Sept. 29--Where there's brine, there's water.
Scientists scouring the Red Planet using NASA's Mars Reconnaissance Orbiter say they've found direct chemical evidence of transient saltwater flowing on the surface today.
Granted, they haven't caught the liquid in the act -- and what they've detected looks less like salty water and more like watery salt. Nonetheless, the discovery of hydrated salts serves as a "smoking gun" for flowing water and helps solve a longstanding Martian mystery.
"This is the first time we've found flowing water on a planet that's not ours," said lead author Lujendra Ojha, a planetary scientist and PhD candidate at Georgia Tech.
The findings, published by the journal Nature Geoscience, also shed fresh light on the potential for life on our planetary neighbor.
"It suggests that it would be possible for there to be life today on Mars," John Grunsfeld, associate administrator of NASA's Science Mission Directorate, said at a briefing Monday morning.
Scientists got a tantalizing hint that there could be liquid water on the surface in 2011, when Alfred McEwen, lead scientist for the Mars Reconnaissance Orbiter's HiRISE camera, along with Ojha and other colleagues, discovered these strange dark streaks on Martian slopes that seem to grow and fade with the seasons. These "recurring slope lineae," which can stretch up to a few meters, extend downward when it gets warm and then later shrink and fade, reappearing each Martian year.
"Ever since the discovery in 2011 ... a number of us have been incredibly excited by the prospect of liquid water on Mars," said Bethany Ehlmann, a planetary geologist at Caltech who was not involved in the paper. Nonetheless, she added, "we try to be cautious -- it's a big thing to say there's liquid water on Mars today."
Granted, Mars' atmosphere is cold and thin -- which means that any pure water that made it to the surface would either freeze or immediately evaporate, depending on the temperature. But a recent study by scientists using NASA's Curiosity rover found that water might indeed be able to exist briefly on the surface -- provided there were enough salts, such as perchlorates, dissolved in the liquid. These salts would keep the water from freezing or evaporating quite as easily and could actually serve to suck moisture back out of the air.
So could liquid water -- very salty, briny water -- really explain these strange dark streaks on Martian slopes?
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Theoretically, the scientists could look for water by using the orbiter's Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM. CRISM can look for different chemicals in a given spot on the surface by studying the telltale signature of dark bands they've left in the light. The problem is, it's hard to check the light's chemical fingerprint at the recurring slope lineae, or RSL, because, according to the study, "few locations exist in which RSL are wide or dense enough to fill even a single CRISM pixel."
So researchers used a method in which they focused on the handful of individual pixels that were mostly filled by the recurring slope lineae. They looked at four different spots with recurring slope lineae and discovered a strong fingerprint for hydrated salts -- salts with water locked into the mineral structure, a clear sign that saltwater likely had flowed there. The hydrated salts included magnesium perchlorate, magnesium chlorate and sodium perchlorate.
"I think it's incredibly exciting, because when we look back at the broad scope of Mars history, it's always in the past where there's evidence for the most water," Ehlmann said. "But if there's liquid water even today, when Mars is supposedly at its driest ... I think that says that there was probably liquid water for all of the last 4.5 billion years, just like there was on Earth. Not in the same quantity, but at least ephemerally, episodically, it's there."
The new findings bolster the idea that astronauts who land on Mars could draw upon the planet's natural resources. Not only could the water potentially be purified and harvested, but the salts known as perchlorates, which are ubiquitous around Mars, could in principle be mined as an ingredient for solid rocket fuel, Grunsfeld said.
The Mars 2020 rover will have one such proof-of-concept experiment on board -- an instrument to try to synthesize oxygen for breathing (and for burning rocket fuel) out of the carbon dioxide in the Martian atmosphere.
"All of the scientific discoveries that we're making on the surface of Mars ... are giving us a much better view that Mars has resources that are useful to future travelers," Grunsfeld said.
The discovery may further whet the appetite of astrobiologists looking to probe past habitable environments on the Red Planet, researchers said.
"To me, the chances of there being life in the subsurface of Mars has always been very high," McEwen said during the briefing.
Still, the water is so incredibly briny that it's difficult to imagine microbes being able to survive with the harsh fluid.
"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."
In the meantime, where exactly the water comes from, how it's released, and how it gets back into the soil to repeat the cycle every year remain open questions, the scientists said. Such questions could be answered by a future orbiting mission to Mars, Ehlmann added.
Ashwin Vasavada, Curiosity's project scientist at Jet Propulsion Laboratory who was not involved in the paper, called the discovery exciting but not surprising.
"What seemed really unbelievable 10 years ago -- that Mars has modern, liquid water -- has slowly become more and more an expectation," Vasavada said. "And to have the evidence that that team found today is fantastic."
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UPDATES
6:38 p.m.: This story has been updated with additional comment from NASA's John Grunsfeld on the implications for future astronauts who might land on Mars.
1:34 p.m.: This story has been updated with additional comment from Curiosity project scientist Ashwin Vasavada.
10:55 a.m.: This story has been updated with additional comment following a Monday morning briefing.
This story was originally published at 8:05 a.m.
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