Is there life on Mars, albeit in a primitive form, and could it make the journey to Earth? It may already be here, reports Martin Ince.

Frequent flyers say that a good landing is one you can walk away from. But are there Martian creatures on the Earth, or earthlings on Mars, saying much the same as they stumble away from interplanetary flights?

This was the intriguing prospect raised at a scientific meeting last week at the Ciba Foundation in London on the apparently arcane subject of "The evolution of hydrothermal ecosystems on the Earth (and Mars?)". The idea has its origin in recent findings in the Earth and space sciences. We now know that meteorites from Mars arrive fairly often on the Earth. One that landed in Nakhla, Egypt, in 1911 is said to have killed a dog. These rocks seem to have been blasted from the surface of Mars by the impact of other meteorites. Because the Earth is more massive than Mars, it would take a bigger explosion to send material from Earth into deep space, but it is still likely that Earth material found its way to Mars long before the first artificial space probes made the journey.

At the same time, those same spacecraft have expanded our knowledge of Mars itself. They have shown that Mars has massive volcanoes. And it has also had abundant water, for its surface is marked by deep river valleys that cannot have been formed by any other force.

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Richard Henley, a geologist based at Deakin West in Australia, told the meeting that wherever heat from within the Earth meets water, the result is hydrothermal activity, the complex interaction of rock, water and heat on and below the surface of the Earth. It occurs on every scale from the warm waters of Bath to massive systems that develop alongside active volcanoes. They have been studied in detail because they mean money, since the interaction of heat and water underground concentrates minerals. Henley has worked on one hydrothermal deposit, Lihir in Papua New Guinea, which is set to be the world's biggest gold mine.

Nobody has yet observed such activity on Mars, but there are areas of pale mineral deposits suggestively similar to those found near earthly hydrothermal systems.

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As Henley sees it, such systems are just what would emerge should a project manager be called on to ensure the origin of life. They have a wide range of chemicals, and enough energy to ensure rapid reaction. There are heavy metals, whose large atoms would be expected to lead to the rapid formation of new complex molecules, and there are minerals like clays whose complex shapes might provide a template for the formation of living molecules, a concept for the origin of life also floated by Richard Dawkins of Oxford University.

At the same time our knowledge of thermophiles, creatures that live in hydrothermal systems, has also been advancing apace. Karl Stetter, of the University of Regensburg and the University of California at Los Angeles, said that thermophiles are ancient - the oldest known are 3.5 billion years old, only a billion years younger than the Earth. More importantly, thermophiles are primitive creatures that occupy many of the lowest branches on the tree of life. They and their relatives are among the direct ancestors of today's animals and plants.

Stetter, who has studied hydrothermal systems in the deep oceans with US submersibles, said the thermophiles we know on Earth can live at 80-113C, and can be found in such hostile environments as crude oil reservoirs, kilometres below the surface. Given that the early Earth may have been very like Mars was in the early life of the solar system, the odds are that the conditions in which thermophiles flourish on the Earth have been present on Mars.

But there is a world of difference between postulating that life can arise on Mars and suggesting that Martians, even extremely primitive ones, can find their way to the Earth. But Paul Davies of the University of Aledaide told the meeting that there are in fact few genuine objections to the idea that the journey can be accomplished safely. The first problem is that being blasted into space is inherently stressful. But it turns out that in a major impact about 1 per cent of the material ejected can be thrown straight up with a minimum of force and find its way into space. Some of this material would be from deep inside the earth where thermophiles live.

Once in space, said Davies, things are again less stressful than one might think. Although it is cold there, thermophiles do not mind: they just go into hibernation until it warms up. More importantly, the radiation in deep space would affect only the outer few centimetres of a meteorite in space. Even the landing would be less grim than one might think; the outer layers of a meteorite are cooked on the way through the atmosphere, but the inner ones remain undisturbed.

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The possibility of life being transmitted from Mars to the Earth is enhanced, in Davies's view, by the fact that major impacts were far more common in the early solar system than they are now. Today we see only the fossils of the once-active rivers and volcanoes of Mars, and any life that may have arisen is probably extinct by now. But at that time they might have been combining to make perfect conditions for thermophiles to arise. As he sees it, it is an equal bet whether the process has led to Earth life finding its way to Mars or vice versa.

There could be cross-contamination, perhaps repeated many times to produce a low-level cross-infection between life on the two planets. Or life that had become extinct on the earth could be reintroduced here via Mars, a kind of Jurassic Park without human intervention.

But Davies pointed out that the US Viking missions to Mars were unsuccessful in their search for life there. They were designed to detect Earth-like life emitting carbon dioxide, and found none.

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Despite these discouragements, Davies insists that finding even one crude fossil in the rocks of Mars would have a fundamental effect on the way we think about ourselves. The basis of previous searches for intelligent life in other solar systems, which rely on detecting their radio emissions, is the "ladder of progress" model whereby something like Earth technology will emerge elsewhere in the universe.

But finding the remains of now-extinct Mars life would "shatter the orthodox paradigm" and reinforce the view that self-organisation is the way life really works. It arises and evolves without any tendency to develop to a higher plane: anyone pretending to observe such a trend is simply guilty of anthropomorphism. All this, as Davies said, "from the simple engineering project of going to Mars".

Not everyone is convinced that the knowledge we have will bear the heavy philosophical weight it is being asked to carry. Stephen Moorbath, a geologist from Oxford University, told a meeting held at the Wellcome Trust to publicise the findings of the Ciba conference that there is simply too little data to support the speculation.

However, as the unlucky dog of Nakhla found out, there is little doubt that pieces of Mars do turn up on Earth, or that it is possible for fragments of the Earth to make the return journey. Given that life has been abundant here for billions of years, the odds may be higher that Mars has received biological material from here than that Earth has received viable Mars life. But if it did, and if it became woven into the Earth's stock of genetic material not long after life started, we would all be walking around today with minute fossil traces of Mars inside us.

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