Devilish weather on Mars

Most people have seen a dust devil on Earth. Unlike true tornadoes, these spinning columns of air are not associated with a storm system, and seem to form and disperse randomly. Like tornadoes, dust devils form when warm air rises and spins as cooler air moves down around it, forming a vortex. It can happen in almost any weather; on calm days little pockets of sun-warmed air rise up to form dust devils, while on windy days they seem to be spawned by little turbulent eddies. They're not very strong or long-lived, and rarely cause any damage. The same conditions seem to spawn dust devils on Mars, too. As the rovers emerged from their first Martian winter, they caught a surprising number of dust devils going by, possibly driven by seasonable winds. Like the Midwestern United States, spring may also be the active tornado season on Mars. But while there are many similarities between Martian and Earthly dust devils, there are also some spectacular differences.

For one thing, the atmosphere of Mars is very different from Earth. Don't even think about breathing it - not only is the air composed almost entirely of carbon dioxide, but it's much too thin to support Earth-like life. Air pressure on Mars is only six millibars, or six thousandths of one Bar. (One Bar is the pressure of our atmosphere at sea level.) In fact, one of the big mysteries is how Mars came to have such a thin atmosphere in the first place. Such low air pressures would never have supported the presence of liquid water, something we know was present in Mars' distant past. Even with fast winds, dust devils in the rarefied atmosphere of Mars pack very little punch, and are too weak for engineers to worry about the rovers getting blown over or bent out of shape by a passing dervish.

But probably my favorite aspect of the Martian dust devils is their size. Give the thin air of Mars a good stir, and it seems to form enormous dust devils. Some of the ones we've observed have funnels almost a mile wide, and were several miles high. Now that's a dust devil! Just think about it; on any given night when you look up at Mars, giant dust tornadoes are spinning around, somewhere in your line of sight. I think it gives Mars a much more interesting personality. Instead of a dead, airless desert, I love to picture what it might be like to stand by the rovers, watching multiple dust devils play in the wind.

The rovers will not last forever. Parts are starting to wear out, and there's always the possibility that one or the other will get stuck in a sand dune, as when Opportunity got mired in a feature now known as "Purgatory Dune." But it does really seem that Mars has been on our side. Not only has an active spring tornado season cleaned up our rovers, but Mars has also avoided one of its colossal, planet-wide dust storms during the past two years. Thin air aside, such a storm might well block out the sun or even partially bury the rovers' wheels. But for now, the rover drivers at NASA have no plans to shut down their Martian operations. A few have joked that they'd like to drive the rovers head-on into each other, maybe even play the first game of interplanetary robot wars (those rock abrasion tools could do a lot of damage). This, of course, is beyond the realm of possibility because the rovers are on opposite sides of Mars, and so far each one has only covered about a mile of terrain. But other things are now in the range of possibility that we would never have imagined. We know that water was once common, and we're very keen to figure out why it disappeared. Not only for Mars' sake, but also our own. And at the very least, we have a fabulous new topic for water-cooler conversation. How's the weather been this year? I hear it was an unusually active tornado season on Mars.

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