Britain is returning to Mars, and it's doing it in something that sounds, as it moves, less like a 21st-century spacecraft than an old sailing ship.
The cause of this antique noise is something that couldn't be more advanced: the prototype ExoMars rover, creaking and popping as its six flexible aluminium wheels crunch across the gritty, dry rocks of a dormant volcano on a test run in the Canary Islands. It's a bit like the groaning of an old tea clipper's rigging as it rocks on the waves, but with a metallic, springy twang. The unreleased video of the run, two weeks ago, shows Bridget (as the scientists and engineers call it) manoeuvring over wobbly rocks and climbing scree slopes at a steep 30 degrees.
In Stevenage, at the centre of Britain's space industry, sophisticated satellites are assembled in hi-tech conditions that look like a James Bond film set. But Bridget resides at the far end of a poky, 1950s-era laboratory, whose whitewashed brick walls are lined with left-over gear from other space projects. A scrap of beige carpet protects the floor from its metal treads, still coated with pinkish - almost Martian - sand from the Canaries. Chris Draper, the mission systems engineer with European space company EADS Astrium, is controlling the prototype rover with a joystick through a laptop computer. Bridget crawls across the lab at a stately top speed of 150 metres an hour before performing its party piece. The four corner wheels swivel 45 degrees, then it slowly pirouettes on the spot. "She's like a mechanical crab," says Mr Draper, proudly.
Beagle 2, Britain's last mission to the Red Planet, was more of an elementary mollusc. Even if it hadn't crash-landed on Christmas Day 2003, it would have been limited to gathering and analysing samples within a metre or so of its clamshell casing. Beagle 2, an add-on to a European Space Agency mission to put a satellite into orbit around Mars, was severely limited by money (£40m), mass (34kg) and time (two years). ExoMars, an ESA mission in its own right, has much higher limits. It has £400m in international funding, a dedicated Soyuz 2b rocket able to lift a 450kg payload, and five years to go until blastoff in 2011. And it is part of a much grander project: Aurora, Europe's plan to send a manned expedition to Mars by 2030.
The US rovers Spirit and Opportunity, by contrast, are more like snails. In the two years since they arrived on Mars, they have covered a mere 10km. ExoMars will be able to do that in a week - partly because it's bigger and more powerful, but mostly because it will be moving autonomously. Scientists on Earth will pick a destination up to 100 metres away and the rover will figure how to get there by itself. Spirit and Opportunity have to radio back to Earth for new instructions after each small manoeuvre, a round-trip delay of 18 to 40 minutes, depending on where the planets are in their orbits.
But what really makes ExoMars exceptional is its science package. More than any other mission, it will be searching for an answer to the most tantalising question about Mars - is it, or has it ever been, home to alien life? Among its suite of experiments will be three aimed at detecting signs of life: Urey - named after a scientist who did groundbreaking work on the origin of life on Earth - the Life Marker Chip and the Gas ChromatographMass Spectrometer. Between them, they will be able to spot everything from DNA and amino acids, the building-blocks of proteins, to ATP, a molecule that on Earth is essential for storing energy in cells. The Life Marker Chip will work like a "lock and key", says Dr Mark Sims from the University of Leicester, the team co-ordinator. Proteins on the chip will bind to specific molecules from the samples and the two together will then fluoresce when hit by light. "We'll just look for the glow," he says. The rover will also be equipped with a two-metre drill for recovering sub-surface samples for analysis, and a package of sensors mounted on the robotic arm designed for Beagle 2.
The idea that "little green men" lived on Mars is the result of a mistranslation. An Italian astronomer, Giovanni Schiaparelli, reported in 1877 that he had seen canali, long, straight features, on Mars. In English this became canals, and inspired his American counterpart, Percival Lowell, to construct an elaborate theory in which a Martian civilisation channelled water from the polar caps to irrigate croplands on a dying world. That, in turn, led to the publication in 1898 of The War of the Worlds, in which H G Wells described an alien invasion from Mars. So thoroughly was the concept implanted in the public imagination that when Orson Welles adapted the story for radio 40 years later, presenting it as news rather than fiction, tens of thousands of Americans thought it was true.
But any life on Mars is likely to be no more complex than bacteria, and will probably shelter beneath the planet's surface. Earth's atmosphere and magnetic field shield life from dangerous ultra-violet and cosmic radiation. But Mars has little magnetic protection and only a wispy atmosphere (so thin that the cacophony from ExoMars would be barely audible). The main reason for doubt, though, is the absence of liquid water, an essential for all cellular life on Earth.
The intriguing thing about Mars, as opposed to the other planets, is that it probably had more air and running water in its geological past. "It's likely that Mars had a thicker atmosphere in the past but lost it as the planet cooled," says Professor Monica Grady from the Open University. Because of the lower gravity on Mars (about one-third of Earth's), volcanic eruptions could have blown much of the atmosphere into space, leaving the air pressure so low that surface water would have turned to gas. America's Mariner 9, the first probe to go into orbit around Mars, sent back images that show drainage patterns, with tributaries feeding into rivers and deltas, leading down from higher altitudes to lower levels. Since then, landers have discovered other evidence, including minerals that, on Earth at least, typically form in the presence of water. So life could have evolved on Mars, and if it started, it's unlikely to have stopped. After all, life can be found in some of the most inhospitable places on Earth.
Hopes that Mars might have harboured life were raised in 1976 when an experiment on the Viking landers found signs that nutrients added to a soil sample were being consumed by something living. Yet two other experiments were negative, and scientists realised that the positive result could have been due to non-biological processes. Enthusiasm soared again in 1984, when a meteorite that had come from Mars was discovered in Antarctica. Early analysis suggested it contained microscopic structures that were formed by life, but these too were later put down to non-biological causes. Which leaves the field open.
And the ExoMars rover, with its physical range and impressive array of tests, is well placed to answer the greatest question about the Red Planet, indeed the greatest question about space: are we alone?
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