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The uncertainty of science: A just-published survey of Mars following the 2018 global dust storm found that there was a significant increase in the seasonal dark streaks that scientists call recurring slope lineae, providing more evidence that these streaks are not caused by some form of water seepage but instead are related to some dry process.

The map to the right is figure 2 from that paper. The white dots show the candidate lineae that appeared following the 2018 global dust storm. About half were new streaks, not seen previously.

From the paper’s conclusion:

Our results suggest that the presence of freshly deposited dust causes or enhances [lineae] formation. This result may resolve the mystery of why [lineae] occur on some slopes but not others that are largely similar (steep, rocky, and warm). Rather than requiring some unseen variable such as groundwater or salt or ripples, the activity may in part be a function of whether or not sufficient dust is deposited over a slope in each year. The otherwise puzzling recurrence and year‐to‐year variability of [lineae] activity can be explained by variable yearly dust fallout.

For years scientists have puzzled over the cause of these seasonal dark streaks. Their look and nature and presence on slopes strongly suggested to our Earth-based eyes that they were caused by the seepage for some form of liquid brine or water.

The data now shows increasingly that they might be a dry Martian process involving dust. According to this new hypothesis, each year dust accumulates on slopes due to the passage of dust devils and wind. When enough dust piles up and encouraged by the warmer summer temperatures, it then flows downhill as a thin avalanche of dust, staining the surface but not changing the topography significantly. I asked Colin Dundas of the U.S. Geological Survey’s Astrogeology Science Center in Arizona and one of the co-writers of this new paper if this study confirms that the cause of recurring lineae is this dry process. His answer,

The association of high lineae activity with dust storms alone could have many causes. However, there are now multiple lines of evidence that lineae are dry grainflows, such as systematically occurring on angle-of-repose slopes. The correlation with dust storms helps us to understand how those grainflows could be working–possibly because sand is also transported on the surface in the storms, and/or because the presence of a coating of dust on the surface helps to cause flows.

Nothing is yet settled, mostly because the cause of these dark streaks is likely complex with a mixture of factors that might even vary from location to location, depending on the material in the ground. Nonetheless, the data seems to shifting to a dry explanation rather than a liquid one. The number of lineae found inside Valles Marineris, in the arid equatorial regions, favors this dry explanation.

At the same time, the preponderance of lineae at the 30 to 60 degree southern latitudes puts them squarely in the region where a lot of glacial features are found. Thus, the science remains uncertain, though that uncertainty is getting narrower and more refined.