I use this image as my desktop, so each day I gaze at it in wonder. It’s natural art and design, somehow mysterious, somehow sensual. This extraordinary – and much-publicised – image is from Mars, captured by the high resolution orbiter, and reveals the tracks of dust devils, whirling miniature tornado dervishes of wind engraving their way over the dunes. But exactly how these dark tracks form has been something of a mystery: is it to do with composition, that light dust is removed to reveal sand of darker materials beneath?
Ironically, more observations had been made of this phenomenon on Mars than on Earth – orbiters and rovers have gathered a gallery of images and movies of dust devils on the Red Planet:
Earlier this year, Dennis Reiss, a geographer at Westfälische Wilhelms Universität Münster in Germany, together with his colleagues, reported on their earth-bound fieldwork in the Turpan Desert of northwestern China. Dust devil tracks are small and ephemeral things here, whereas the Martian designs are larger and last far longer. But the workers in China were able to observe dust devils at work and examine the character of the dark trails left behind. What they showed was that the appearance of the tracks was dependent on the size of the grains. The sand grains outside the tracks were coated with fine dust that brightly reflects the light. Within the tracks the blast of the passing dust devil blew the sand along, tumbling and saltating, with the result that the dust was picked up by the vortex and clean sand grains were left behind. Smaller grains appear lighter than coarse ones – they reflect more light and their albedo is higher – and so the tracks appear darker than the surrounding sand.
So far, so good. But there are examples, as recently reported by Mary Pendleton Hoffer and Ronald Greeley at Arizona State University, of dust devil tracks on Mars that are lighter than their surroundings:
Reiss and his colleagues have now reported on light-coloured tracks from the Turpan Desert. These appeared in sand that had been darkened by a brief rain shower the previous night. But invoking rain on Mars is clearly a non-starter, so what is going on? Well, it turned out that the sand within the light tracks was no different from usual – millimetre-sized grains cleansed of finer material. But, the darkening of the surrounding sands was a result of the grains being cemented together by the rain into clumps up to a centimetre across. Big clumps = low albedo = dark appearance. The dust devil had broken these up, brightening the sand left behind in its track.
OK, but there’s still no rain on Mars. However, there are forces that can clump sand grains together – electrostatic forces. Such clumping has been observed by the field geologists on Mars, the indomitable rovers.
In 1979, Greeley conducted lab experiments showing that charges build up on dry, wind-blown sand particles in a similar manner to the way charge builds up on a balloon when you rub it on your hair. Just like the charged-up balloon can stick to the wall, charged sand grains pull together to form delicate, “popcorn ball” aggregates.
“The destruction of aggregates on Mars would lead also to bright dust devil tracks,” Reiss said.
Greeley thinks the idea makes sense. “This is a plausible model for the formation of the bright tracks on Mars,” he said. “This is a very nice study, a very nice result.”
So, through questions raised on Mars we are better understanding processes on our own planet. The wonder of Martian calligraphy is, however, by no means diminished.
[See the amazing gallery of Mars high resolution images at the HiRISE site. Images courtesy of NASA/JPL/University of Arizona HiRISE]