filled with liquid hydrocarbons, astronomers have said.

The picture, snapped by the Cassini spacecraft, shows a dark shape 235km by 75km in the moon's southern polar region (the red dot marks the pole itself). Scientists are hesitating to pronounce the object a lake with any kind of certainty, because no open bodies of liquid have yet been found on the moon, despite predictions that they should be there.

However, according to the imaging team: "The shore-like smoothness of its perimeter and its presence in an area where frequent convective storm clouds have been observed by Cassini and Earth-based astronomers make it the best candidate thus far."

Other possible explanations of the feature include it being a sinkhole, or an old volcanic caldera, filled with dark, solid hydrocarbons. It could also be an old lake that has dried up, leaving behind dark hydrocarbon deposits.

One way of finding out for sure whether or not the feature is a lake will be to look for glints of reflected sunlight on the surface. So far, however, the space craft has not been in the right position to see any reflections. A further 39 flybys are planned, and the imaging team says it will be looking at the area again in greater detail. ®

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