Scientists have confirmed that water-soaked salts likely create dark seasonal lines on Mars.
Something’s been drawing dark streaks on Mars. These recurring slope lineae (RSL) appear on sloping ground — such as crater walls — during warmer months on the Red Planet. Growing several meters long (or much longer) during the late spring and early summer, they then fade as the temperature cools, only to reappear the following year, as persistent as daffodils. In equatorial regions, they can appear any time of year the ground gets enough sunlight.
The narrow lines look like they’re created by downhill flows. But at 250K (−9°F) and up, these areas are too warm for carbon dioxide frost and too cold for pure liquid water. Instead, planetary scientists suggest salty water is to blame: salts lower water’s freezing point enough that water could melt in these conditions, creating briny flows just beneath the surface.
So far, scientists haven’t found definitive signs of liquid water or waterlogged salts in areas containing RSL. Lujendra Ojha (Georgia Institute of Technology) and colleagues decided to narrow the search, using spectral observations by NASA’s Mars Reconnaissance Orbiter. Instead of averaging spectra over large areas containing RSL, as they and others had done before, the researchers homed in on only the pixels dominated by the streaks. They looked at four RSL sites: the craters Palikir, Horowitz, and Hale, and the valley Coprates Chasma.
The team found spectral matches in all four locations to salts that have been soaked in water. (The team didn’t detect liquid water itself, but that’s unsurprising, given that the spacecraft observed during local mid-afternoon, when conditions are likely to be driest.) Based on the spectral lines seen, the researchers report in Nature Geoscience, the salts appear to be perchlorates and chlorates.
Perchlorates are likely all over Mars; the Phoenix lander detected them at its northern landing site, and other lander and rover data support their presence elsewhere. They're also particularly good at dissolving in water, compared with other salts (such as sulfates) we might find on Mars. The water probably picks up the salts from the soil, then as it evaporates deposits them again on the surface, but in higher concentrations than they were before. As the salt deposits build up, they would alter the soil grains around them, changing the surface’s color and creating the low-reflectivity streaks.
Planetary scientist Briony Horgan (Purdue) says she’s convinced that the features Ojha’s team sees are from hydrated salts. Work done in desert, Mars-like valleys in Antarctica by Joseph Levy (University of Texas, Austin) and others have found that just a few percent by weight of water in the soil can lead to similar dark features on slopes. There, she says, the water is probably pulled up from the subsurface by capillary action — the “climbing” movement of water through a porous material thanks to a combination of cohesion, adhesion, and surface tension (it’s the same reason water creeps up a paper towel).
But it’s still unclear where the Martian water comes from. It might be from subsurface ice, although it’d be a little weird for that to be buried at such shallow depths near the equator, which is generally where these streaks appear. Salty grains on the surface might also grab so much water from the atmosphere that they dissolve and form a solution, but scientists don’t know if the planet’s atmosphere has enough water vapor for that to happen. (NASA planetary science head Jim Green did say in a Monday press conference that the rovers have found Martian air is more humid than expected.)
Wherever it’s coming from, liquid water does seem to be flowing on Mars.
“This is a big deal right now,” Horgan says. Next month, researchers are coming together in Houston for a workshop on picking a landing site for a future human mission to the Red Planet. The main science goals for such missions will be finding signs of past or present life on Mars. “The latter hasn’t been a target previously for rover missions, because of planetary protection concerns,” she explains. “But it looks like it is going to be a target of interest for human missions.” Because RSL are probably the first confirmed aqueous environments on Mars, these areas might make their way onto a list of proposed landing sites.
Reference: L. Ojha et al. “Spectral evidence for hydrated salts in recurring slope lineae on Mars.” Nature Geoscience. Published online September 28, 2015.
Senior Editor Kelly Beatty contributed to the reporting for this blog: additional factoids were added on Tuesday, September 29th based on NASA's press conference.
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