One year ago today, an alien craft screamed out of a pink, dusty sky and bounced down on the rocky floor of the giant Gusev Crater on Mars. Spirit, the first of NASA's two golf-cart-sized Mars exploration rovers, had landed.
Despite our requisite public confidence, in private many of us in the planetary science community were crossing our fingers. A previous Mars lander had failed totally in 2000, and nobody really knew what had gone wrong. And the scrappy, innovative and ambitious (but terminally underfunded) British Beagle had disappeared without a trace while attempting to land on Christmas Day, 2003.
We knew that the two new rolling robots had been more carefully designed and thoroughly safety-checked than either of these failed craft. We knew that the scary landing scheme, in which the precious cargo of instruments and transmitters hits the ground and bounces hard, cushioned by air bags, had worked once before, on the smaller and more compact Mars Pathfinder, with its tiny Sojourner rover, in 1997. We also knew that the reputation and the future of the Mars program were on the line.
So when Spirit's lander finally deflated its air bags and the rover extended its camera mast and took the first pictures of its new Martian home, we breathed a sigh of relief and thought "even if it dies tomorrow, at least we have these." And again, a few days later, when Spirit, fully awake and unfolded, rolled off its landing platform and made its first tracks in Gusev's red dirt, we thought "at least we have this."
Spirit's landing was followed three weeks later by the successful bounce-down of its twin craft, Opportunity, on the opposite side of Mars, the Meridiani desert. In a fantastic stroke of luck, Opportunity hit an interplanetary hole in one, rolling to rest in a small crater with ancient, layered bedrock -- just what we most wanted to study -- staring it right in the face.
Each rover was supposed to last 90 days; each was meant to travel only half a mile. Now a year has passed and, incredibly, defying all expectations, they are both still crawling away from their landing sites, photographing new vistas on the Red Planet, grinding and sniffing the rocks and sorting through the deep past of Mars.
What have we learned?
The primary goal of these craft was to look for signs of surface water in Martian history. We seek water intensely because we are water. As far as we know, alien life will also be water-based. Of course, "as far as we know" isn't very far, and that's why we explore. Though Mars today is too cold and its atmosphere too thin to support surface water (underground may be a different story), the planet sports what look like primordial riverbeds and lake beds. But we need ground truth: Rocks don't lie. Their chemistry and detailed textures reveal the story.
Opportunity hit the first real pay dirt of the mission. That convenient outcrop right where it landed turned out to be largely made up of finely layered, sulfur-rich salt deposits, apparently the remnants of an evaporated sea. This tells us that Meridiani was once soaking wet, and that Mars once had all the requirements for life as we know it.
It also tells us that we must go back to Mars with equipment to look for fossils or chemical signs of past Martian life. (The next launch is set for 2009.) Before Opportunity, such a search would have seemed far-fetched; now it is not only reasonable but obligatory. The payoff would be cosmic perspective on our own evolutionary history. If Mars was wet but never "alive," then perhaps the conditions we believe to be necessary for life are not sufficient. In that case, dumb luck might have played a larger role in our existence than we like to think, and the universe might be a lonely place indeed for inhabited worlds like ours.
After thoroughly exploring the little crater where it landed, Opportunity set out across flat, eerily featureless plains toward the stadium-sized Endurance Crater. A month later, it arrived and paused at the rim, awaiting instructions. Should we let it enter the steeply walled depression? Would it be able to climb out again? Did it matter, if the buried treasures were valuable enough?
The rover team risked the descent. It was the right decision. Endurance, with its layered cliffs and fissured surfaces, confirmed Mars' watery past and provided clues, still being sifted through, to the precise nature of its changing environments. Now, after spending six months rooting about in the crater, Opportunity has just safely climbed back out onto the surrounding plains and headed south, seeking new terrain.
Spirit, which landed first but soon lost the limelight to Opportunity's dramatic discoveries and heroic exploits, has its own dogged story to tell. From space, Gusev Crater looked just like a dried-up lake bed. But what Spirit found was that the rocks on the ground were volcanic, not water-formed sediments.