When two rovers landed on opposite sides of Mars a little over a year ago, they carried a 90-day warranty: that was all the working lifetime the designers of Spirit and Opportunity would promise. "In my secret heart of hearts," says Stephen Squyres, the Cornell planetary scientist who heads the rover mission, "I was hoping to stretch it out to six months."
But now, more than 13 months after landing, both Mars cars are still going strong. They've taken pictures of astonishing clarity, complementing the equally clear but long-distance images from both the Mars Global Surveyor and the European Mars Express orbiter. Just last week news leaked that Express had found what may be huge slabs of ice from a frozen sea buried under a thick layer of dust.
The rovers, meanwhile, have drilled into Martian rocks to analyze their composition and, a few weeks ago, discovered the first meteorite ever found on another world. Opportunity confirmed almost immediately after landing what scientists have long believed: water flowed on Mars ages ago--water that could have nourished Martian life. But the rovers' probing has revealed even more than that. Says Squyres: "Not only can we say there was water, but we've made substantial strides in understanding what conditions were actually like."
That's true, at least, for Meridiani Planum, the plain near the Martian equator, where Opportunity set down. By pure good fortune, the rover landed inside a small crater gouged by an ancient meteorite. This natural excavation exposed ancient layers of crust laid down by a shallow lake that had periodically dried up and refilled. Spirit landed half a world away in giant Gusev Crater, on what looked like a lake bed but turned out to be lava. Frustrated, mission scientists ordered the rover to move on. "We were already at 80 days, but we decided to put the pedal down and go like hell," says Squyres. Their goal: the Columbia Hills, some 2 1/2 miles away. Spirit didn't find tidy sediments there either. Still, says Squyres, "there's lots of evidence that water once soaked the ground."
That doesn't mean life once thrived on Mars, but it's a tantalizing sign. So is methane found in the Martian air. It could be volcanic, but methane is more often produced by living organisms. If bacteria still live under Mars' surface, this could be their calling card. Opportunity and Spirit can't detect bacteria directly, but they're not finished unearthing secrets. Spirit's discovery of an intact meteorite was totally unexpected, and the rover should get a dramatic panoramic view in a few weeks when it reaches the crest of the Columbia Hills. "I have no idea how much longer [the rovers] will last," says Squyres. "So you plan for the long term--but each day you drive like there's no tomorrow." •