Mars’ Streaks of Flowing Water May Actually Be Sand

Scientists have debated for years if—and how much—water could exist on the Red Planet

mars water
The dark, narrow streaks were once thought to be caused by flowing water. But a new study suggests they are just rolling sand. NASA/JPL/University of Arizona

Is water flowing on Mars? As Ryan F. Mandelbaum reports for Gizmodo, scientists have concluded that any claims about flowing liquid water on Mars are likely a stretch.

In 2011, researchers noticed strange streaks down several Martian slopes that resemble paths of flowing water. Known as “recurring slope lineae,” or RSL, researchers believed these were some of the most compelling evidence yet for flowing water on the Red Planet. They published their work in 2015 in the journal Nature Geoscience.

But a new paper, also published in Nature Geoscience, contradicts this idea. The most likely culprit for the ebb and flow of the streaks, according to the study, are flowing grains of sand.

Researchers have identified thousands of these features at more than 50 sites, according to a press release. “They're found on steep, rocky slopes on the darkest areas of Mars: the equator, the northern plains, the southern mid-latitudes,” reports Ashley Strickland of CNN. They change in intensity, recurring during the warmest Martian seasons and fade into winter, she writes. Since their discovery, however, scientists have debated the source of the streaks and whether these lines were hints of flowing water—or even larger reservoirs of liquid beneath the surface of the planet.

In the latest analysis, however, scientists examined 151 dark streaks at 10 sites using observations from the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter. They also created 3D models of slope steepness to examine trends in the data. What they found is that the streaks are limited to steep slopes, nothing less than 27 degrees. They all end on what's known as the "angle of repose" for sand on Earth—the maximum angle in which the grains can be piled before slumping down.

"It can't be a coincidence," Alfred McEwen, HiRISE Principal Investigator at the University of Arizona, Tucson, and co-author of the new report, says in the press release. The researchers conclude that there’s no longer enough evidence to support the idea that these streaks represent flowing water.

They don’t rule out the possibility that water exists on the planet at all, however; the new study adds doubt to how much flowing water there is. “Liquid water involved is likely to be low volume with low activity, inhospitable to known terrestrial life, alleviating planetary protection concerns,” researchers state in the paper.

It’s well-established that water previously flowed on Mars—the Red Planet may have once even been peppered with lakes, evidenced by geologic formations shaped by flowing water and even meteorite composition. But the extent of the amount of water that once existed on the planet, and whether water could exist in the planet’s climate today, has been debated for years.

It seems the new paper in Nature Geoscience adds to the growing number of studies on the topic—but still doesn’t offer a definitive conclusion about the Martian landscape. There are many questions that still remain unanswered.