THE world got its first peek at the surface of Saturn鈥檚 moon Titan last week. The images were taken as NASA鈥檚 Cassini-Huygens spacecraft swept past the moon at a distance of less than 1200 kilometres, the first of many fly-bys planned in the next few years.
The images show a landscape that is clearly still being shaped. Although Titan must have suffered numerous meteor impacts in the past, its surface today is largely crater-free. Somehow these scars must have been eroded or filled in. 鈥淲e are seeing a place that is alive, geologically speaking,鈥� says Charles Elachi, head of the team running Cassini鈥檚 radar instrument.
Titan鈥檚 basic geology is unique. The moon is thought to have a thick crust of water ice mixed with ammonia, but evidence is emerging that this may be covered by another layer of organic material. During the fly-by on 26 October, Cassini picked up microwaves from the surface that look like the thermal glow of hydrocarbon molecules. 鈥淭itan really is covered in organics,鈥� says radar team member Ralph Lorenz of the University of Arizona in Tucson. Scientists believe these hydrocarbons are created in the atmosphere when methane is broken down by sunlight and its components recombine into more complex molecules 鈥� a theory supported by the detection last week of benzene and acetylene high in the atmosphere.
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Titan鈥檚 surface has thrown up other puzzles too. Infrared and radar images reveal bright 鈥渋slands鈥� surrounded by darker material, often crossed by long narrow features. These long lines 鈥� perhaps canyons, ridges or cracks 鈥� are up to 100 kilometres long but less than 200 metres wide. Just what these features are and how they formed is the focus of intense discussion.
鈥淭he images show a landscape that is clearly still being shaped. We are seeing a place that is alive, geologically speaking鈥�
The images also show bright lobe-shaped areas like volcanic flows that may have formed when the water-ammonia crust melted, flowed onto the surface and refroze into icy lava sheets.
One disappointment is the lack of definite evidence for oily lakes and seas that most scientists had expected to see. Fluid on the surface should reveal itself in images by bright glints where surface waves reflect light from the sun, but so far there are no signs of this. However, the probe鈥檚 radar has found areas that look very dark, which might mean that its beam hit a flat, mirror-like lake surface and bounced away from the radar receiver. 鈥淭hese might be lakes of liquid.鈥� says Elachi.
Titan also appears to have lost much of its original atmosphere. The moon has an unusually high abundance of nitrogen-15, compared with the lighter isotope nitrogen-14. That could be explained if most of the atmosphere had evaporated into space, a process in which the nitrogen-14 would have escaped more easily than nitrogen-15. What could cause such a loss is unknown, but it would mean that Titan once had an atmosphere 40 times as thick as Earth鈥檚 鈥� making it a dwarf version of a gas planet.
鈥淭his bizarre world may be far more complex that we have begun to imagine,鈥� says Larry Soderblom of the US Geological Survey in Flagstaff, Arizona.