Salty Flows on Mars!

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.


Image 1: RSL flowing downhill on the steep slopes of Palikir crater in the southern mid-latitude of Mars. Credits: NASA/JPL/University of Arizona.


Image 2: RSL flowing downhill from the central peaks of Horowitz crater. HiRISE image PSP_005787_1475. Credit: NASA/JPL/University of Arizona

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.


Animation 1: Horowitz Crater, Mars. These dark, narrow, 100 meter-long streaks called Recurring Slope Lineae (RSL) flowing downhill are inferred to have been formed by contemporary flowing water. Recently, hydrated salts called perchlorates were detected on these slopes, corroborating the original hypothesis that RSL are formed by liquid water. The image is produced by draping an orthorectified image (PSP_005787_1475_RED) on a digital terrain model of the same site produced by the High Resolution Imaging Science Experiment (University of Arizona). Vertical exaggeration is 1.5. Image credit: NASA/JPL/University of Arizona.

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.


Image 3: (a) RSL emanating from bedrock exposures at Horowitz crater’s central peak. Part of HiRISE image PSP_005787_1475. (b) A different section of the same HiRISE image as a, showing RSL activity at a different peak (scale same as in a). (c) Black spectra corresponds to the highlighted areas in (a) and (b). Colored spectra are also shown from spectral mixing between the Martian soil and a variety of salts (as specified in the figure).

Further Reading:

McEwen, Alfred S., et al. “Seasonal flows on warm Martian slopes.” Science333.6043 (2011): 740-743. doi:10.1126/science.1204816

McEwen, Alfred S., et al. “Recurring slope lineae in equatorial regions of Mars.” Nature Geoscience 7.1 (2014): 53-58. doi:10.1038/ngeo2014

Ojha, Lujendra, et al. “HiRISE observations of Recurring Slope Lineae (RSL) during southern summer on Mars.” Icarus 231 (2014): 365-376. doi:10.1016/j.icarus.2013.12.021

Ojha, Lujendra, et al. “Spectral evidence for hydrated salts in recurring slope lineae on Mars.” Nature Geoscience 8.11 (2015): 829-832. doi:10.1038/ngeo2546

Stillman, David E., et al. “Observations and modeling of northern mid-latitude recurring slope lineae (RSL) suggest recharge by a present-day martian briny aquifer.” Icarus 265 (2016): 125-138. doi:10.1016/j.icarus.2015.10.007

Dundas, Colin M., and Alfred S. McEwen. “Slope activity in Gale crater, Mars.” Icarus 254 (2015): 213-218. doi:10.1016/j.icarus.2015.04.002

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