The Huygens probe which landed on Saturn's giant moon, Titan, on Friday has carried out its audacious mission far more successfully than anyone had expected, sending back extraordinary images and sounds. The data will take time to interpret properly, but the early signs are that Titan's surface is unique.
After the detailed but ambiguous maps produced by the probe's NASA mothership, Cassini, the images from Huygens seem strikingly real. Dark, winding features crossing one bright region are probably drainage channels that cut through icy highlands. There are flat, dark expanses that may be flood plains, according to the leader of the imaging team, Marty Tomasko.
The European Space Agency probe also recorded the sound of its harrowing descent through the Titan's atmosphere, and has produced an audio clip of the probe's radar blips, with the aid of The Planetary Society.
And at ground level, Huygens captured images of a field of rounded rocks, which might have been eroded into shape by liquid flow, like pebbles in an Earthly stream.
If these tentative hypotheses hold up, this new world has been largely shaped by liquids. At the temperatures on Titan's surface, around -170°C, liquid ethane or methane could form floods or streams. There are few clouds, but that may simply mean that rainstorms are rare but intense, scientists say.
And at the Huygens landing site, the liquid may not be far away. When the probe hit Titan's surface, a metal rod instrument attached to its base penetrated 15 centimetres into the ground. During that fraction of a second, the force of the impact peaked very quickly and then fell back to a roughly constant level, suggesting that the surface has a hard crust with softer material underneath - a consistency that one of the mission's surface science team compared to a crème brûlée.
"You get the feeling that maybe this area was wet not so long ago and maybe liquid hasn't penetrated too far into the surface," says Tomasko.
The basic material at the landing site, as with other dark areas of Titan, is probably tar-like material that has settled out of the atmosphere, but scientists are not sure. Spectral data taken by Huygens may reveal more when they are properly analysed and compared with lab measurements.
Scientists will have to wait to find out more about the complex organic chemistry of Titan's atmosphere, which they hope will give them some clues about the origin of life on Earth. Two instruments have sampled gases and aerosols in the atmosphere, but it could take months to analyse the data.
So far, the chemical analysis teams have only reported their measurements of one gas: Huygens' gas chromatograph and mass spectrometer instrument detected a puff of methane on impact, implying that there is some kind of liquid or solid methane reservoir on the surface, perhaps in the form of frost or dew. And the camera spotted what might be a fog bank which could also be composed of methane.
The picture of Titan may change radically over the coming months, but should only get more interesting. "It's nothing like Mars or Venus," says Titan expert Ralph Lorenz. "It has a flavour all of its own."
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