Steve Connor: We're desperate to believe that there is life on Mars

The Red Planet has such a hold on popular imagination that we seize upon any new fact

What we want to see and what we actually see tend to be different concepts when it comes to looking at Mars. The Red Planet has had such a hold on the popular imagination that we seize upon almost any new fact as further evidence of Martian life. It is as if we are desperate to discover that We Are Not Alone.

We can expect further Martian mania later this week when the results of the Mars Odyssey space probe are published in a scientific journal. It was reported over the weekend that the Nasa probe has found evidence of vast oceans of water ice buried just a few feet under the surface of the dry, dusty and unbearably cold planet. Proof of a watery world on our doorstep – if proof it is – will no doubt kindle our deepest suspicions that extraterrestrials exist.

Our obsession with life on Mars goes back to the observations of a Milanese astronomer called Giovanni Schiaparelli, who produced some of the earliest and best maps of the planet in the 1870s. In some of them he identified linear structures he called canali, which in Italian means simply "channels". However, in one of those almost comic mistranslations that have changed the course of history, many English-speakers assumed he had discovered "canals", evidently made by aliens.

Although Schiaparelli claimed to be agnostic on the issue of whether these features were natural or artificial, a turn-of-the-century American astronomer, Percival Lowell, firmly plumped for the latter. As the science author Oliver Morton says in his latest book, Mapping Mars, to be published next month, Lowell's reasoning was that Mars is habitable, but its aridity makes the habitability marginal. If there were intelligent life-forms on Mars, they would do something about this unfortunate state of affairs and build a network of canals for diverting its most precious resource: water.

"With this leap of the imagination, Lowell created one of the most enduring tropes of science fiction: Mars as a dying planet," says Morton. "It would live on in the works of HG Wells, Edgar Rice Burroughs, Leigh Brackett and many, many others."

Better telescopes and more detailed observations may have failed to support the idea of canali criss-crossing the surface of Mars, but this did not stop the Martian hare running. The War of the Worlds, written in 1894, took the idea of desperate aliens from a water-starved planet to its logical conclusion – a full-scale invasion of Earth.

Orson Welles, in his famous 1938 radio broadcast of the Wells classic, brought Martian hysteria to a mass audience. Thereafter, a generation of science fiction writers exploited the idea of aliens from Mars for all it was worth, until the first visiting spacecraft "proved" that it was after all a lifeless planet.

The images sent back from the two Viking probes which landed on the Red Planet in 1976 showed a desolate terrain dotted with extinct volcanoes, a place that was colder than Antarctica and drier than the Sahara desert. No trees, no canals, no Martians and not even a decent atmosphere.

But this turned out to be yet another false image of Mars. Subsequent missions revealed evidence to suggest that Mars was once a warmer, wetter place with lakes, perhaps an ocean, and even an atmosphere thick enough to support life. We lapped it up in the hope that perhaps life on Mars did at least exist in the past.

Scientists have since shown that Mars had lots of water a few billions of years ago, but something had happened in the meantime for it to have disappeared. It is almost certain that much if not all of the Martian water evaporated into space as its atmosphere got thinner. But there was also a strong possibility that some was still there, submerged below the dusty surface in frozen, underground aquifers. If true, then life on Mars might still be there today, buried below a dry and highly corrosive Martian surface.

This brings us to perhaps the most dramatic "seen it" story about Mars. In August 1996 Nasa scientists said they had found evidence for ancient microscopic life on a Martian meteorite that had landed on Earth. Nasa said it was the most convincing evidence yet in support of extraterrestrials. Who cared about the caveats, we all wanted to believe it. Nasa said it, President Clinton confirmed it (it was an election year after all), and the public believed it.

"There is not any one finding that leads us to believe that this is evidence of past life on Mars," said David McKay, a Nasa scientist, at the time of the announcement. "None of them in itself is definitive, but taken together the simplest explanation is early Martian life."

Perhaps not surprisingly, other scientists begged to disagree. If none of the findings on their own was definitive, why is the case any stronger when they are all put together, they argued? The sceptics concluded that the "fossilised lifeforms" seen in the Martian meteorite were simply artefacts. We didn't like it, of course, because we wanted to believe in the far more exciting story.

After the initial euphoria of this week's findings dies down, no doubt other sceptics will emerge to question Nasa's interpretation of the Odyssey findings. Until then, we will keep hoping that the space probe's instruments really have divined water. And that life beyond Earth may one day be found.

s.connor@independent.co.uk