BioEd Online

More images from Titan have confirmed scientists' expectations that a complicated cycle of weather is shaping the surface of Saturn's largest moon.

"It looks as if there are rivers, cliffs, lakes and clouds," says Tony McDonnell, part of the Huygens probe's surface-science team from the Open University, Milton Keynes, UK. "It's the full range of geography."

The pictures show terrain that is covered with sinuous drainage channels winding around a range of hills towards a dark area that looks like an ocean.

The interpretation of these pictures should be obvious to anyone who has seen an aerial view of Earth, says Peter Smith from the University of Arizona, Tucson, who helped to develop Huygens' descent imaging system. "We see the same features every time we fly over a coastline."

"There's no evidence yet of liquid in these rivers, they look more like Arizona's dried-up river beds," Smith adds. "But those canyons didn't carve themselves."

The surface rubble snapped by Huygens also bears the marks of flowing liquid hydrocarbons such as methane and ethane: "They're very smooth, there's definitely some fluvial process that shaped them," says Andrew Ball, another member of the Open University team.

Scientists had speculated that Titan could be a cold analogue for conditions on the primordial Earth, and these early discoveries bear that out, says McDonnell.

"There's no evidence for life," he cautions. "But we do see all three phases of matter, and transport mechanisms between them: evaporation, clouds, rain and rivers. This provides the opportunity for the development of complex organic molecules."

Fresh surface

Titan's surface also seems to be remarkably fresh. Counting the number of craters on a moon and comparing it with the number of meteoroids in the area gives astronomers a way to calculate the age of a moon's surface. The younger the crust, the fewer craters are visible.

"One thing's for sure: Titan's not cratered. I haven't seen a single one," says Smith.

Unlike the other moons in the Solar System, Titan has a thick atmosphere, so smaller meteors may burn up before they hit the ground. This makes it more difficult to estimate the age of the surface, but "it's probably less than 10 million years old", Smith says.

"In some areas, the surface is very bright," he adds. This suggests that liquids wash dark tars from the icy surface quite regularly. This might be a sign of methane rain.

Smith is now piecing together images taken in the last kilometre above the landing site, which was illuminated by a lamp on the bottom of the probe.

Big bang

In the meantime, McDonnell has been studying results from Huygens' impact sensors. "It had quite a big bang, hitting the surface at about 3.5 metres per second," he says. That's equivalent to 12.6 kilometres per hour.

The probe has made a dent in Titan's surface that is a few centimetres deep. The soil seems to be made up of grains of water ice glued together by sticky hydrocarbons such as methane, giving it the consistency of putty, says McDonnell.

Huygens hit the ground just 8° from vertical, suggesting that winds were not strong enough to blow it around much. The mosaic image released today confirms this. Because Huygens' on-board camera was looking down at a slight angle, it took pictures of the ground beneath looking in all directions as the falling probe spun round. But it was not able to see directly downwards.

If the probe had been drifting during its descent this wouldn't have mattered, but the dark patch in the centre of the mosaic means that the probe must have remained over the same area of ground as it fell.

Smith admits he was surprised that Huygens survived its crash landing. "We had more than an hour of extra data from the surface, and that's a wonderful addition," Smith says. "Thank God the parachute didn't fall on top of the probe."

Mission scientists expect to release more details on 21 January. A series of scientific meetings over the next two months will allow the different teams to compare their data to get a better understanding of Titan's geography.