Post contributed by Lujendra Ojha, Georgia Institute of Technology.
Recurring slope lineae (RSL) are dark, narrow features forming on present-day Mars that have been suggested to be a result of transient flowing water. RSL extend incrementally downslope on steep, warm slopes, fade when inactive, and reappear annually over multiple Mars years (Images 1 and 2). Average RSL range in width from a few meters (<5 m), down to detection limit for the High Resolution Imaging Science Experiment (HiRISE) camera (~0.30 m/pixel). The temperatures on slopes where RSL are active typically exceed 250 K and commonly are above 273 K. These characteristics suggest a possible role of salts in lowering the freezing point of water, allowing briny solutions to flow.
Initially, RSL were only found to be in the southern hemisphere of Mars. It was thought that this was because slopes in the southern hemisphere experience higher peak surface temperatures than those in the north, as perihelion (the planet’s closest approach to the sun) occurs during southern summer. RSL are now known to be widespread in the equatorial regions of Mars and have also been discovered in the northern hemisphere. Although geomorphic, visual and temporal data support the liquid hypothesis for RSL, spectroscopic evidence had been equivocal until recently. Spectroscopic confirmation of the “wet” hypothesis for RSL requires either detection of absorption bands typical of liquid water, or of hydrated salts precipitated from that water.
The mineralogic composition of RSL and their surroundings can be investigated using orbital data acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) which acquires spectral data over wavelengths between 0.4 and 3.92 µm. Both liquid water and hydrated salts have diagnostic absorption bands at 1.4 and 1.9 µm and a broad absorption at 3.0 µm. Analysis of the spectral data from CRISM at four different locations where RSL are observed showed evidence for hydrated salts. The hydrated salts were only observed in the seasons when RSL are most extensive, which suggests that the source of the hydration is RSL activity. The hydrated salts most consistent with the observed spectra are magnesium perchlorate, magnesium chlorate, and/or sodium perchlorate (Image 3). These finding strongly support the hypothesis that RSL form as a result of contemporary water activity on mars.
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