Monsters from outer space

Could martian research samples carry diseases? Seth Shostak hopes not

Aids, mad cow disease, and avian flu stalk the globe, and they're problem enough. But some space scientists are suggesting that a new menace might soon join the pantheon of pandemics threatening your bodily wellbeing: bugs from space.

The exotic warning appeared last week in Science, where researchers reported on discoveries made by Nasa's Mars Exploration Rovers. In the last year, these small, motorised geology labs have beamed back convincing evidence that water once formed pools and puddles on the red planet.

Ask any astrobiologist (yes, there are such people), and they will tell you that liquid water is the essential ingredient of life. So it's possible that when Mars was a kinder, gentler and wetter world, perhaps billions of years ago, single-celled living beings made an appearance there. Admittedly, contemporary Mars is brutally cold and dry. But those microbes - if they ever evolved - could still be around, pursuing a spartan lifestyle in underground aquifers.

The problem is this: sometime in the next decade, Nasa hopes to use robots to dig up samples of Mars, and bring them back to Earth. The agency argues, rightly, that this may be the only way to decide whether the red planet has, or had, life. Robotic rovers - as clever as they are - can never match the wits or laboratory equipment of earthly biologists.

But Jeffrey Kargel, one of the rover researchers, and a scientist at the US Geological Survey in Arizona, writes that a sample return mission demands caution, and for obvious reasons. Just as the plague came to Europe from Asia's distant habitats, so too might Nasa unwittingly import extraterrestrial pathogens for which we have no defence. It might be The Andromeda Strain for real.

Well, it should comfort you to know that Nasa has already thought of this. In the 1970s, when men were going to the moon, Nasa worried about lunar infection, even though the experts were thoroughly convinced that our cratered neighbour was as dead as mutton. Even so, the rocks brought back by the early Apollo astronauts, as well as the astronauts themselves, were quarantined for weeks. (The rocks didn't complain, although the astronauts did.)

Margaret Race, a biologist at the Seti (search for extra-terrestrial intelligence) institute who works closely with Nasa, says far tougher and subtler tests would be applied to samples from Mars: "In the Apollo days, we would grind up the rocks and put them on plants, or feed them to Japanese quail and white mice. But today, you wouldn't have to use whole organisms. You could expose microscopic amounts of the rocks to tissue from various cell lines - invertebrates, vertebrates, and so forth."

"We are no longer looking for a lab rat to clutch its throat and kick over," says Nasa's planetary protection officer, John Rummel. "We have very finely tuned procedures to look for genetic changes caused by any pathogen."

The protocol for dealing with the martian samples has been worked out by hundreds of scientists, worldwide. But while such measures are important, they are extremely precautionary. The chilling idea that alien microbes could obliterate Homo sapiens ignores the fact that pathogens and their hosts co-evolve. They are attuned by nature to one another. Dutch elm disease attacks elms, but shows rather little interest in humans. "We aren't invaded by tobacco mosaic virus," says Race.

Rummel adds: "It's unlikely that any human pathogen will be found on Mars, because you can't make a living on that planet as a human pathogen.

"Frankly, the fear is not so much that any martian microbes would be pathogenic, but that they might find some useful resource in us or our environment." Imagine if they formed inert lumps in our bodies, or functionally displaced other kinds of earthly organisms. That is an inconvenience that Nasa strongly hopes to avoid.

Of course, all exploration is dangerous. Still, you might wonder why anyone would chance bringing Mars into their home. There are millions of uncatalogued bacteria on Earth, so what's the incentive to search for a bit of metabolising smudge on someone else's world? Why should we care, even to the point of taking a risk that could conceivably prove catastrophic?

It's because to find life on Mars would answer a deeply important question, and solve a puzzle worth solving: is biology some sort of miracle, or a common happenstance - a natural and frequent phenomenon? Discovering microbes on Mars would strongly suggest the latter.

In addition, finding a second "genesis" - another world where life sprang up - would undoubtedly give us clues to biology's deeper workings, and quite possibly be useful in dealing with problems in human genetics and disease.

Life that comes to Earth from Mars is an old idea. In HG Wells's War of the Worlds, the invaders came to slake their need for water that was disappearing from their own planet. These unpleasant Martians were done in - not by the military - but by terrestrial bacteria.

Well, call it bad planning. If the Martians had bothered to send robotic rovers first, maybe they would have known about such dangers. Frankly, they were careless; they showed up on Horsell Common without spacesuits or decent air filters for their war machines.

Nasa intends to do a better job.

· Seth Shostak is the senior astronomer at the Seti institute, California