Mars is a pretty barren place, and although vast oceans likely once covered the surface, nowadays any remaining water seems to be locked in ice at the poles or underground. But in 2011 the Mars Reconnaissance Orbiter (MRO) spotted dark streaks in the dust that appear in summer and vanish in winter, hinting at the influence of flowing water. On closer inspection, a new NASA report suggests the answer may not be so simple.
Thousands of examples of these dark streaks have been found on Mars, from the equator to the middle latitudes. Dubbed recurring slope lineae (RSL), these marks appear on rocky slopes in the late Martian spring, slowly crawl downhill during the warmer months before fading away by winter, only to reappear the same time next year. Similar patterns on Earth are associated with seeping water, leading the original discovery to be heralded as a clear sign of the wet stuff flowing on the Red Planet.
The MRO has continued to observe the streaks over the years, and the new report has used that data to determine that water, if it's involved at all, plays a much smaller part in the phenomenon than it was previously given credit for. Instead, the effect may just be caused by dry grains of sand and dust descending the slopes.
"We've thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand," says Colin Dundas, lead author of the report. "This new understanding of RSL supports other evidence that shows that Mars today is very dry."
The report used data gathered by the MRO's High Resolution Imaging Science Experiment (HiRISE) camera, which produced 3D models of the steepness of slopes at 10 sites. The researchers looked at 151 RSL features at those 10 locations, and found that almost all of them occurred on slopes steeper than 27 degrees.
A dead giveaway that dry sand is largely responsible was that the flows tended to end on slopes that match sand dunes' dynamic angle of repose – the steepest angle that a given material can be piled up before it begins to slump. If liquid water was responsible, the dark marks should extend down to the shallower slopes, but that wasn't the case.
"The RSL don't flow onto shallower slopes, and the lengths of these are so closely correlated with the dynamic angle of repose, it can't be a coincidence," says Alfred McEwen, co-author of the report.
Dry sand is a neater explanation than liquid water in other ways, too. Scientists weren't quite sure how enough liquid water to sustain those patterns could exist in the thin, arid atmosphere of Mars. But the new theory still has its holes: Why do the streaks appear so regularly? How do they grow? And why do they fade so quickly afterwards?
While the report doesn't have all the answers, it does put forward some possible explanations. Hydrated salts have been detected in the sand in those areas, and the researchers suggest that these salts may hydrate themselves by absorbing water vapor in the atmosphere. This creates small droplets of salty water, which may expand, contract or release water to set the dry sand grains tumbling downhill, creating the patterns. Then, if the amount of water in the air changes seasonally, it could be help explain the cycle.
That's just an informed guess for now, but the researchers believe that the phenomenon is likely a unique product of a perfect storm of environmental conditions that only exist on Mars. Studying the streaks up close could help unlock the mystery, which in turn could reveal useful details for future human settlers and explorers on the Red Planet.
"Full understanding of RSL is likely to depend upon on-site investigation of these features," says Rich Zurek, an MRO Project Scientist. "While the new report suggests that RSL are not wet enough to favor microbial life, it is likely that on-site investigation of these sites will still require special procedures to guard against introducing microbes from Earth, at least until they are definitively characterized. In particular, a full explanation of how these enigmatic features darken and fade still eludes us. Remote sensing at different times of day could provide important clues."
The report was published in the journal Nature Geoscience.
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