Latest Mars finding hurts water hopes
New research indicates Red Planet’s streaks don’t behave like flowing liquid
In 2015, NASA announced it had “the best evidence yet” for water flowing on the surface of Mars: Dark, apparently damp streaks spotted on the Red Planet contained salts associated with liquid water.
The news had NASA’s head of planetary science, Jim Green, imagining future astronauts slurping up the salty sludge as they explored the Red Planet.
“Mark Watney could have taken advantage of this discovery,” he said at the time, referring to the central character of the movie The Martian. Others suggested the streaks might harbour microbial life.
But then U.S. Geological Survey scientists decided to take a look.
In a study published recently in the journal Nature Geoscience, the USGS says that those promising streaks are merely marks made by flowing sand or dust.
“This new understanding … supports other evidence that shows that Mars today is very dry,” lead author Colin Dundas said.
The finding is the latest disappointment for scientists who hoped the streaks, technically called recurring slope lineae (RSL), might indicate Mars isn’t the desolate desert wasteland it’s commonly made out to be.
While the RSL weren’t ever direct evidence of water, they seemed like a strong indicator.
Scientists had noticed that the streaks grew and shrank in response to the seasons — almost as if salty water was being heated by the sun and then flowing down ridges and hills.
Not only that, but the streaks contained perchlorates — molecules that help water stay liquid over a broader range of temperatures.
But when Dundas and his colleagues examined images of dozens of RSL at multiple sites, they found the “streaks” didn’t behave like flowing water.
For one thing, they existed only at the tops of very steep slopes.
For another, the streaks all seemed to end when their slopes matched the dynamic “angle of repose” — the steepest angle at which a given material can be piled without slumping.
If you’ve ever tried to build a sandcastle, you’re familiar with this concept. It’s why dry sand — which has a very shallow angle of repose — tends to slide out of shape, but wet sand — with a steeper angle of repose — can be piled into towers and turrets.
“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,” co-author Alfred McEwen of the University of Arizona, Tucson, told Phys.org.
McEwen is the principal investigator for HiRISE, a camera on the Mars Reconnaissance Orbiter that was used to image the RSL.
The RSL aren’t created by water, Dundas and McEwen both concluded.
Instead, they resemble the markings left by dry grains that slide down the sides of a sad, slumping sand castle.