EARTH must take precautions to avoid contamination from lifeforms that must now be presumed to exist on Mars, leading scientists gave warning yesterday.

Potentially deadly microorganisms could be returned to Earth on a probe which is being planned to collect samples from the Martian surface.

The warning comes after a detailed scientific analysis of data sent back by the roving vehicle Opportunity which landed on Mars on January 25.

Jeffrey Kargel of the US Geological Survey said that protection of our own planet from alien forms of life requires the assumption that Martian life exists. “Before proceeding with sample returns or human missions to Mars, we must review measures for planetary biological protection.”

His warning appears in Science magazine in an article accompanying the first formal publication of the mass of data from Opportunity, which continues to operate on the Martian surface.

The search for life on Mars, now more than a century old, is still not finally resolved. But the odds that life existed there and may still exist are shortening, according to planetary experts, Dr Kargel said.

Nobody any longer expects Martian life forms to be anything like those on Earth. But there remains a possibility that bacteria or other microscopic organisms may survive in regions where there is still water. On Earth, almost every imaginable habitat, including deep underground, has specialised bacteria — called extremophiles — living and thriving.

The risks are twofold: probes sent from Earth may contaminate Mars with terrestrial bacteria, wrecking future studies of Martian life; or, more important, bacteria brought back from Mars may contaminate the Earth with unpredictable effects.

Similar precautions were taken at the time of the Apollo Moon landings. Astronauts returned to Earth were kept in quarantine after they landed for fear they might be infected with a lunar bug. None was.

Although the presence of water on the red planet can be considered proved, of life there are only hints. One is the presence of the gas methane, which might be produced by forms of life. On Earth, life can exist in areas as acidic and salty as Meridiani Planum, where Opportunity landed — examples are the ancient mines of Rio Tinto in Andalucia, Spain, or the salty Permian Basin in Texas. But few earthly species survive in environments that are at the same time very cold, very acidic, and very salty — and none that do survive in such conditions produce methane.

“But maybe on Mars they do,” says Dr Kargel, the author of a recent book on the latest ideas about Mars. Or maybe, he suggests, the organisms that produced the Martian methane live in areas more hospitable than Meridiani Planum.

Analysing the data collected by Opportunity, a team led by the rovers’ principal investigator, Dr Steve Squyres, of Cornell University, concludes that the sedimentary rocks found by Opportunity preserve a record of environmental conditions different from any on Mars today.

“Liquid water was once present intermittently at the Martian surface at Meridiani, and at times it saturated the subsurface,” the team concludes. “Because liquid water is a prerequisite for life, we infer that conditions at Meridiani may have been habitable for some period of time in Martian history.”

Opportunity has explored two craters, Eagle and Endurance, near its landing site. In both areas, layers in the bedrock showed that it had been laid down as sediments, implying past oceans and voids in the rock were probably caused by the dissolution of salt.

Opportunity also found quantities of small spheres, named “blueberries” — even though they are grey, not blue. These marbles consist of the iron-rich mineral haematite. Similar spheres have been found in the deserts of southern Utah, formed as iron-rich water seeped through sandstone.

The first idea that Mars might contain life came from the astronomer Giovanni Schiaparelli, who thought that he saw canals when he looked at Mars through a telescope at the end of the 19th century. H.G.Wells popularised the idea in The War of the Worlds, published in 1898.

NATIVES OF THE RED PLANET

Some time in the next decade Nasa plans to bring back samples from Mars that may answer questions about Martian life

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