Study casts doubt on water flows as cause of streaks on Martian slopes

Images taken of Mars from orbit dating back as far as the 1970s have captured curious dark streaks running down the sides of cliffs and crater walls that some scientists have construed as possible evidence of flows of liquid water, suggesting that the planet harbors environments suitable for living organisms.

A new study casts doubt on that interpretation. Examining about 500,000 of these sinewy features spotted in satellite images, the researchers concluded they were created probably through dry processes that left the superficial appearance of liquid flows, underscoring the view of Mars as a desert planet currently inhospitable to life - at least on its surface.

The data indicated that formation of these streaks is driven by the accumulation of fine-grain dust from the Martian atmosphere on sloped terrain that is then knocked down the slopes by triggers such as wind gusts, meteorite impacts and marsquakes.

"The tiny dust particles can create flow-like patterns without liquid. This phenomenon occurs because extremely fine dust can behave similarly to a liquid when disturbed - flowing, branching and creating finger-like patterns as it moves downslope," said Adomas Valantinas, a postdoctoral researcher in planetary sciences at Brown University and co-leader of the study published on Monday in the journal Nature Communications.

"It's similar to how dry sand can flow like water when poured. But on Mars, the ultra-fine particles and low gravity enhance these fluid-like properties, creating features that might be mistaken for water flows when they're actually just dry material in motion," Valantinas added.

The study examined about 87,000 satellite images - including those obtained between 2006 and 2020 by a camera aboard NASA's Mars Reconnaissance Orbiter - of slope streaks, which form suddenly and fade over a period of years. They average roughly 1,970-2,540 feet (600-775 meters) long, sometimes branching out and going around obstacles.

The slope streaks were concentrated mostly in the northern hemisphere, particularly in three major clusters: at the plains of Elysium Planitia, the highlands of Arabia Terra and the vast Tharsis volcanic plateau including the Olympus Mons volcano, towering about three times higher than Mount Everest.

The researchers said limitations in the resolution of the satellite images mean they account for only a fraction of slope streaks. They estimated the actual number at up to two million.

Water is considered an essential ingredient for life. Mars billions of years ago was wetter and warmer than it is today. The question remains whether Mars has any liquid water on its surface when temperatures seasonally can edge above the freezing point.

It remains possible that small amounts of water - perhaps sourced from buried ice, subsurface aquifers or abnormally humid air - could mix with enough salt in the ground to create a flow even on the frigid Martian surface. That raises the possibility that the slope streaks, if caused by wet conditions, could be habitable niches.

"Generally, it is very difficult for liquid water to exist on the Martian surface, due to the low temperature and the low atmospheric pressure. But brines - very salty water - might potentially be able to exist for short periods of time," said planetary geomorphologist and study co-leader Valentin Bickel of the University of Bern in Switzerland.

Given the massive volume of images, the researchers employed an advanced machine-learning method, looking for correlations involving temperature patterns, atmospheric dust deposition, meteorite impacts, the nature of the terrain and other factors. The geostatistical analysis found that slope streaks often appear in the dustiest regions and correlate with wind patterns, while some form near the sites of fresh impacts and quakes.

The researchers also studied shorter-lived features called recurring slope lineae, or RSL, seen primarily in the Martian southern highlands. These grow in the summer and fade the following winter. The data suggested that these also were associated with dry processes such as dust devils - whirlwinds of dust - and rockfalls.

The analysis found that both types of features were not typically associated with factors indicative of a liquid or frost origin such as high surface temperature fluctuations, high humidity or specific slope orientations.

"It all comes back to habitability and the search for life," Bickel said. "If slope streaks and RSL would really be driven by liquid water or brines, they could create a niche for life. However, if they are not tied to wet processes, this allows us to focus our attention on other, more promising locations."