If life exists on Mars it would lie deeperunder the surface of the planet than current probes are drilling, according to research led by University College London.
The planet's thin atmosphere and lack of a global magnetic field expose Mars to radiation that would kill any life too close to the surface, the researchers said in a study published in the journal Geophysical Research Letters.
Lead author Lewis Dartnell said any hopes of finding a dormant living cell lie in looking deeper into the planet.
"It just isn’t plausible that dormant life is still surviving in the near-subsurface of Mars — within the first couple of metres below the surface — in the face of the ionizing radiation field," said Dartnell, a post-graduate student at UCL.
"Finding life on Mars depends on liquid water surfacing on Mars, but the last time liquid water was widespread on Mars was billions of years ago. Even the hardiest cells we know of could not possibly survive the cosmic radiation levels near the surface of Mars for that long," he said.
The team looked at the known radiation resistance of life forms on Earth and combined this information with the annual radiation doses to calculate the survival time of dormant populations of the cells.
The study's authors suggest looking for living cells within the Elysium Basin — which some say was home to a sea in the last five million years — or in recent craters with similar signs of water activity.
Water would have provided a potential life form added protection from the radiation, the authors contend, in addition to being a necessary condition for life.
"We have found that this suspected frozen sea in Elysium represents one of the most exciting targets for landing a probe, as the long-term survival of cells here is better than underground in icy rock," said Dartnell.
Even at a site like Elysium, the researchers estimate life could only survive for long periods of time if it was at least 7.5 metres below the surface. The European Space Agency's ExoMars rover, which is not scheduled to launch until 2011 at the earliest, is not expected to be able to drill more than two metres into the planet's surface.
Scientists have been analyzing images and data sent back from the planet by orbital probes and planetary rovers in search of signs of liquid water they hope will lead to the discovery of life.
NASA's next major mission to Mars will be the Phoenix lander, which is scheduled to launch in August 2007 and land on the planet's icy pole. The Canadian Space Agency contribution to the lander is the Meteorological Station, which will record the daily weather of the Martian northern plains.