Water pulled from the atmosphere by salts, or mechanisms with no flowing water involved, remain possible explanations for the occurrence of RSLs in part of the Valles Marineris region near Mars' equator.
"There are so many of them, it's hard to keep track," said study led author Matthew Chojnacki from the University of Arizona.
"The occurrence of recurring slope lineae in these canyons is much more widespread than previously recognised. As far as we can tell, this is the densest population of them on the planet, so if they are indeed associated with contemporary aqueous activity, that makes this canyon system an even more interesting area than it is just from the spectacular geology alone," he added.
The team examined the geological context of canyonland RSL sites and also calculated how much water would need to be present if the streaks are due to liquid water seeping through a thin surface layer to darken the ground.
Many of the sites where RSLs were previously identified are on inner walls of impact craters. At that type of site, a conceivable explanation could be that an extensive underground layer holding water was punctured by the crater-forming impact long ago and still feeds warm-season flows.
If it is seeping water that darkens RSLs, the amount of liquid water required each year to form the streaks in the studied portion of Valles Marineris would total about 10 to 40 Olympic-size swimming pools (about 30,000 to 100,000 cubic metres), the researchers estimate.
However, no such underground layer fits the ridge or peak shapes at several of the RSL sites in the new study.
Another possible mechanism previously proposed for RSLs is that some types of salts so strongly pull water vapour out of the Martian atmosphere that liquid brine forms at the ground surface.
The new study results, reported recently by NASA, bolsters the link between RSL and salts. Some sites bear bright, persistent streaks near the dark, seasonal ones. The bright streaks might result from salt left behind after evaporation of brine.
"There do seem to be more ways atmosphere and surface interact in the canyons than in blander topography, such as clouds trailing out of the canyons and low-lying haze in the canyons," Chojnacki said.
Another factor added by the new study is that RSLs not only darken the surface, but are also associated with material moving downslope. The research documents slumping and other three-dimensional changes at some RSL sites, occurring seasonally in tandem with the streaks.
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