Dark streaks on Mars may not be evidence of water after all

Dark streaks on Mars may not b...
These dark streaks on Mars were once thought to be evidence of flowing liquid water, but a new NASA study suggests that might not be the case
These dark streaks on Mars were once thought to be evidence of flowing liquid water, but a new NASA study suggests that might not be the case
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These dark streaks on Mars were once thought to be evidence of flowing liquid water, but a new NASA study suggests that might not be the case
These dark streaks on Mars were once thought to be evidence of flowing liquid water, but a new NASA study suggests that might not be the case

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.

Source: NASA

Just when we put a human on the red planet, those many 'if', or 'may' will be resolved. The best part is that a human in the red planet would be able to perform all tests and walk what all robots have done in decades, in a day. The thing that still need a lot improvement it propulsion, to be able to get there and come back way faster than technology today permit.
@Aussie_2017 For the effort a manned mars mission would take we could deploy an army of all kinds of exploring robots on the planet. Mars is a freezing cold desert with no water, life, atmosphere, and tons of solar radiation. Everyone wants to have hope but even if we successfully terraformed mars and had oxygen in the atmosphere solar winds would strip it away and Mars is probably our best shot. The next closest solar system with a potentially habitable planet would take thousands of years to reach and who knows if we'd make it or what we'd find when we got there. We're learning from long ISS missions that sustained time in zero gravity environments is hard on people. We could sustain a base on mars but it's probably only slightly more habitable than the ISS itself.
I suspect that, if there is any water on Mars, it will be in deep underground aquifers, that will need drills hundreds of feet at least, or even several thousands of feet to reach. When water was first accessed in Arizona, the wells only needed to be a hundred or so feet deep, now, its well over a thousand. Given the long term aridity of Mars, a thousand feet would be optimistic at best. With water on Mars, I'd rather be a disappointed pessimist, than a disappointed, and dying of thirst optimist.
Many years ago my dad as a boy asked my great grandpa if people would ever go to the moon (back in the 50's). My great grandpa said..." well if there is anything on the moon that is worth while or can be messed up then God won't let us go there...but since I don't think there is anything to mess up or worth while then sure, man will go to the moon."
I am guessing the same is true for mars. There is nothing there that can't be obtained for much less cost here on earth. I don't see life ever being sustainable on mars without constant help from earth...and that would cost a fortune both on our environment as well as pocket book. It is fun exploring other planets in our solar system but we are not going to find much IMO. Even if we found solid gold I wonder if it would cost more to get it then to mine it on earth. It cost about $16,000 per pound of gold here on earth and it costs about $10,000 per pound to orbit, let alone from even solid gold from mars would be a tough sell.
@Daishi, mate, we already have an 'army' of robots out there, all of them with good results but take too long to get any results. A human in Mars, repeating... means that that person would be able to perform all of those robots experiments in a fraction of the time and better human ingenuinity would be able to observing and combining results or experiments to extract more information than just through a robot. YES there's several problems (we need to start looking into each one and invest more $$$ and research on it for we get there), but nobody is saying anything in terraform Mars, would be great if we could (have heard about not having all eggs in one basket?). it's just to get there and get the answers, like I wrote before, better propulsion systems that can take us there and back way faster, it would mitigate most of the radiation, food suply and gravity problems. We need explore to learn and grow as civilization. Just robots so far is clear not the answer. Too limited by the tools it has built in, communications are too slow, moving super slow too, so again a human there would be the best. Honestly just be a little more imaginative that you will find possible solutions don't just look down and think we belong to the dirty.
@Aussie_2017 As robotics and automation improve they are threatening jobs here on earth that can be completed by humans for minimum wage. The ability to be on mars and not die is a bonus. In the amount of time and money it would take to get a person on mars with useful tools our rovers should improve significantly and the gap between what robots can do on mars and what humans can do might not be that significant. We could probably build a GPS network for mars, a base there, and a drill rig that takes core samples before the first human shows up. At worst we could send all the parts and tools for a base so if the human can't get back off the planet they can carry out the rest of their time building out the base station.