Mars may be a frigid desert, but perchlorate salts in the planet's soil are lowering the freezing temperature of water, setting up conditions for liquid brines to form at equatorial regions, new research from NASA's Curiosity rover shows.
The discovery of subsurface water, even a trickle, around the planets warmer equatorial belt defies current climate models, though spacecraft orbiting Mars have found geologic evidence for transient liquid water, a phenomenon termed "recurring slope lineae."
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The findings, published in this week's Nature Geoscience, are based on nearly two years worth of atmospheric humidity and temperature measurements collected by the roving science laboratory Curiosity, which is exploring an ancient impact basin called Gale Crater near the planet's equator.
The brines, computer models show, form nightly in the upper 2 inches of the planet's soil as perchlorates absorb atmospheric water vapor. As temperatures rise in the morning, the liquid evaporates.
The levels of liquid, however, are too low to support terrestrial-type organisms, the researchers conclude.
"It is not just a problem of water, but also temperature. The water activity and temperatures are so low in Mars that they are beyond the limits of cell reproduction and metabolism," Javier Martin-Torres, with Lulea University of Technology, in Kiruna, Sweden, wrote in an email to Discovery News.
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The realization that conditions were suitable for liquid brines to form stemmed from an unrelated effort to include a brine-manufacturing in-situ resource demonstration on the next Mars rover, slated to launch in 2020. The proposed instrument was not selected, but in the course of its development Martin-Torres and Maria-Paz Zorzano, with the National Institute of Aerospace's Astrobiology Center in Madrid, Spain, analyzed the humidity and temperature data coming back from Curiosity.
"We realized were seeing conditions where brines should be formed," Martin-Torres said.
The discovery has implications for Mars' global environment.
"As perchlorates are widely distributed on the surface of Mars, this discovery implies that the rest of the planet should possess even more abundant brines owing to the expected greater atmospheric water content and lower temperatures," the scientists write in Nature Geoscience.
The scientists hope to include the brine-manufacturing instrument on Europe's ExoMars mission.
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Perchlorates on Mars were discovered by NASA's Phoenix lander in August 2008, a finding that prompted a re-analysis of the 1970s-era Viking experiments that looked for organics on Mars.
Scientists repeated a key Viking experiment using perchlorate-enhanced soil from Chile's Atacama Desert, which is considered one of the driest and most Mars-like places on Earth, and found telltale fingerprints of combusted organics -- the same chemicals Viking scientists dismissed as contaminants from Earth.
"Contrary to 30 years of perceived wisdom, Viking did detect organic materials on Mars," planetary scientist Christopher McKay, with NASA's Ames Research Center in California, told Discovery News at the time. "It's like a 30-year-old cold case suddenly solved with new facts."
The follow-on study didn't mean that Viking found life. Indeed, the ubiquitous presence of highly corrosive perchlorates makes conditions more challenging for life.
The new research, "looks to be against the presence of life, as we know it, in the Martian soil," Martin-Torres said.