Devilish weather on Mars

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.

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