The latest paper by members of the Group of Atmospheric Science (namely, Anshuman Bhardwaj, Lydia Sam, F. Javier Martín-Torres, and María-Paz Zorzano), published in the Reviews of Geophysics journal, presents Salar de Uyuni, Bolivia, as a novel Mars analogue site which shows exceptional conditions for studying Slope Streaks, an intriguing kind of Martian surface features whose formation mechanisms are quite unknown and controversial. The possibility that deliquescence is involved in the phenomenon poses interesting questions regarding the assessment of the habitability of the planet’s surface, and counting on this new place permits to develop proper investigations to deepen in its comprehension.
Slope streaks are prevalent surface features on present-day Mars, which were identified for the first time from imagery gathered by the Viking orbiters in 1970s. Since then, they have been better and better characterized as a different type of feature from other existing ones, such as Recurrent Slope Lineae (RSL) or avalanche scars for instance, though the processes involved in their formation are far from being clear.
Although slope streaks show a wide variety of morphologies (linear, curved, fan-shaped, or splitting/branching), they share a series of common characteristic that have been well-determined so far. They usually start in a point from where they gradually widen downslope, show sharp margins, dark colour with respect to the surrounding terrain, and typically lack of any surface relief and debris accumulation at the end.
Slope streaks can be found in equatorial and subequatorial regions of Mars (most densely in between ±30º latitudes) covered by dusty regolith, their appearance doesn’t show any seasonal pattern, and they form in gentle slopes, being able even to overcome obstacles of a few meters height in their course. Their lengths range from a few meters to several kilometres. In addition, several common physicochemical characteristics of the areas in which slope streaks form have been also defined, namely, chlorine concentrations above the average values of the Martian soil, high hydration levels in the regolith and in the overlapping atmospheric column, and high concentrations of Fe.
The published study develops a systematic review (as the journal’s title implies, it is specialized in this sort of works) of the literature devoted to describing the morphological and flow characteristics of slope streaks, and to discuss the possible processes involved in the appearance of these common topographic traits of Martian ground. The proposed mechanisms can be basically classified in two groups: dry and wet ones. Based on this thorough analysis, the authors strengthen their idea of the involvement of transient liquid water in triggering the formation of slope streaks, reinforcing the arguments they already posed in the previous study “Martian slope streaks as plausible indicators of transient water activity” for supporting the linkage of the phenomenon with the concurrence water in the liquid state as a result of brine formation on contemporary Mars.
However, and despite the current prevalence of models based on wet processes to explain the formation of slope streaks, the authors conclude that all the proposed ones, both dry and wet ones, and the different operating ways of each of them, should be taken into account in order to cover the wide variety of particular features displayed in the total amount of slope streaks localized so far.
Salar de Uyuni; a novel proposal for in situ study of the case
In the light of the foregoing, it appears that the matter gets more and more complicate rather than becoming clear. But there is a unique chance of going into depth in the subject by performing studies in one place on Earth in which the geochemical surface and atmospheric conditions regarding the occurrence of deliquescence and its consequences are similar to those on Mars: Salar de Uyuni, the largest salt flat on Earth with an area of ~10,000 square kilometres, at an altitude of 3,653 meters above sea level.
The Group of Atmospheric Science performed a field campaign last year at the northern area of this region, from which this new study stems, consisting of an analysis of imagery gathered by means of an Unmanned Aerial Vehicle to characterize the dynamics of the deliquescence, its visual effects on the terrain, and the further comparison with the observations from Mars.
One of the highlights of the present paper is that it proposes for the first time this place as an optimal Mars analogue to investigate the presence of transient liquid water mediated by deliquescent of salts such as chloride and sulphate ones, which are the dominant species on the soil. Due to its particular environmental conditions (rainfall regime, dryness, average temperature and soil composition), the brines in Salar de Uyuni, and the resultant slope streaks formed there are solely a result of salt deliquescence, so the in-depth study of the area can provide important clues about the formation and flow of the proposed Martian brines, not only with relation to slope streaks but to other surface features of the planet which have also been linked to the transient occurrence of liquid water produced by deliquescence of hygroscopic salts.
Dedicated investigations in Salar de Uyuni could cast new light on the discussion about the nature of slope streaks on Mars, clearing out some crucial keys regarding the mechanisms involved in their formation. The subject deserves the effort, since the possible concurrence of transient liquid water in form of brines in the origins of the feature would entail important implications for the future exploration of the planet regarding the assessment of the habitability of the surface and the delimitation of Special Regions in accordance with the Planetary Protection policies that should be observed.