Still more about Martian water

A recent study published in Scientific Reports, an open access Nature magazine’s journal, points to the likely possibility that Slope Streaks, one of the most conspicuous topographic features of the surface of Mars, have deliquescence as their root cause. The paper,entitled Martian slope streaks as plausible indicators of transient water activity and led by the Atmospheric Science Group with Anshuman Bhardwaj as first author, enhances the importance of the ground-atmosphere water exchange cycle and its liquid state phases in the Martian environmental dynamics, a matter that will be directly investigated by means of the HABIT instrument for the first time during the ExoMars 2020 mission.

Slope Streaks at Arabia Terra imaged by HiRISE
Credit: NASA/JPL/University of Arizona

Slope Streaks are a particular kind of feature frequently observed on the Martian surface which have been described and differentiated from others with similar appearance, such as the Recurrent Slope Lineae (RSL). Slope Streaks are formed at low latitudes on terrains with low thermal inertia and high albedo, which are covered by a relative abundant amount of fine dust, and they can reach lengths of up to 1,000 m and widths of 100 m.
Whereas RSL’s are thought to be landslides of loose materials caused by salty liquid flow, the most accepted explanations of the formation of Slope Streaks resort to different kind of “dry” processes. Now, this new study includes them among the topographic features determined by the occurrence of liquid state phases within the daily water exchange cycle between the atmosphere and the ground, contributing a solid justification for both its origin and some of its most intriguing observed characteristics: they form without showing any seasonal pattern, and don’t imply downslope movements of materials.
The root phenomenon causing the formation of Slope Streaks is depicted as a sudden and short-term process of deliquescence prompted by the presence of highly hygroscopic chlorine salts during favourable environmental conditions of temperature and relative humidity. Due to the typical low thermal inertia of the soils in which Slope Streaks are produced, these conditions concur sometime along the wide diurnal variations of temperature irrespectively from the season. According to the scenario proposed in the paper, once the brines are formed, they propagate through the dust, being capillarity the main factor favouring the process. Taking into account the relatively weak Martian gravitational pull, this mechanism would as well explain the observed capability of slope streaks to overcome obstacles up to 3 meters high along its course.
Firstly, the study is based mainly on the analysis of images gathered by ConTeXt (CTX) imager, and the High-Resolution Imaging Science Experiment (HiRISE) camera, both on board Mars Reconnaissance Orbiter, (MRO). From this huge collection of images, covering almost the entire Martian surface, it has been possible to map a global distribution of the Slope Streaks. Then, starting from the hypothesis of a wet formation mechanism, the authors have looked for the areas in which the necessary elements to give place to deliquescence are present by using data from rovers and remote sensing instruments on board orbiters. They found a clear correlation between slope streaks and presence of chlorine salts, regolith hydration levels, and atmospheric water content in the overlying boundary layer. These two latter, in addition, reveal a strong ground-atmosphere coupling regarding water activity. On the other hand, it is proposed that the brines trigger a chain of chemical reactions involving other components of the soil, giving place to the formation of certain by-products which would explain the dark shade of the slope streaks.

Map showing the coincidence of Slope Streaks distribution
with the presence of the considered deliquescence factors.
Credit: Anshuman Bhardwaj.

As for the particular salts responsible for deliquescence, with relation to the environmental parameters (basically temperature) which could permit its occurrence according to the eutectic temperature of the solutions they form, it has been established that iron chloride (FeCl3), calcium chloride (CaCl2), and magnesium chloride (MgCl2) are the most likely ones.
The conclusions of the study have major implications in several aspects for the further exploration of Mars. First, the catalogue of zones in which liquid water is present (though in a very transient way) has been extended. These zones are considered “Special Regions” for the risk of biological contamination with terrestrial organisms the availability of liquid water suppose. Then, and assuming that water is a necessary requirement for any kind of life, these regions are among the best candidates to host possible extant Martian life. Moreover, the investigation of the deliquescence process on Martian ground could result affected by the identification of particular salts involved in the phenomenon, and the geochemistry within the soil should be revised under the light of this work, which could pose clues for the definition of strategies to apply in the scheduled exploration missions regarding the investigation of the Martian water cycle and for the better design of future exploration instruments.