About five years ago, scientists noticed something unusual on Mars. Images taken at different times of the Martian year by the Mars Reconnaissance Orbiter’s HiRISE camera showed dark areas developing on steep slopes in the summer and then vanishing during the Martian winter. The pattern of these dark areas looked like what you’d expect from a liquid flowing downhill.
At the time, water seemed like an obvious explanation for the dark areas, which would make this the first evidence of liquid water in the present of Mars. But to confirm this theory, scientists needed to get a reading on the chemical composition of the dark streaks. Now, researchers have overcome some major technical hurdles to get these readings, and the results indicate that the streaks contain water-rich salts.
The dark features have picked up the name “recurring slope lineae,” or RSL. They appear on steep slopes, such as crater walls, and form braided patters that look like water flows. RSL are absent in the winter and spring, and only appear during the near the Martian equator during the summer, at which point the temperatures in the area can often climb above the freezing point of water. Any liquid water should evaporate into the sparse Martian atmosphere quickly, but dissolved salts will inhibit evaporation, and can lower the freezing point of water by as much as 80K.
The people who initially discovered the RSL favored salty brines flowing downhill as the most likely explanation for their seasonal appearance, but they lacked direct evidence for the presence of water.
It turned out that obtaining any such evidence was exceedingly difficult. The RSL are typically only a few meters in width, and that’s well within the range of the HiRISE, which has an astonishing resolution of a quarter meter per pixel. The Mars Reconnaissance Orbiter also carries a spectrometer (Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM) that can separate out different wavelengths of light and use these to infer the chemistry of what it’s imaging. But CRISM’s resolution is relatively poor, at 18 meters per pixel.
(Just take a moment to savor the fact that we’re calling 18m/pixel poor resolution when we’re talking about imaging the surface of Mars from orbit.)
Frustrated by CRISM’s limits, a team of researchers has spent time figuring out how to get data from individual pixels on the CRISM instrument. The analysis was even able to identify six pixels within the instrument that were especially sensitive at the wavelengths where water salts absorb light. They also identified areas where RSL were broad or grouped together in order to focus imaging on areas where the dark material would largely fill a single pixel. Then, they imaged the same regions during the Martian summer and at times when the RSL weren’t present.
The results seem clear: all of the sites they imaged have hydrated salts present in the darkened materials. In two of them, the salts were a mixture of magnesium perchlorate, magnesium chlorate, and magnesium chloride; at a different one, sodium perchlorate provided the best match for the data. Perchlorates have already been found on other areas of Mars, and these salts are known to latch strongly onto water—magnesium perchlorate has been used commercially to remove water from gas mixtures.
While there was no evidence of liquid water, the appearance of these salts is clearly tied to the seasonal cycling of the RSL. This means that either they are carried into place by the flows and then precipitate out, or that the flows carry enough water to change the hydration state of salts already present. Either way, liquid water has to be involved.
There’s a remaining mystery, in that it’s not clear how the water gets onto these slopes in the first place. There’s probably not enough water in the Martian atmosphere for the salts to pull it from there directly. The equatorial regions of Mars aren’t expected to have much ice, which is expected to largely have shifted towards the poles. The fact that they appear near the tops of slopes also seems to rule out an underground aquifer.
The authors wrap up with what appears to be a plea: send something to the surface to look at these features more carefully. (In the language of the scientific literature, this is phrased as “The detection described here warrants further astrobiological characterization and exploration of these unique regions on Mars.”) They also point out that, on Earth, salts like these are the only source of water in the core of the Atacama desert, where they pull it from the atmosphere. Despite the meager supply, these regions are capable of supporting some salt-loving microbes.
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