vie mars microorganismes methane

Life on Mars: Methane-producing microorganisms are believed to be the cause of the planet’s uninhabitability

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Due to its proximity and its many similarities to Earth, the planet Mars has always aroused special interest in the field of astrobiology. Despite the absence of irrefutable evidence, many researchers are convinced that when it was younger, it could have supported life, when its atmosphere was denser than it is today. But why has the red planet become uninhabitable? A new study suggests that life became extinct due to a period of rapid cooling induced by methane-producing microbial populations. The latter would have ended up consuming all the hydrogen and CO2 on the planet, making it uninhabitable due to the depletion of resources and the cold.

According to scientists, Mars would have had a much denser atmosphere in its early days – some 4 billion years ago. Rich in hydrogen and carbon dioxide, this atmosphere would have helped preserve liquid water on the surface, as potentially demonstrated by long ridges in the planet’s floor (likely ancient rivers) and other important clues.

Being farther from the sun than Earth, it would already have experienced cool temperatures at the core, although warm enough to keep the water liquid. ” We think Mars may have been a bit cooler than Earth back then, but not as cold as it is now, with average temperatures most likely above the freezing point of water. says Boris Sauterey, a former postdoctoral fellow at the University of Arizona and co-senior author of the new study. According to the latter, in its primitive state, the planet was a rocky planet with a porous and moist crust.

Its water would have been extremely salty, based on spectroscopic readings of the rocks on its surface. However, if the water could have remained liquid at more or less temperate temperatures, methanogenic microorganisms could have survived there. This type of microbial population is especially known for its ability to survive in areas with extreme conditions, living off the chemical energy produced by the transformation of hydrogen and CO2 into methane. These microbes exist on Earth in hydrothermal vents deep in the abyss, enduring crushing pressures, subzero temperatures, and total darkness.

Because Mars was once home to large amounts of hydrogen, researchers in the new study, which appeared in nature astronomy, suggest that these methane-producing bacteria could have existed on the planet when it was not yet so cold. These large concentrations of hydrogen and CO2, two powerful greenhouse gases, would have allowed the planet to maintain enough heat to keep water liquid, while feeding its microbial population.

Many hypotheses, such as the loss of its magnetic field, have been proposed to explain why the planet has become uninhabitable. For the first time, a theory suggests that this loss of habitability could have been induced by life itself. ” According to our results, the atmosphere of Mars would have been completely modified by biological activity very quickly, in a few tens or hundreds of thousands of years. Sauterey explains. ” By removing hydrogen from the atmosphere, the microbes would have significantly cooled the planet’s climate. “, he adds.

Microorganisms would have lived inside the crust.

To test their hypothesis, the researchers simulated in the laboratory the conditions in which methanogenic bacteria on Mars might have lived, reproducing temperatures on the surface and in the crust, as well as atmospheric conditions. They then introduced a microecosystem that catalyzes hydrogen and CO2. The crustal simulation also respected the planet’s mix of rocks and saltwater.

Our goal was to create a model of the Martian crust, let gases from the atmosphere diffuse into the ground, and see if methanogens could live with that. “, explains Régis Ferrière, professor in the Department of Ecology and Evolutionary Biology at the University of Arizona and also a co-author of the study. This simulation allowed us to evaluate the possibility that microbes could have lived inside the crust, and how this would have modified the chemistry of the soil as well as that of the atmosphere.

Result: in the presence of hydrogen and CO2, microbes could very likely have lived in the Martian soil. And since the latter was still relatively cold on the surface, microbial life forms would have sunk (up to 100 meters deep) below the surface to survive.

The researchers also found that if these methanogenic life forms had been able to thrive, they would have released so much methane into the Martian atmosphere that it would have caused global cooling of the planet. Then the microbes would have been forced to sink lower and lower, until they disappeared. Furthermore, the Martian atmosphere would have gotten thinner and thinner until it could no longer provide them with enough gas to assimilate.

However, it is still very difficult to say how long Mars could have remained habitable. Other unexplored Martian sites could one day reveal traces of these life forms, including Hellas Planitia, a vast plain carved out by a large comet or asteroid impact when the planet was still in its earliest stages.

Source: Nature Astronomy

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