SpectroscopyNOW- Mars on the run: Liquid water spotted

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Water, water...

NASA's Mars Reconnaissance Orbiter (MRO) has provided perhaps the strongest evidence we have so far that liquid water still flows, albeit intermittently, on the Red Planet, opening up the possibility that microbial life may one day be discovered there.

The imaging spectrometer onboard the MRO has allowed scientists to reveal the chemical signatures of hydrated minerals on slopes where previously inexplicable streaks are apparent on the Red Planet. These dark streaks seem to wax and wane over time, ebbing and flowing in a tantalising way as if they are seasonal changes driven by water. This is especially poignant to those researchers who hoped to find water on Mars as the streaks darken and appear to flow down steep slopes during warm seasons when water might be liquid and recede and fade in colder periods. Similar streaks are known to exist in several places on Mars when temperatures are above minus 23 Celsius but are not seen during the Martian "winters".

“Our quest on Mars has been to ‘follow the water,’ in our search for life in the universe, and now we have convincing science that validates what we’ve long suspected,” explains astronaut John Grunsfeld, who is associate administrator of NASA’s Science Mission Directorate in Washington. “This is a significant development, as it appears to confirm that water - albeit briny - is flowing today on the surface of Mars.”

Seasonal flow

Technically these seasonal downhill flows have been labelled, as recurring slope lineae (RSL), and many observers have suggested that the simplest explanation for their presence is liquid water; however proof was lacking. The discovery of hydrated salts on these slopes would be associated with the colligative property of minerals dissolved in water to lower the freezing point of a liquid brine, just as salting the winter roads down here on Earth is used to nudge the freezing point of ice below the ambient temperature or the temperature of the road and allow it to melt. Scientists suggests that the RSLs are actually a manifestation of a shallow subsurface flow, with enough water wicking to the surface to explain the darkening.

"We found the hydrated salts only when the seasonal features were widest, which suggests that either the dark streaks themselves, or a process that forms them, are the source of the hydration. In either case, the detection of hydrated salts on these slopes means that water plays a vital role in the formation of these streaks," adds team member Lujendra Ojha of the Georgia Institute of Technology (Georgia Tech) in Atlanta.

It was as a University of Arizona undergraduate student in 2010 that Ojha first noticed these puzzling features in images from the MRO's High Resolution Imaging Science Experiment (HiRISE). HiRISE observations now have documented RSL at dozens of sites around Mars. The new study, published to much media fanfare in the journal Nature Geoscience couples the HiRISE images with mineral mapping by MRO’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). The spectrometer observations show signatures of hydrated salts at multiple RSL locations, but only when the dark features were relatively wide. When the researchers looked at the same locations and RSL weren't as extensive, they detected no hydrated salt.

Ojha and his colleagues suggest that the spectral signatures due to hydrated minerals are down to perchlorates, and more specifically, magnesium perchlorate, magnesium chlorate and sodium perchlorate. It is well known that several perchlorates can maintain solvents, including water, in the liquid state at temperatures as low as -70 Celsius.

Remote sensing

NASA's Phoenix Lander and Curiosity Rover previously found evidence of perchlorates in the Martian soil and there is some evidence that as long ago as the Viking missions of the 1970s, the signatures of these salts was observed. The new study of perchlorates detected in hydrated form in RSLs is in different areas to those explored by the landers. Moreover, this is the first time perchlorates have been identified from orbit. As a matter of historical fact, MRO has been examining Mars since 2006 with its six science instruments.

"The ability of MRO to observe for multiple Mars years with a payload able to see the fine detail of these features has enabled findings such as these: first identifying the puzzling seasonal streaks and now making a big step towards explaining what they are," explains Rich Zurek, MRO project scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. For Ojha, however, the new findings are more proof that the mysterious lines he first saw darkening Martian slopes five years ago are, indeed, present-day water.

"When most people talk about water on Mars, they're usually talking about ancient water or frozen water," he said. "Now we know there’s more to the story. This is the first spectral detection that unambiguously supports our liquid water-formation hypotheses for RSL." Michael Meyer, lead scientist for NASA’s Mars Exploration Program at the agency’s headquarters in Washington, adds that, “It took multiple spacecraft over several years to solve this mystery, and now we know there is liquid water on the surface of this cold, desert planet. It seems that the more we study Mars, the more we learn how life could be supported and where there are resources to support life in the future.”