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NASA had dangled there a few days ” major scientific discovery “ on Mars. Since then, speculation was rife about the webosphère, always ready to ignite when it comes to the Red Planet: detection of a life form of liquid water sources? The suspense ended with putting online an article presented at the European Planetary Science Congress, held in Nantes until 2 October. Meanwhile, the journal Nature Geoscience published Monday, September 28, the work of the same team detailing the observations conducted by the US Mars Reconnaissance Orbiter (MRO).
What was so it “Martian mystery” , finally resolved, evoked by the American agency – to whom it could happen to “oversell” certain discoveries? It concerns the nature of dark seasonal flows observed on slopes of Mars, when the temperature becomes more lenient there. According Lujendra Ojha (Georgia Tech, Atlanta) and colleagues, these flows are made up of different compositions brines made of chlorate and magnesium perchlorate and sodium perchlorate, mixed with a little water.
A salt mixture to remain in the liquid state
The discovery of such brines hardly surprising specialists: MRO and its predecessor, Mars Global Surveyor have long observed that castings gully Mars, in summer, on the slopes exposed to the sun. It is known that the conditions of temperature and pressure to the surface of Mars do not allow stagnation or flow of liquid water surface, which is frozen, either immediately evaporates. It was therefore necessary to devise combinations of salts that maintain the mixture in the liquid state: iron sulfate, sodium salts and ammonium salts, etc. Several models have been proposed in recent years
Meanwhile, the Phoenix lander of NASA. – Active near the north polar cap of Mars in 2008 – had confirmed the presence of perchlorates, also on subodorée landing sites Viking missions in new analyzes of data from the late 1970s Finally, in May 2015, the journal Nature Geoscience already announced the presence of such brines in the environment the Curiosity rover.
Could it be sources of life, knowing that on Earth, life was possible thanks to the presence of liquid water? “If we combine these observations with the thermodynamics of the formation of these brines and our current knowledge of terrestrial organisms, is it possible for organisms to survive on Mars these brines? questioned Vincent Chevrier (University of Arkansas), who co-authored the study. My answer is no. “ His appreciation is now more nuanced: it welcomes the comments of his colleagues, that ” provide a direct link between the theoretical models and geomorphological observations “, it also invites to address the question of life on Mars “with large tweezers” , both the environment on this planet, in terms of temperature and humidity, remains inhospitable.
The new study Nature Geoscience says the same thing: “The water activity in perchlorate solutions would be too low to be used to support life as we know it on Earth, “, she remembers, even if it mentions a kind of like on Earth, the Atacama Desert, where extremophiles bacteria manage to survive. But they are not subject to such hostile and intermittent conditions than those encountered on Mars.
The study details how a new method of analysis of data collected by the spectrometer MRO determined the composition of intermittent flows. “What the spectrometer analyzed, these are not liquid flows themselves says Marion Massé, Planetology and Geodynamics Laboratory (CNRS-University of Nantes), co-signatory of the article but the signature salt once the water evaporated. “
The imagined mechanism called” decay “is: chlorates and perchlorates in the soil would act as the salts used on roads Prevent ice, or those used to fight against certain condensation in humid rooms. They absorb the dew of the morning and evening, in sufficient quantity to burden the soil and cause it to slow flow on slopes above 30 °. So do not imagine spring streams rushing down the slopes on Mars, but impregnating the first centimeters of a spongy ground. “We will try to reproduce in the laboratory phenomenon” says Marion Massé, which emphasizes the need to verify that the low water content of the Martian atmosphere does not contradict this theory. And to proceed with caution, perchlorates having a tendency to explode …
Other explanations are advanced, such as the presence of underground water ice reservoirs that become liquid when in contact with salt the temperature rises. But it would explain how they are recharged, moreover at the top of the slopes, so that more easily imagine the water in the valleys.
Contrary to the implied NASA, mystery is not fully resolved. These results cover four sites, including the signatories of the study believe that it would be “justified” to explore them to validate this hypothesis. No doubt that the US agency, still seeking credit for its exploration missions, make sure to relay this request. Although she is likely to face in the usage precautions for protection of Mars: the scientific community wishes indeed prevent accidental contamination of the Red Planet by terrestrial microbes. To the extent that it is difficult and costly to fully sterilize spacecraft, they are in principle banned from the most likely land approach to host life. It might then be possible to determine whether organic matter detected do not come from stupidly Earth
These precautions will fly anyway shattered the day a human mission will land on Mars. Impossible to disinfect completely Homo sapiens that could not survive without the billions of microbes with which it lives in symbiosis, and he constantly excretes.
But until this colonization, which does is not expected before the 2030s, several robots are surveying Mars. Curiosity itself may try to get close, walking distance, a dropping on the sides of Gale Crater, said James Green, Director of Planetary Science at NASA, on the sidelines of the press conference, reports the New York Times. It would take two years to go see him.
hervenirom
morin@lemonde.fr
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