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From dust to more dust. On Mars, a dust devil鈥檚 own shadow can help the sandstorm grow.
Dust devils are a nuisance on Earth, but can be helpful on Mars when they clean off rovers鈥� solar panels. Because Mars has a much thinner atmosphere 鈥� which means lower atmospheric pressure 鈥� the dervishes function differently there.
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On Earth, a light breeze is enough to lift dust grains into the air, but on Mars, storm-speed winds are necessary, says at the University of Duisburg-Essen in Germany 鈥� and those are rare in Mars鈥檚 low atmospheric pressure. That means something must give Martian dust grains a helping hand.
鈥淐onditions in dust devils are just on the edge to lift grains, so any supporting effect helps,鈥� he says.
Wurm suspected that temperature differences between sunlit and shadowed soil could kick extra dust grains into the air. To test this, he dropped a capsule full of Mars-like sand grains down a . This put the sand grain experiments in free fall, allowing Wurm to eliminate any side effects from Earth鈥檚 gravity.
Shadow boost
To study how temperature differences affect the dust devils, he shone a laser on the dust grains as they fell and then switched it off. The light made the grains bounce around their compartment, but when it was turned off, they bounced even more readily, by a factor of 10, he says.
On Mars, Wurm argues that the temperature difference caused by a dust devil鈥檚 own shadow could increase the amount of dust in the devil. The temperature changes could cause movement of gases in the Martian soil, Wurm says.
鈥淭his gas motion builds up a pressure below the surface, and if this is strong enough, the upper layer kind of explodes,鈥� he says. The explosion lifts sand grains into the air, where they join the dust devil. This feedback effect might contribute to the raging dust storms seen on the Red Planet.
Paradoxically, this effect increases with lower atmospheric pressure, Wurm adds. 鈥淚t is only important on Mars, not on Earth. This is in contrast to the force of wind, which decreases with decreasing pressure.鈥�
But the energy from the lasers in Wurm鈥檚 experiment is much greater than the effect of sunlight on Mars, notes of Johns Hopkins University in Baltimore, Maryland. He argues the effect may be too slight to matter there.
Journal reference: Icarus, DOI: 10.1016/j.icarus.2016.02.049
