Is there really life on Mars?

For years, scientists have been convinced that it is impossible for life to survive on the Red Planet - now it seems they may have been wrong

Early next Wednesday morning, an unmanned spacecraft will slip into orbit around the Red Planet. Mars Odyssey – named in honour of Arthur C Clarke and the year 2001 – will check how wet Mars is. Starting from next January, when its orbit has been brought down to a good height for surveillance, Odyssey will look for signs of ice under Mars's soil – or even pools of liquid water on the warmer parts of the planet.

It's the curtain-raiser to a series of probes that will investigate Mars for signs of life. At Christmas 2003, Britain's Beagle 2 will land on the planet's red desert and check if the soil or atmosphere have been modified by living cells. In 2011, a joint Nasa-French mission will take a sample of Mars's soil and send it back to Earth so that microbiologists can check it out for bugs.

But could this all be a waste of money? After all, in 1976 Nasa landed a pair of vastly expensive probes on Mars specifically to look for life. After months of analysis, Nasa published its official conclusion: Mars is dead.

As scientist and writer, I've always toed that pessimistic line. When Nasa researchers in 1996 claimed there were "fossil microbes" in a meteorite from Mars, I was among the disbelievers. But now I've experienced something of a conversion. It was sparked by the decision – along with my colleague Heather Couper – to write a book on our present knowledge about the Red Planet. As we travelled to interview dozens of researchers – American, British, Russian – we found a wave of new thinking about Mars. Bruce Jakosky, a seasoned planetary scientist at the University of Colorado, Boulder, sums it up: "The Mars I'm studying today is not the Mars that I was studying five or 10 years ago."

The breakthrough has come largely because of a Nasa probe that's currently orbiting the planet. The Mars Global Surveyor has had so little media interest that scientists have nicknamed it the "stealth mission". The spacecraft carries a powerful camera built by Mike Malin, a former Nasa employee who has now set up his own company in San Diego.

Malin is difficult to track down: his company lists no street address. It's because his camera has revealed that the so-called "Face on Mars" is not a giant artificial sculpture but a naturally eroded plateau, and he's constantly pestered by "face-freaks" convinced of some conspiracy to cover up signs of an alien civilisation. "Mars is neither the Earth with craters nor the Moon with an atmosphere," Malin explains. "Mars is Mars. Mars is its own unique planet that has its own unique way of telling us things about itself."

And some of the things it's telling us are turning our preconceptions on their head. Mars has huge volcanoes, which geologists have always thought are extinct. But the detailed pictures from Mars Global Surveyor show fresh lava flows on the flanks of the biggest volcano, Olympus Mons.

But the biggest surprise from the new pictures was water-worn gullies running down the sides of canyons and craters. They appeared so fresh that they could have been eroded yesterday. "They are incredibly young, geologically speaking," says Malin. "We could be talking about a couple of million years ago, or it could be right now."

Ken Edgett, a colleague of Malin's at Malin Space Science Systems, is also a natural sceptic. "I had to be dragged kicking and screaming to this conclusion," Dr Edgett says. He is convinced that there are so many gullies all over Mars that they couldn't all have flowed until a couple of million years ago, and then all switched off in unison. Like the volcanic eruptions, water must still occasionally well up from within Mars, and tinkle down the desert soil to erode fresh gullies.

But temperatures on Mars are well below freezing point. How can liquid water flow? Nasa researcher Nathalie Cabrol thinks the answer is natural antifreeze. "You can have liquid water there if it's a superbrine," she explains, "that's water loaded with salts of potassium, magnesium and so on. You can depress the freezing point of water to as low as minus 60C."

Cabrol says she's found places on Mars where she believes shallow lakes exist even to this day. In the bottom of some craters she has found deposits of sediments that look fresh. "That's why we believe there is liquid water on Mars today."

Mars may be wetter than scientists previously thought. But it's hardly Eden. The soil is filled with reactive compounds called peroxides; the surface is blasted by lethal ultraviolet radiation from the Sun. But over the past few years, biologists have come to realise that life is far hardier than anyone thought. It might be able to cope with the harsh conditions on the Red Planet.

