Washington, May 30 : A new analysis of Martian rock, that had earlier given hints of water on the Red Planet, has suggested that the water was more likely a thick brine, far too salty to support life as we know it.
Scientists at Harvard University and Stony Brook University in the US have made this latest finding.
Together with co-authors Andrew H. Knoll and Scott M. McLennan, Nicholas J. Tosca from Harvard University analyzed salt deposits in four-billion-year-old Martian rock explored by NASA's Mars Exploration Rover, Opportunity, and by orbiting spacecraft.
It was the Mars Rover whose reports back to Earth had created excitement over water on the ancient surface of the Red Planet.
The new analysis suggests that even billions of years ago, when there was unquestionably some water on Mars, its salinity commonly exceeded the levels in which terrestrial life can arise, survive, or thrive.
"Our sense has been that while Mars is a lousy environment for supporting life today, long ago it might have more closely resembled Earth," said Knoll.
"But this result suggests quite strongly that even as long as four billion years ago, the surface of Mars would have been challenging for life. No matter how far back we peer into Mars' history, we may never see a point at which the planet really looked like Earth," he added.
Tosca, Knoll, and McLennan studied mineral deposits in Martian rock to calculate the "water activity" of the water that once existed on Mars.
Water activity is a quantity affected by how much solute is dissolved in water; since water molecules continuously adhere to and surround solute molecules, water activity reflects the amount of water that remains available for biological processes.
The water activity of pure water is 1.0, where all of its molecules are unaffected by dissolved solute and free to mediate biological processes.
Terrestrial seawater has a water activity of 0.98. Decades of research, largely from the food industry, have shown that few known organisms can grow when water activity falls below 0.9, and very few can survive below 0.85.
Based on the chemical composition of salts that precipitated out of ancient Martian waters, Tosca and his colleagues project that the water activity of Martian water was at most 0.78 to 0.86, and quite possibly reaching below 0.5 as evaporation continued to concentrate the brines, making it an environment uninhabitable by terrestrial species.
According to Knoll, life that could originate and persist in such a salty setting would require biochemistry distinct from any known among even the most robust halophiles on Earth.