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Recurring slope lineae
These dark streaks, shown flowing downhill in a false-color image from NASA’s Mars Reconnaissance Orbiter, are inferred to have been formed by contemporary flowing water. (Credit: NASA / JPL / Univ. of Arizona)

For years, scientists have puzzled over dark streaks that appear and disappear on the surface of Mars – and now they’re confident enough to assert that the streaks are caused by trickles of salty water.

Their findings, published Monday in Nature Geoscience, serve as the best evidence yet that liquid water still occasionally flows on the Red Planet. The research is likely to spark a new wave of speculation about life on Mars – but it’s not likely to justify the breathless reports that circulated in advance of the study’s release.

“Has NASA found life on Mars?” one headline asked over the weekend. The short answer is no. Nevertheless, NASA thought enough of the study to call it a “major science finding” and schedule a news briefing about it.

John Grunsfeld, NASA’s associate administrator for space science, said the results make it “even more imperative that we send astrobiologists and planetary scientists to Mars, to explore the question, ‘Is there current life on Mars?'” NASA’s long-range plan calls for astronauts to start visiting Mars and its moons in the 2030s.

The streaks, known as recurring slope lineae, show up on imagery from NASA’s Mars Reconnaissance Orbiter. During warm seasons on Mars, the dark streaks appear on steep slopes – extending up to 16 feet (5 meters) in width, and tens or hundreds of feet in length. They eventually fade away but reappear the next year in places where temperatures rise to the toasty level of 10 below zero Fahrenheit (-23 degrees Celsius).

Scientists have long suspected that the streaks were caused by flows or seeps of salty water flowing down the Martian surface, but until now, they lacked direct evidence.

In the latest study, Georgia Tech researcher Lujendra Ojha and his colleagues analyzed single pixels of imagery gathered by the orbiter’s Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM. They didn’t detect the actual water, but they did see the chemical signature of salts that are typically left behind by brine, including several different flavors of perchlorates.

The researchers theorize that such salts would make it possible for water to exist in a liquid state, even if the temperature stays below the freezing point for pure H2O. “These results strongly support the hypothesis that seasonal warm slopes are forming liquid water on contemporary Mars,” they write.

They suggest three possibilities for the source of the water: melting ice ( which they say is “highly unlikely”), seasonal discharges from underground aquifers, or the absorption of atmospheric water vapor (which could form a thin film of salty water on the surface through a process known as deliquescence). Ojha and his colleagues draw a parallel to Chile’s Atacama Desert, where deliquescence supports communities of salt-loving microbes in one of the driest places on Earth.

“The detection described here warrants further astrobiological characterizations and exploration of these unique regions of Mars,” the researchers write.

Such findings could turn recurring slope lineae into potential targets for future missions to Mars. Scientists already have developed instruments capable of detecting microbes like the ones found in the Atacama Desert. However, the challenge of putting a lander or rover on a steep slope would be daunting.

“It would be trivial for an astronaut in a spacesuit to go up and investigate,” Grunsfeld said. “But it’s very hard for a rover, so we’re a little ways off.”

Future missions – such as NASA’s InSight lander, due for launch next year, and the European Space Agency’s ExoMars 2018 rover – will be equipped with drills to sample the Martian subsurface. Although the latest findings are intriguing, Grunsfeld said recurring slope lineae might not be the best places to look for Martian life.

“If I were a microbe on Mars, I would probably not live near one of these RSLs,” Grunsfeld said. “I would want to live probably further north or south, higher latitudes, under the surface, quite far under the surface, and where there’s more of a freshwater glacier. We only suspect those places exist, and we have some scientific evidence that they do. And that’s the subject of future exploration.”

Chris McKay, an astrobiologist at NASA’s Ames Research Center who wasn’t involved in the research, agreed that the dark streaks aren’t the best places to look for life.

“All life on Earth needs liquid water to grow or reproduce,” he explained in an email to GeekWire. “Life can be dormant in the dry state. Brines of sodium chloride (normal salt) are OK for life, too. However, there are brines on Earth that are too salty for life. The most famous is Don Juan Pond in Antarctica. This is the saltiest liquid water on Earth and is composed of saturated calcium chloride. Nothing can live in this brine.”

McKay noted that the briny traces observed on Mars are saturated solutions of perchlorate. “This brine is even saltier than the calcium chloride brine in Don Juan Pond,” he said. “Such a brine is not suitable for life and is of no interest for biology. The result is of interest geologically.”

In addition to Ojha, the authors of “Spectral Evidence for Hydrated Salts in Recurring Slope Lineae on Mars” include Mary Beth Wilhelm, Scott Murchie, Alfred McEwen, James Wray, Jennifer Hanley, Marion Masse and Matt Chojnacki.

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