Water molecules will quickly bond with any soluble substances present, so water activity reflects the amount of pure water that remains available for biological processes.
The water activity of pure water is 1, where all of its molecules are free to foster life. Seawater, which is loaded with salt, has a water activity of 0.98.
Research has 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 the figure could have reached below 0.5 as Mars went through climate changes and evaporation concentrated the brines.
"This doesn't rule out life-forms of a type we've never encountered," Knoll said.
"But life that could originate and persist in such a salty setting would require biochemistry distinct from any known among even the most robust halophiles [salt-loving organisms] on Earth."
The team's results appear this week in the journal Science.
Ben Clark, a Mars expert at Lockheed Martin Corporation who was not involved in the study, said the area at the Martian equator sampled by the rovers for this work is already known to be unusual.
The region, called Meridiani Planum, was chosen partly for its high content of hematite—an iron oxide mineral—which makes it chemically unique to begin with. (See images of Mars.)
Regardless, he said, no single place should be seen as a global representative of Mars's mineral composition.
"It is very difficult to simulate actual Martian conditions," he said. "Whether organisms could evolve to survive or propagate under near-saturated conditions of [salts] is difficult to fully evaluate."
Lead author Tosca argues that Meridiani Planum might have been one of the least harsh environments in early Martian history. Past samples of other locales have revealed even higher mineral concentrations.
"Meridiani Planum was perhaps the best place for life in terms of salinity, but it was still so salty that only a handful of known microbes on Earth would be able to survive," he said.
"The most surprising result was that, at the ancient Martian surface this 'extreme' environment by Earth's standards may have been the norm by Mars's standards."
Lockheed's Clark countered that a uniformly harsh Mars may actually mean that life was more likely to have evolved.
"Mars probably has had a far greater extent of environments of this type," he said. "Thus, there would have been more opportunity for niches to occur and be exploited."
But the authors of the new study point out that the handful of known Earthly halophiles descended from ancestors that first evolved in fresher waters.
"We can't rule out the presence of dilute waters earlier in Martian time or elsewhere on the planet," lead author Tosca said. "But we can say that if there was a window for life on Mars, it was short."
Clark noted that the Phoenix lander, which was co-developed by Lockheed Martin, has a wet chemistry lab on board for analyzing soil composition and how it reacts with liquid water.
"These results could provide substantial evidence for or against the relevance of these speculations about the habitability of life on Mars," he said.
(Related: "On 'Dream' Terrain, Mars Lander Readies for Experiments" [May 26, 2008].)
For his part, Tosca hopes to delve more deeply into the sulfates and chlorides on the Martian surface.
"Our future research will be devoted to understanding under what conditions these 'keystone minerals' can form in the laboratory," he said.
"They can continue to be used as markers for determining the chemistry of ancient Martian water. Only then can we begin to make careful inferences about how habitable ancient environments may have been."
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