PUZZLING Martian gullies, thought to have been carved by flash floods, may actually be patterns formed by 鈥渞ivers鈥� of dust particles.
In 2000, the Mars Global Surveyor photographed several features that look a lot like terrestrial gullies cut into the soil by water. There are no impact craters near these gullies, implying that these features are less than a million years old. But Mars has been colder than a Siberian winter for most of its life, so any water would have been rock hard.
To explain these gullies, Mike Malin of Malin Space Science Systems in San Diego, California, who led the camera team that photographed the gullies, argued that flash floods might occasionally scour the Martian landscape, fed by high-pressure underground aquifers that kept the water liquid (New Scientist, 1 July 2000, p 6).
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But the explanation could be simpler. Troy Shinbrot and his team at Rutgers University in New Jersey believe mere dust might be enough to form the features seen on Mars. Their experiment was prompted by last year鈥檚 suggestion from Alan Treiman of the Lunar and Planetary Institute in Houston, Texas, who reasoned that dust particles should settle very slowly on Mars because its gravity is only about a third that of Earth. So if a steep dune face collapses, the swirling sand might behave like a fluid long enough to form those puzzling gullies.
To test this, Shinbrot鈥檚 group filled a tray with hollow glass beads with diameters ranging from 4 to 90 micrometres. These light beads fall slowly though the air, mimicking the movement of dust particles in the low Martian gravity. When the researchers tipped the tray to get the beads flowing, the results were startling. The beads could flow, slosh around, and reflect from boundaries in a liquid-like manner, forming patterns that were often indistinguishable from the features on Mars. These miniature landscapes matched not just the Martian gullies, but also some longer channels and fan-shaped 鈥渁prons鈥� where the flows run out of energy.
While such a resemblance isn鈥檛 proof that rivers of dust created the Martian features, Shinbrot thinks it鈥檚 more than mere coincidence. He points to similar small-scale experiments that have successfully mimicked volcanic deposits on Earth, for example.
Nick Hoffman of the University of Melbourne in Australia, who suggested last year that evaporating carbon dioxide might carve the gullies (New Scientist, 18 January 2003, page 15), agrees. 鈥淥ur instinctive feel that Martian channels and gullies must have been carved by water gets yet another knock by the demonstration that dry particle flows can reproduce many of the essential elements of the gullies and debris flows of Mars,鈥� he says.
The only problem is the planet鈥檚 extremely rarefied atmosphere, says Hoffman. Its atmospheric pressure is very near the limit below which dust particles cannot create a stable, fluidised flow.