Scientists say they have found evidence of freshly flowing water on the surface of Mars - but caution that there is a "long way to go" before they can say if the Red Planet could support life.
Chemical analysis of dark streaks on the surface of the planet has identified the presence of hydrated salts that are the signature of liquid salty water.
"We can safely say that some sort of liquid water activity is involved in the formation mechanism of these," researcher Lujendra Ojha of the Georgia Institute of Technology in Atlanta said.
"It's been three years of trial and error to get these readings."
Since the discovery of the dark streaks on Martian slopes over a decade ago, astronomers have debated about what caused the structures, dubbed "recurring slope lineae".
Are they really a sign of flowing water or are they simply caused by dry grains of material rolling down the slopes of a freeze-dried planet surface?
To date, observations have lacked the fine resolution needed to determine the chemical composition of the streaks, which are typically less than five metres wide.
But today, in findings reported in the journal Nature Geoscience, Mr Ojha and colleagues have solved this problem by combining data from two different instruments aboard NASA's Mars Reconnaissance Orbiter spacecraft.
Mr Ojha and colleagues analysed the chemical composition of streaks at four different locations on the Martian surface.
"By using the HiRise visible light camera and the CHRISM spectrometer aboard the same spacecraft, we could look at the same surface and acquire both pictures and ... spectra data to tell us about the chemical information at the same time," Mr Ojha said.
The researchers found hydrated salts, which precipitate from water, on all the streaks they examined.
"We detected magnesium perchlorate, magnesium chlorate and sodium perchlorate," Mr Ojha said.
By contrast, the terrain surrounding the transient streaks lacked any spectral signatures for salts.
Mr Ojha and colleagues argue the range of surface temperatures over which the streaks appear supports the hypothesis that water is seeping down slopes on Mars during warm seasons and drying up when the seasons get colder.
On Mars, water freezes into ice at 0 degrees Celsius, and boils at just 10 degrees Celsius because of relatively low air pressure.
But, said Mr Ojha, water containing salts like sodium perchlorate can stay liquid down to minus 70 degrees Celsius, and will not start boiling until temperatures reach as high as 24 degrees Celsius.
Mr Ojha said the perchlorates were also "hydroscopic".
"They can absorb atmospheric moisture, a really amazing process called deliquescence, where they absorb the water vapour from the atmosphere and create a water flow," he said.
Life on Mars?
Earlier this year NASA's Mars Curiosity discovered calcium perchlorates in the soils of Gale Crater, unrelated to the dark streaks.
Those findings, also reported in Nature Geoscience, indicated that the right temperature and humidity conditions existed in Gale Crater at night and just after sunrise during the Martian winter for moisture in the atmosphere to be absorbed by the salts and potentially form salty liquid water.
Mr Ojha said this new discovery meant some places on Mars were more habitable than previously thought, but that did not mean there was life on Mars.
"We're just starting to scratch the surface about these features, and there's a long way to go before we can safely say it is a habitable place or it has biological potential," Mr Ojha said.
"However, if humanity ever goes to Mars, this is probably an excellent source of water for consumption or rocket fuel."
NASA's planetary science director Jim Green described the discovery as revolutionary.
"Mars is not the dry, arid planet we thought of in the past ... it once had extensive water resources," he said.
"Mars suffered a major climate change and lost its surface water. Today we're revolutionising our understanding of this planet."
Astrophysicist Professor Geraint Lewis of the University of Sydney said the new findings were "critical evidence" for the streaks on Mars having formed by flowing, "or at least dribbling" water.
"This new result bolsters the argument for water on the surface of our planetary companion," he said.
Dr Amanda Bauer from the Australian Astronomical Observatory added: "I can simply say that the potential presence of liquid water on the surface of Mars is incredibly exciting because water is essential for life as we know it to exist.
"More work is needed to understand how exactly these seasonal salty water streams are formed on Mars, but the new evidence for water flow is certainly an exciting step forward in understanding water activity on Mars."
Originally Posted by Gary Corbett
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