London, September 24 : A group of 150 astronomers, geologists and biologists has picked what it considers the three best places to look for life on Mars.
According to a report in New Scientist, using new data from the Mars Reconnaissance Orbiter, which can detect the composition of the surface of the planet, participants at the Third Mars Science Laboratory (MSL) Workshop, held in Monrovia, California, voted on the three sites they thought most likely to show signs of habitability.
Their recommendations, which will help to guide NASA's selection of a landing site for the MSL, are all craters thought to have once contained ancient bodies of water.
The MSL mission is scheduled to drop a six-wheeled rover onto the Red Planet in October 2010.
The multibillion dollar vehicle's suite of instruments will photograph the surface, detect radiation that could be harmful to future human explorers, and collect samples of soil and rock.
Its primary goal will be to test the soil and rock samples for chemical signs of life, such as carbon atoms and organic molecules that could be built into proteins.
The discussion of where best to perform those tasks focused on sites that have associations with water.
Candidate sites included trenches with deep hydrothermal vents that might have protected subterranean organisms from radiation at the surface.
However, in the end, the idea of looking for the remains of photosynthetic bacteria in lakes and rivers carried the day.
"The majority view was that looking for signs of surface life is the more productive way to go," said Roger Buick of the University of Washington in Seattle.
Images of the surface of Mars show many paths of erosion created by ancient waterways.
The top three sites chosen are Gale Crater, Holden Crater and Eberswalde Crater, which appear to have once been an isolated lake, a lake connected to a system of rivers, and a delta, respectively.
Since detecting signs of life even in a once-watery environment might be difficult, the researchers felt they should go for the sites with the most chance of sheltering microorganisms.
The chosen sites are all rich in clays, called phyllosilicates, which were deposited in water. These slowly-deposited sedimentary rocks could coat and shield microorganisms within.
The Gale Crater also contains sulphate deposits - used as food by some organisms on Earth and thought to be able to preserve specimens.
All three sites are considered 95 percent safe for the rover to navigate.