BioEd Online

Saturn's giant moon has rain and rivers very similar to those on Earth, the planetary probe Huygens has discovered. But on Titan they consist of liquid methane rather than water.

The mission scientists presented their results at the European Space Agency's head office in Paris, exactly a week after the craft's spectacular landing on 14 January.

Photographs of the distant world revealed that Titan's rain has shaped a system of river beds and basins, by draining from the ridges to the plains. The rivers and lakes appear to be dry at the moment, but methane rain may have fallen within the past few weeks.

As the probe landed, its penetrometer pushed 15 centimetres into the ground, which has the consistency of loose sand. Its impact generated enough heat to release methane gas, which was detected by the craft's mass spectrometer. The presence of methane at the surface hints that liquid methane rain must have fallen recently.

On Earth, methane is generated mainly by biological agents such as bacteria. But on lifeless Titan, it must have come from elsewhere. Tobias Owen of the University of Hawaii's Institute for Astronomy, Honolulu, thinks that the gas probably bubbles up from the moon's insides.

This theory is supported by the analysis of atmospheric gases during Huygens' descent. Whereas the concentration of nitrogen (the main component of Titan's atmosphere) was stable, the methane concentration increased during the final three minutes of the craft's journey. Mission scientists also suspect that the white haze seen in the first picture is mainly methane.

Hard facts

The composition of Titan's solid ground has thrown up a shock: it's made of water ice rather than rock. Photographs of the landscape that show the same region from two different angles have highlighted a clear distinction between the bright hilltops and darker valleys.

Hydrocarbon particles may have been washed from the ice by methane rains, pooling in recesses such as river basins to give the darkened effect, speculates Martin Tomasco of the University of Arizona, Tucson, who is responsible for Huygens' on-board camera system.

The mission has also found signs of volcanic activity on Titan. An analysis of the element isotopes found at the surface suggests that the volcanoes spewed out water and ammonia, rather than molten lava.

More results can be expected as scientists continue to scrutinize the wealth of data beamed back by Huygens, which now lies abandoned on Titan's frozen surface. The mother ship Cassini, which carried Huygens on its seven-year journey, will continue to explore Saturn and its moons for another three years at least.