In the realm of microbes, scientists have discovered a whole range of extremophiles – bugs that thrive where humans would never dare to go. Nasa researcher Jonathan Trent fishes them out of the boiling poisonous springs of Yellowstone National Park in America. Other extremophiles enjoy life in ponds as corrosively acid as a car battery, or equally alkaline. Most importantly, Trent reveals, "There's just been the discovery of a species in Antarctica that lives exclusively at low temperatures. It doesn't just survive; it actually grows at temperatures below the freezing point of water."

Trent is optimistic that some extremophiles could cope with the chemicals on Mars. Even human cells have some tolerance. If you put hydrogen peroxide on a wound to sterilise it, an enzyme, catalase, from our cells breaks the peroxide down into harmless oxygen and water. "Many organisms produce catalase," says Trent, "and it isn't inconceivable that they could cope with the peroxide levels on the surface of Mars."

So microbes could cope with cold and chemicals; but what about the blistering ultraviolet radiation? Mars has no ozone layer, and so it's perpetually blasted by the Sun's most damaging rays.

But Trent has an extremophile for all seasons. Now he pulls out Deinococcus radiodurans. This bug has the greatest radiation resistance of any microbe on Earth. It was first discovered in cans of meat that went off, even after they'd been irradiated to sterilise them. Expose most cells to radiation, and their DNA is smashed to pieces and the cells die. "But Deinococcus radiodurans can repair its own DNA," explains Trent. "Cut its DNA up into tiny pieces, and it will completely repair it in hours. So it can cope with radiation levels thousands of times higher than most organisms."

Charlie Cockell, at the British Antarctic Survey, thinks the Martian microbes could also be aided by their own sunscreens. Cockell came into the public eye during the 1990 general election, when he stood against John Major in his Huntingdon constituency, as leader of the Forward to Mars party.

In his scientific life, Cockell researches how life survives in the regions of Earth that are most like Mars – the Arctic and Antarctic. "In our field trips," he says, "we see microbial mats where the organics in the upper layer screen out the ultraviolet radiation and protect the microbes below. On Mars, you could have living microbes protected under a dead layer of microbes or under a very thin layer of dust."

So where does that leave the "no-life" outcome of the 1976 Viking landers? We have discovered that the experiments never gave any clear-cut answers. Two experiments reacted to the chemicals in Mars's soil, but revealed nothing about possible biology. Another appeared to be positive. Nasa's official negative attitude was based entirely on the fourth experiment, which looked for carbon atoms in the soil. It found no carbon. And, as living cells are made largely of this element, Nasa interpreted "no carbon" as "no life".

Now, researchers at the Scripps Institution of Oceanography in La Jolla have re-analysed that fourth experiment on Viking, and uncovered the startling truth. It wasn't anything like as sensitive as Nasa claimed. "There could have been cells in the Martian soil," says Scripps researcher Danny Glavin, "but Viking wouldn't have seen them. It could have missed as many as 30 million microbes in a gramme of soil."

No one has actually demonstrated a living Martian bug – yet. But the circumstantial evidence is mounting in favour of life on Mars today. All the pillars on which I built my old sceptical attitude have been knocked away. The Viking experiments were ambiguous, at best; Mars is more benign than we believed; and life is far hardier than we ever suspected.

In fact, there's one final line of thought that has swung the balance for me. Meteorites are always being blasted from one planet to another, and there must have been much more of this traffic when the planets were young and the Solar System was filled with interplanetary rubble.

"The early Earth was covered in abundant microbial communities," Cockell says, "and at the same time Mars had water and roughly the same atmospheric composition as the Earth. And meteorites would have been transferring microbes across the Solar System." So terrestrial microbes must have landed on Mars; and we've learnt from the extremophiles on Earth that evolution is extremely adaptable. The chances are that life on early Mars would not have died out, but changed to fit the evolving planet.

"So the greatest scientific discovery of the millennium," Cockell concludes, "would be the discovery that there's no life on Mars!"

'Mars: The Inside Story of the Red Planet', by Heather Couper and Nigel Henbest, Headline, £25