Published online 21 February 2005 | Nature | doi:10.1038/news050221-2


Titanic complexity pleases planet scientists

Cassini beams back data on ammoniac lava and 'cat scratch' formations.

Cassini's views of Titan show craters and scratches. Click here for more pictures  type='special.Cassini's views of Titan show craters and scratches. Click here for more pictures' type='special.© NASA/JPL/Space Science Institute

Titan is turning out to be just as complicated as scientists had hoped. A fly-by of the saturnian moon has revealed varied terrain, including two impact craters and some mysterious parallel lines. And data sent back from the European Space Agency's Huygens probe hint that a complex mix of organic molecules could be present in its ice.

In a close fly-by of Titan on 15 February, the Cassini spacecraft, on which Huygens hitched a lift, scanned a strip of ground around the moon's equator, equivalent to about 1% of its surface.

"We got some stunning data," says Cassini project scientist Denis Matson. He and his colleagues discussed their findings on 18 February at the annual meeting of the American Association for the Advancement of Science (AAAS) in Washington DC.

A previous fly-by on 26 October 2004 revealed features that seem to have been caused by volcanic flows, which were probably a frosty mixture of ammonia and water and had the consistency of basalt lava.

Similar features showed up in the latest pass, says Jonathan Lunine of the University of Arizona, Tucson, who is a member of the Cassini-Huygens team. But this time, he says, he was struck by the diversity of the terrain. "The rest of the strip is very different."

Titanic circus

The most impressive feature is a huge impact crater, around 440 kilometres across, which Lunine and his team have named Circus Maximus. The crater has been considerably eroded over time, with roughened edges, and various hills and channels scarring its centre.

Further east lies a smaller crater, just 60 kilometres across. "It's beautiful," says Lunine. "This one is much better preserved." The scientists estimate that the crater was created when a comet or asteroid roughly 5 to 10 kilometres wide slammed into Titan.

Icy Enceladus is covered in cracks and ridges. Click here for more pictures  type='special.Icy Enceladus is covered in cracks and ridges. Click here for more pictures' type='special.© NASA/JPL/Space Science Institute

The main message from the craters, however, is that Titan has much fewer of them than expected for its age. Volcanic flows and tectonic activity are continually reshaping the terrain, concludes Lunine. He estimates from the number of craters found so far that the surface is less than a quarter of the age of the Solar System.

Elsewhere lie dark plains and channels, like the ones the Huygens probe saw on its descent to the moon's surface on 14 January (see Latest Titan pictures show details of geography). And there are some intriguing dark lines. Perfectly parallel, they have been dubbed "cat scratches", but scientists are mystified about how they formed. Lunine speculates that they could be sand dunes shaped by the wind.

Chilly Enceladus

In a busy week for the spacecraft, Cassini made its first close fly-by of Saturn's moon Enceladus on 16 February. It snapped pictures of the icy moon's surface, which is the most reflective surface in the Solar System.

The craft flew just 1,180 kilometres away from the moon, and images reveal terrain fractured by cracks and covered in ridges. The lack of craters suggests that, like Titan, the surface of Enceladus is constantly being refreshed by a lava-like mixture of frozen chemicals.

Huygens surprises

Scientists analysing the data returned by the Huygens probe are also finding surprises. Although Huygens has confirmed the presence of large amounts of methane on Titan, they have found very little of the ethane they had expected.

“The photochemistry goes further than the models suggested, to more complex things.”

Toby Owen
Jet Propulsion Laboratory, Pasadena, California.

When methane molecules are broken down by ultraviolet radiation that has penetrated the atmosphere, two of the fragments generated can potentially combine to form ethane. But the relative lack of ethane suggests that the fragments may be reacting further, for example, by combining with the nitrogen in the atmosphere.

Toby Owen, Cassini's interdisciplinary scientist for the atmospheres of Titan and Saturn, presented the results to the AAAS meeting. "The photochemistry goes further than the models suggested, to more complex things," he says. "Methane and nitrogen are being broken apart in the atmosphere and are forming larger molecules."

The chemistry may resemble that of the early Earth, before life began. "That has us very excited," Owen says. Huygens detected organic molecules at its landing site, but Owen says it is not yet clear how complex they are. He's not expecting to find life because, "It's much too cool." But he says we might find some of the precursor molecules of life frozen in the ice, forming a "primordial ice cream". 

Jet Propulsion Laboratory, Pasadena, California.