Scientists have their first evidence that trickles of liquid water play a role in sculpting mysterious dark streaks that appear during summertime months on Mars, a finding that has implications for potential life on Mars, as well as planning for future human expeditions.
The discovery, reported Monday in the journal Nature Geoscience, follows years of speculation and studies to learn why the faces of some cliff walls on Mars are streaked with narrow dark slopes, some more than 300 feet long, that appear when temperatures are warm and then vanish during the winter chill.
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The streaks, known as recurring slope lineae, or RSL, were first reported in 2011 in the Martian southern highlands, but have since been found throughout the planet's equatorial region, particularly within deep canyons.
Using data collected by NASA's Mars Reconnaissance Orbiter and a new analysis technique, scientists were for the first time able to detect the telltale chemical fingerprints of hydrated salts in dozens of RSL sites.
"That implies that there was liquid water there very recently to leave this residue of hydrated salts. It confirms that water is playing a role in these features," University of Arizona planetary geologist Alfred McEwen told Discovery News.
In a press conference advisory, NASA said that the research "solves" a Mars mystery, but it actually opens the door to another, potentially more challenging puzzle: Where is the water coming from?
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"We haven't been able to pinpoint the source," lead researcher Lujendra Ojha, a graduate student at Georgia Institute of Technology, told Discovery News.
"Water could form by the surface/subsurface melting of ice, but the presence of near-surface ice at equatorial latitudes is highly unlikely," Ojha and colleagues write in Nature Geoscience.
Another option is that salts absorb water vapor directly from the Martian air, though scientists are at a loss to explain how they could trap enough water from the tiny amount available in the atmosphere to seep down hill slopes and form the streaks.
"It's just not clear that the atmosphere can supply enough water to do that," McEwen said.
Whatever the source of the water, its seasonal appearance on the surface of Mars raises the prospect that life might be present on the planet today.
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Scientists, however, are quick to note that the RSL water is briny and possibly so dense that no terrestrial organisms, at least, could survive there.
NASA, which is working toward a human expedition to Mars in the 2030s, has another interest in the water. Future crews will need water for drinking and to produce oxygen, both for breathing and for rocket fuel.
For now, far more information is needed about the chemistry of the water as well as its source.
"We're just starting to scratch the surface," Ojha said.