Gleaming red in the night sky, the planet Mars has always seemed a bit malevolent to us. In ancient times we named this strange, wandering celestial presence after the god of war, and even today science fiction fills our imaginations with Martian invaders, dead cities, and just the general sense that something up there is a bit wrong. But now, for the first time, NASA has removed all doubt. There is indeed something up there, two things in fact, that are beginning to verge on the eerie and unnatural. Yes, gentle reader, we are speaking of the rovers that wouldn't die.
The Mars Exploration Rovers were cautiously estimated to last 90 days on the Martian surface when they landed in January 2004. After passing the three-month milestone with flying colors (and finding evidence of past surface water on Mars), NASA engineers began to relax a bit and plot out a more thorough exploration of the rovers' surroundings. Then a year passed, and now... two. And Spirit and Opportunity are still ambling through the Martian desert, gathering even more evidence that Mars was once much warmer and wetter.
What can explain the amazingly long life spans of the rovers? The rovers were, of course, very well-designed and built. But even the best technology has limits. In the case of the rovers, NASA engineers figured that the limiting factor would be how long the rovers could keep their batteries charged. Both rovers run on solar power, and eventually the combined factors of dust building up on the solar panels and the low temperatures of the approaching Martian winter would finally run the batteries down, millions of miles from the nearest jumper cable. Or perhaps a giant dust storm would block sunlight long enough for the batteries to lose their charge. In short, Martian weather would eventually kill the rovers. But nothing of the kind has happened. Two years into the mission, the rovers have survived their first Martian winter, and their batteries are almost as fully charged as when they landed.
Surprisingly, help for the rovers arrived in the form of a particularly active tornado season on Mars. For quite a while now, astronomers have known that weak tornado-like vortexes, or dust devils, routinely scour the dusty surface of Mars. The Martian desert is permanently dry; as far as we know, there has been no rain for millions, perhaps billions of years. Not only does that make the surface of Mars very dusty, it also means that the tracks left by dust devils stay around for a while, giving us a chance to image them. Even from high above in orbit, the Viking orbiter was able to image long, feathery shadows of dust devils winding around the Martian twilight. Back in 1997, the Sojourner rover measured a precipitous drop in local air pressure that scientists hypothesized might have been due to a passing dust devil. These days, the Mars Global Surveyor satellite routinely images the tracks of thousands of dust devils. Some tracks stay for years, while others are wiped out almost immediately as other dust devils blow through.
But even with thousands of dust devils blowing around, most scientists thought that the chance of actually imaging one was very small. At first, excitement bubbled when Spirit's camera imaged a distant white streak that might have been a passing dust devil. How fortuitous! Then another came by, even closer. In the end, engineers were taking panoramic shots of the plains of Gusev Crater, watching as many as three dust devils ramble by at a time. Yes, the rovers had found themselves in the midst of a veritable dust devil party. And it gradually began to dawn on the Earth-bound rover operators what was happening: wind from the passing dust devils was blowing away any dust that had accumulated on the solar panels, giving the rovers a new lease on life with each passage.
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.