If life ever got going on Mars, it may have been exterminated 4 billion years ago by a buildup of salt. Evidence that the planet is poisonously salty comes from a study of minerals near the Martian surface.
While exploring Mars's Meridiani plain, the rover Opportunity discovered ancient deposits of magnesium sulphate that appear to have been left behind by salty water.
Nicholas Tosca of Harvard University in Cambridge, Massachusetts, US, and colleagues have calculated the likely salt content of that water, based on the types of mineral deposited and the chemistry of the Martian surface.
They use a fairly abstruse measure of saltiness called "water activity", which decreases as you add salt to water. Pure water has an activity of 1; seawater 0.98. Few terrestrial organisms can survive at a water activity below 0.85.
Tosca and his group found that the waters flowing across the Meridiani plain had an activity of at most 0.86. That value would rule out most forms of Earthly life, and the team's analysis suggests that the water activity was probably even lower, perhaps below the survival thresholds of any salt-resistant organisms known on Earth. (The current record holder is a fungus that can still grow with a salt activity as low as 0.61.)
The deposits on the Meridiani plane are known to be between 3.5 and 4 billion years old, implying that Martian water was already an unhealthily salty soup by that time, less than a billion years after the planet formed.
"What this work establishes is that if there was an early window for life on Mars, it was indeed short," Tosca told New Scientist.
As Mars has continued to lose atmospheric water to space since these minerals were laid down, the salt has probably been concentrated further. That means any liquid water remaining today beneath the surface might be even less palatable.
The salty waters weren't limited to the Meridiani area. An instrument called OMEGA on Europe's Mars Express spacecraft can sample Martian chemistry remotely by analysing the spectrum of light bouncing off the surface rocks. OMEGA has found evidence that the same minerals are widespread on Mars.
In fact, Meridiani might have been one of the least hostile environments on Mars. "We looked at a variety of other localities, including some Martian meteorites," says Tosca. "They are all much, much worse."
It is possible, he admits, that some extremophile organisms will be discovered on Earth that can survive even saltier environments. Or it could be that Martian life started out with very different chemistry from ours. "The important question is whether life could have originated at high salinity," says Tosca.
Rover team member Harry McSween of the University of Tennessee in Knoxville, US, agrees with the study's findings.
"It's a sobering conclusion, but I find it very convincing," McSween told New Scientist. "They present a cogent argument that Mars is and maybe always has been sterile."
Tobias Owen, a planetary scientist at the Institute for Astronomy in Hawaii, has reservations about the work. "I find the arguments compelling; the question is whether they are complete," he says. "Is the appearance of water at the Martian surface always governed by the same processes, so we can be sure of its subsequent history? I would say that it is probably too early in our state of investigation of Mars to reach the sweeping conclusion they have."
Journal reference: Science, vol 320, p 1204
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