for National Geographic News
If life exists on Mars, it won't be found close enough to the surface to be easily discovered by robotic probes, a team of European scientists has concluded.
That's because any bacteria that may once have lived on the surface have long since been exterminated by cosmic radiation sleeting through the thin Martian atmosphere.
Billions of years ago the red planet may have been remarkably Earthlike, said astrobiologist Lewis Dartnell, author of Life in the Universe: A Beginner's Guide and a graduate student at University College London.
Then the Martian climate collapsed, producing the cold, dry world we see today (images of Mars).
If Earthlike life survived the climate shift, it would most likely be bacteria that live close to the surface.
Such bacteria would spend most of their time dormant, waiting for rare geological events to bring water from deeper in the planet.
Unfortunately, Dartnell said, such a scenario would have left the bacteria vulnerable to years of radiation damage.
In a study published January 30 in the journal Geophysical Research Letters, Dartnell's team found that even the most radiation-tolerant Earthly bacteria would survive only 18,000 years at the surface.
Bacteria living at greater depths could survive longer.
But even at 6.5 feet (2 meters)—the greatest depth at which a Mars probe scheduled for launch in 2013 will be capable of reaching—survival time would be 90,000 to half a million years, depending on the type of rock.
The best places to look, the team concluded, are frozen crater lakes or gullies in the sides of craters.
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