Bright sand streaks can form in the wake of desert cyclones when the whirlwinds break up popcorn-ball-like clumps of sand, a new study shows. The first-ever sighting of bright, instead of dark, dust-devil tracks on Earth could help decipher how these funny features form on Mars.

"This is the first observation and analysis of bright dust-devil tracks on Earth," said geologist Dennis Reiss of Westfälische Wilhelms Universität Münster in Germany, who led the new study. "They are known from Mars, but their formation mechanism is unknown."

Scientists have spotted dust-devil tracks in satellite images of both Earth and Mars, and the Mars rovers Spirit and Opportunity have even seen the dusty cyclones whipping past.

Most of these streaks are darker than the surrounding sand. The coarser the grains of sand are, the darker they appear. When dust devils swish by, they clear their paths of smaller grains, leaving dark tracks like eerie, swirly tattoos. But occasionally, cameras orbiting Mars have caught glimpses of bright streaks on dark sand.

"They have posed a problem," said planetary scientist Ronald Greeley of Arizona State University, who has researched how dark dust-devil trails form on Mars. "How would that mechanism" – blowing small grains aside – "work with the bright streaks? That's been the puzzle."

Reiss and colleagues may have found an answer in the Turpan desert in northwestern China. The crew went there to hunt for dark dust-devil tracks on the ground. Until their field study, such tracks had been seen only from orbit. They found several of the dark tracks and published an "up close and personal" analysis in the July 28 Geophysical Research Letters.

But on April 18, the team saw several active dust devils leaving surprisingly bright tracks. "It was good luck," Reiss said.

When they looked closer, the researchers realized the streaks weren't actually any brighter than usual – they were filled with the same coarse, millimeter-size sand that dust devils normally suck clean. But the surrounding sand had been darkened by five minutes of rainfall the previous night.

The rain had cemented bits of sand, silt and clay into clumps up to a centimeter wide. When the dust devils came through, they destroyed the fragile clusters, revealing the finer sand grains below. The dust devil tracks appeared bright in contrast to the rain-darkened background. The results will be published in an upcoming issue of the journal Icarus.

While there is obviously no rain on Mars, landers and rovers have observed similar clumps that are held together by electrostatic forces. 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."

*Images: 1) Bright dust-devil tracks in China. Dennis Reiss/Icarus. 2) Dust-devil track imaged by the HiRISE camera on the Mars Reconnaissance Orbiter. NASA. 3) Microscope images of the clumps of sand, silt and clay that darken the landscape around the bright tracks. Dennis Reiss/Icarus.

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