Brines and hydrated salts

Water won't flow at Martian temperatures, even in sun-facing slopes in summer, unless it contains some salt: a brine, in other words. Research published in 2015 found hydrated salts in the surface composition of RSL sites, and these salts increased during the season when streaks are active. Hydrated salts hold water molecules affecting the crystalline structure of the salt.

"Our findings are consistent with the presence of hydrated salts, because you can have hydrated salts without having enough to affect the measured seasonal temperature," Edwards said. "Salts can become hydrated by pulling water vapor from the atmosphere, with no need for an underground source of the water."

But the scientists point out that if RSLs were seasonal flows of briny water followed by evaporation, there would be an annual buildup of crust-forming salt, and this should affect temperature properties. So the lack of a temperature difference between RSL slopes and non-RSL slopes is evidence against evaporating brines.

Another theory for RSLs has proposed that they are mini-landslides of dry material with different thermal properties than the pre-existing slope material. Such might be the case with annual avalanches of powdery dust that accumulates from dusty air. But again the lack of a temperature difference argues against this.

Possible habitats for life

In the end, the mechanism that causes RSLs remains unknown. But Christensen said the discovery that RSLs are extremely dry has wider implications besides pointing the way forward for scientists as they try to understand how these features form, grow and fade each Mars year.

"By constraining the amount of water they contain — and by placing a very low upper limit on it — the THEMIS results help scientists determine how likely these seasonal streaks are as habitats for possible Mars life," he explained.

"Because RSLs are so dry," Christensen continued, "that makes them much less likely to be habitats for life."

Although this may be disappointing to researchers, he said, "It means missions to Mars designed to find biological traces and other markers for life could remove RSL sites from their target list."

Robert Burnham

Science writer, School of Earth and Space Exploration