This month the Mars Reconnaissance Orbiter has witnessed Dust Devils (sediment laden whirlwinds) rising up to 20 km above the surface of the Red Planet. This is not a complete surprise, as dust storms have been jinxing Mars probes since the earliest attempts to reach the planet's surface; burying landers, covering solar panels, and obscuring cameras. They are however a spectacular sight wandering the Martian plains, more distinctive events than the harmattan-style events that many scientists had expected.
Animation of a Dust Devil on the Amazonis Planitia plains of the Martian Northern Hemisphere. NASA/JPL.
While these events are spectacular, they would be far less devastating to humans or human-made objects than tornadoes on Earth. Mars has only about 0.6% of the surface atmospheric pressure of Earth, so an astronaut standing in the path of a Martian Dust Devil would be in no danger of being knocked over, let alone picked up. The storms are only able to support their sediment load due to the low atmosphere of Mars - 38% of that of Earth.
This does not mean that the dust storms are without significant effect on the surface of Mars. The low atmospheric pressure would not stop the storms from ionizing the particles within them, a process that on Earth leads to lightning, something the thin atmosphere of Mars probably cannot sustain. On Mars this effect probably leads to charged sediment particles being deposited back on the surface, where they would act as oxidizing agents, which may account for the reddish colour of the planet, derived from oxidized iron compounds. On Earth iron compounds are also typically oxidized, but by free oxygen in the atmosphere (the Earth is not red, because it also has oceans and an active biosphere); this is absent on Mars so another explanation for the oxidation seen at the surface must be sought.
See also Fragments of Mars found in Morocco, Fobos-Grunt crashes into the Pacific Ocean, Running water on present day Mars? NASA sets its sights on Gale Crater, Mars and Mars on Sciency Thoughts YouTube.
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