A frozen sea found on Mars is one of the most promising places to look for life on the Red Planet, scientists say.
But planned missions designed to search for microbes below the Martian surface will not drill deep enough to find living cells, the UK team has said.
Researchers at University College London say that microbes in the first couple of metres of Martian soil would be killed off by intense radiation.
Life might survive deeper down, where conditions are more benign, they think.
It just isn't plausible that dormant life is still surviving in the near-subsurface of Mars
Lewis Dartnell, UCL
But these depths were beyond the reach of drills envisaged for missions to Mars, said Lewis Dartnell, from UCL's Centre for Mathematics and Physics in the Life Sciences & Experimental Biology (Complex).
These missions - such as Europe's ExoMars rover - could find hints that life once existed there - such as proteins, DNA fragments or fossils, explained Mr Dartnell; and that "would be a major discovery in itself".
But he added: "The Holy Grail for astrobiologists is finding a living cell that we can warm up, feed nutrients and reawaken for study.
"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.
"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."
Survival times
The study, published in the journal Geophysical Research Letters, maps out the likely cosmic radiation levels at various depths, taking into account different surface conditions on Mars.
Unlike Earth, Mars is not protected by a global magnetic field or thick atmosphere and for billions of years it has been laid bare to radiation from space.
Exomars rover Image: Esa
Can Europe's rover drill deep enough to find life?
Mr Dartnell's calculations suggest survival times near the surface reach only a few million years. This would mean that the chance of finding life with the planned probes is slim.
Scientists would need to dig deeper and target very specific, hard-to-reach areas such as recent craters or areas where water has recently surfaced.
The research suggested that one of the best places to look for living cells on Mars would be within the frozen sea in Mars' near-equatorial Elysium region.
This is because the ice is relatively recent - it is believed to have surfaced in the last five million years - and so has been exposed to radiation for a relatively short amount of time.
Icy secrets
Water provides an ideal shield of hydrogen to protect life on Mars from destructive cosmic radiation particles.
Ice also holds an advantage because it is far easier to drill through than rock.
But, even here, surviving cells might be out of the reach of proposed drills. Other ideal sites include recent craters, because their surfaces have been exposed to less radiation, and the gullies recently discovered in the sides of some of these craters, as they are thought to have flowed with water in the last five years.
The discovery of a vast frozen sea just below the Martian surface was announced by scientists in 2005.
Their assessment was based on pictures from Europe's Mars Express that showed plated and rutted features across an area 800km by 900km.
John Murray, from the Open University, Jan-Peter Muller, from UCL, and others said a catastrophic event probably flooded the landscape five million years ago. The floodwaters then froze out and were covered with dust or ash, they argue.
Some researchers point to the lack of "ground truth" about the radiation environment on Mars' surface to assess life's chances there.
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