SPIRIT and Opportunity, NASA鈥檚 twin rovers on Mars, have made it look almost too easy. They were designed to work for about 90 days, but this month they will have soldiered on for a year without appreciable deterioration. Their mission was to look for evidence of past water, which they did with time to spare.
Luck was undoubtedly on their side. Landing zones for the rovers were accurate only to within tens of kilometres, yet Opportunity managed to score a cosmic hole-in-one by landing in a fascinating crater. Here it struck gold as soon as it opened its eyes: the layered bedrock that looks so much like sedimentary rock on Earth that there can be little doubt that water helped to shape it (see 鈥淢ounting evidence for a wet planet鈥�).
This good fortune, combined with meticulous planning, has made the mission a victory for robotic exploration. But it has also highlighted robots鈥� limitations. There was the painstaking months-long process Opportunity went through just to reach out and examine rocks a few metres away. And between them, the twins have travelled a mere 6 kilometres. A human explorer would have accomplished all these tasks on the first day.
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And from here on, answering the questions we need to ask will only get tougher. We now know Mars once had water, but how far did it extend and how long was it around for? The answers will be critical for finding out whether life appeared on Mars, and if so whether it persisted and adapted to the drastic changes in Martian climate. We also want to know if unambiguous fossils exist, or even remnants of life 鈥� perhaps dormant spores or thriving microbes in a sub-Martian oasis.
Answering these questions will need more complicated procedures than any used so far, such as dating studies using isotopic ratios from carefully selected sites 鈥� the most interesting of which may only be reached by deep drilling. Finding the right rocks and acquiring and returning the data to Earth will mean that just about everything will have to go perfectly. Will robots be capable of such feats? And if anything goes wrong, will they have the ability to improvise: to do things that have not been thought out ahead of time?
Then there is human nature to contend with. When evidence for extraterrestrial life has been advanced in the past 鈥� such as from experiments aboard a Viking lander 鈥� it has been greeted with alternative interpretations and intense debate. The chances are that our deepest questions will not be answered with certainty until humans scuff their boots in the ruddy sands and see the evidence with their own eyes. In that case, and assuming that no leap in robot intelligence is in the offing, we can only hope that President Bush鈥檚 pre-election vision of a space programme ending with humans on Mars is more than just talk. We鈥檙e waiting.
