The researchers analysed narrow, down-slope trending surface features on Mars that are darker than their surroundings, called Recurring Slope Lineae, or RSL. The feature would generally form during the warm seasons and gradually flows to downhill and eventually dry out during winter.
So, the streaks behave like tumbling sand grains, not like flowing water.
RSL are mostly found on steep rocky slopes in the dark regions of Mars, such as the southern mid-latitudes, Valles Marineris near the equator and in Acidalia Planitia on the northern plains.
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The researchers discovered these after observations using the Mars Reconnaissance Orbiter's High-Resolution Imaging Science Experiment (HiRISE) camera.
A new study suggests that dark streaks on Mars are signs of flowing sand - not water.
The team found that RSL occur on slopes steeper than 27 degrees, and the flow ends on reaching an "angle of repose" where dry sand accumulates and forms a lump, something which is commonly seen on dunes on Earth. "This new understanding of RSL supports other evidence that shows that Mars today is very dry". Liquid water would have readily extended to less steep slopes.
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Monday's news throws cold water on 2015 research that indicated these recurring slope lines were signs of water now on Mars. Another report published in 2016 create doubts about the possibilities of underground water at RSL sites. It could be limited to traces of dissolved moisture from the atmosphere and thin films of water.
However, RSL remain puzzling. Seasonal changes in hydration of salt-containing grains might result in some trigger mechanism for RSL grainflows, such as expansion, contraction, or release of some water. If atmospheric water vapour is a trigger, then a question is why the RSL appear on some slopes but not others.
Scientists, however, are optimistic about the RSL as a unique feature of Mars. That's bad news in the hunt for microbes, unfortunately. "Remote sensing at different times of day could provide important clues".
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