Pockets of salt water discovered beneath the surface of Mars

The presence of liquid water on Mars is a story full of twists and turns. There are those dark streaks that appear in summer and disappear in winter, which we first heard about in 2011 : is it seasonal salt water flows, of dry sand slides caused by the Sun or … a computer artifact ? While this question has excited astrogeologists for almost ten years, another lead has emerged that seems to be gaining credibility: there could be lakes of salt water beneath the surface of Mars.

The announcement of this great discovery dates from the summer of 2018: a team of Italian scientists explained, in the review Science, to have probed the Martian subsoil by sending radar signals, and by analyzing the time they take to rise to the surface and their power on arrival. They have thus collected clues on the nature of the soil and claim that 1.5 km below the surface, where the radar echo becomes very sharp, it must pass through a liquid layer. The lake of sedimentary water, very cold, would measure 20 kilometers wide and less than a meter thick.

Radar echo of the area probed at the south pole of Mars. The horizontal line at the top is the area. Underneath, deposits of ice and dust. Deeper still, highlighted in light blue, the vivid echoes that suggest liquid water. Image ESA / NASA / JPL / ASI / Univ. Rome; R. Orosei et al 2018

“Other wetlands”

The Italians have continued to dig into the question … and the soil of Mars, programming new radar measurements in the region where they suspect the existence of this lake, near the south pole. They always use the instrument for these purposes Marcians of the European probe Mars Express, in orbit around the red planet since the end of 2003. And to be sure not to go astray, planetologists have also implemented a new method of data analysis, “Based on signal processing procedures usually applied to land ice caps.”

Bingo! Not only the Martian region ofThe slope last seems to still harbor a lake with the new method of analysis, but other interesting places are discovered within the extended prospecting perimeter. The results are published this week in Nature. They “Reinforce the announcement of the detection of a body of liquid water in Ultimi Scopuli, and indicate the presence of other wetlands in the vicinity”say Sebastian Emanuel Lauro and his colleagues.

“This means that we are not dealing with an isolated discovery, underline for IFLScience Elena Pettinelli, co-author of the study. It is a system. And that changes things. ”

Map of the Martian region surveyed by radar, near the South Pole.  Over an area of ​​120 by 90 kilometers, the areas in blue suggest the presence of liquid water.

Map of the Martian region surveyed by radar, near the South Pole. Over an area of ​​120 by 90 kilometers, the areas in blue suggest the presence of liquid water. Lauro et al, Nature 2020

Hypersaline

But what are these Martian lakes made of? “The most plausible form of liquid water under the Martian surface, under current physical conditions”, is an aqueous hypersaline solution. On Earth, such liquids (also called brines) “Persist at temperatures well below the freezing point of water.”

While closely studying this idea inspired by terrestrial phenomena, the Italian researchers recall that it is only a hypothesis: “The unique combination of physical, geological, climatic and topographical conditions that could favor the formation of liquid water and its long-term sustenance in an underground lake at the South Pole is, at best, a matter of speculation today. “

But there are arguments to go in this direction. The American probe Phoenix, which arrived on Mars in 2008, revealed salts called perchlorates in the ground. It has since been known that perchlorates absorb water molecules present in the Martian atmosphere and thus make it possible to form hypersaline solutions, through the phenomenon of deliquescence.

“The possibility of large masses of hypersaline water on Mars is particularly promising because of the potential for mircrobian life”, like to remind the authors of the study in conclusion. Bacteria extremophiles such as we find on Earth or organisms anaerobic are not afraid of such conditions. This is enough to put the red planet back in the spotlight, while the hope aroused by the Venusian phosphine falls like a souffle.


Camille Gevaudan

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