The discovery of dark streaks on the red planet’s slopes in 2011 was an exciting prospect for astronomers. It had served as a potential proof of subsurface water on Mars.

The presence of water further meant that the planet could support life, even if on the level of microbes. Now, however, it seems all that excitement could have been in vain.

Recurring Slope Lineae On Mars

A research team, comprising of members from U.K.’s Durham University and U.S.A.’s University of Arizona, the Planetary Science Institute, and the United States Geological Survey, conducted a detailed study of the recurring slope lineae, known as RSL, on Mars.

The new findings, published in the journal Nature Geoscience on Nov. 20, suggest that the dark streaks, interpreted as water, may actually be caused by granular flows like dust and sand.

Scientists have been puzzled by RSL ever since they were first discovered. Thousands of them recur annually during the red planet’s hottest season. They grow darker and longer before fading in winter. The RSL are found on rocky, steep slopes of the red planet’s darkest areas including the northern plains, the equator, and the southern mid-latitudes.

The seasonal appearance of the RSL had looked like flowing water to scientists, however, they were perplexed at the same time about how water could form only at the steep slopes’ tops, which inclined more than 27 degrees.

After studying 151 RSL over 10 sites, the research team observed that all of the dark streaks ended at similar points, irrespective of the slope’s length. The presence of liquid, however, would have made the streaks and slopes longer.

A more detailed analysis then showed the scientists that the streaks behaved just as dry sand grains do on active dunes, settling at the same angle of repose.

"We've thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand," Colin Dundas, lead study author, said. "This new understanding of RSL supports other evidence that shows that Mars today is very dry."

Red Planet Is Not Completely Without Water

According to Dundas, just because the dark streaks are sand does not mean the red planet is completely without water. The planet has a lot of ice at the poles and subsurface ice, and water can be drawn out from the atmosphere by deliquescent salts to create liquid under certain conditions.

The NASA rovers have also made various discoveries on Mars indicating a range of environments that had liquid water in the past. Hydrated minerals within rocks have also been found. Dundas, however, added there may be little surface liquid on the red planet today. The researchers also said if water is present, it is likely a small amount, which is not conducive to life.

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