AP

The “stream” is to the bottom right

A PICTURE may be worth a thousand words. But when the picture in question is of an alien world, it is difficult to be sure what those thousand words should be. And in the case of the images that have arrived from Titan, Saturn's largest moon, that world is very alien indeed.

On January 14th Huygens, a space probe built by the European Space Agency (ESA), landed on Titan and began to deliver its precious cargo of data to anxiously waiting scientists. The most striking finding so far is a picture taken as the probe descended. It appears to show pale hills criss-crossed with drainage channels containing dark material, leading to a wide, flat darker region. The landing site itself produced less striking, but still significant images. It is flat, strewn with rounded pebbles and appears to be a dry riverbed.

On Earth, or even on Mars, drainage channels and rounded pebbles would be taken as evidence for the erosive effects of liquid water. But at -180°C, Titan is too cold for water to be liquid. It is, however, not too cold for various hydrocarbons to be so (indeed, the most likely candidates, methane and ethane, are gases at terrestrial temperatures). Many people have suggested that Titan's dark regions might be lakes made of such hydrocarbons, or of tar that is composed of hydrocarbons which are too cold to be truly liquid, but have not frozen solid. The presence of hydrocarbons in Titan's atmosphere was confirmed on the probe's journey through it. Huygens's instruments detected both methane and ethane. But the pebbles in the picture probably are made of water—in the form of ice.

Because all of the raw images from Huygens were immediately made available to the public via the internet, amateurs have been racing ESA and its American cousin NASA to create processed, composite images. Some scientists say that a glitch led ESA to publish more data than it had originally intended, something that ESA denies. Nevertheless, a few minutes after the Huygens data were published on one website, they were mysteriously yanked off the web again.

Space The European Space Agency has more pictures from Huygens.

The availability of the data, though, has led to the publication on the internet of a short movie compiled from a series of 80 still images taken of the landing site. This five-second film appears to show movement, with small white objects crossing the camera's field of vision.

ESA's scientists were quick to point out that any movement seen was likely to be an artefact that owed its existence to nothing more than the fact that the images had not been put together correctly. Whether that interpretation is correct should be clear when ESA's own “official” movie is released, which had not happened as The Economist went to press.

Nevertheless there is, privately, a debate among planetary scientists as to whether the white blobs are an artefact, or pieces of ice being carried past the lander on a thin stream of liquid hydrocarbon a few centimetres deep.

That would be exciting. A stream is a stream, whether it is made of water or hydrocarbon. At the moment, Earth is the only body known to have them. But, as Ralph Lorenz, a planetary scientist at the University of Arizona, points out, the lesson from places such as Mars—and indeed Arizona—is that features created by liquids may exist, but the processes that carved them may be transitory or long gone. It is possible that rare but violent events, rather than continuous erosion, are responsible for shaping Titan's landscape. Whether Huygens has collected enough data to tell the difference remains to be seen