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Recurring slope lineae
Recurring slope lineae appear as dark streaks in this picture from the HiRISE camera on NASA’s Mars Reconnaissance Orbiter. (NASA / JPL / Univ. of Arizona / Lujendra Ojha et al. / Geophysical Research Letters)

Scientists have long been intrigued by what seem to be wet streaks that appear on the slopes of Martian craters in warm weather, and disappear in winter. Now a research team reports that the best explanation is that they’re not wet streaks at all, but streaks of dust and sand.

The findings, published today in Nature Geoscience, are likely to disappoint those who hoped that the features known as recurring slope lineae, or RSLs, point to sources of liquid water beneath the Red Planet’s surface.

“This new understanding of RSL supports other evidence that shows that Mars today is very dry,” study lead author Colin Dundas of the U.S. Geological Survey’s Astrogeology Science Center said in a news release.

Some astrobiologists had hoped that the areas around the RSLs just might harbor subsurface life. That’s why NASA has said the thousands of potential RSL sites, including a smattering of prospects near the Curiosity rover, should be off-limits for the time being due to concerns about contamination.

The report in Nature Geoscience is based on an analysis of 151 streaky features at 10 sites. Nearly all of the streaks appear on slopes that are steeper than 27 degrees, which would be consistent with the behavior of tumbling sand. If the streaks were caused by water seeping from the subsurface, they should be seen on slopes that are less steep, the researchers say.

What’s more, all of the streaks stop at a place matching the dynamic “angle of repose” that applies to sand dunes on Mars and on Earth.

“It can’t be a coincidence,” said the University of Arizona’s Alfred McEwen, the principal investigator for the High Resolution Imaging Science Experiment on NASA’s Mars Reconnaissance Orbiter.

So why do the streaks gradually appear starting in the spring, and fade away in the winter? Researchers suggest that hydrated salts in Martian soil might pull traces of water vapor out of Mars’ thin atmosphere when the weather warms, triggering flows of sand. The darkening and lightening of the streaks could be linked to changes in hydration over the course of the Red Planet’s seasons.

Why the streaks appear on some slopes and not others is a puzzle yet to be solved.

“RSL probably form by some mechanism that is unique to the environment of Mars, so they represent an opportunity to learn about how Mars behaves, which is important for future surface exploration,” McEwen said.

Rich Zurek, project scientist for the Mars Reconnaissance Orbiter at NASA’s Jet Propulsion Laboratory, said a full understanding of how RSLs bloom and fade would probably require on-site investigation. But that investigation will still have to be conducted carefully, he said.

“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,” Zurek said.

In addition to Dundas and McEwen, the authors of “Granular Flows at Recurring Slope Lineae on Mars Indicate a Limited Role for Liquid Water” include Matthew Chojnacki, Moses Milazzo, Shane Byrne, Jim McElwaine and Anna Urso.

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