Life may have thrived at the beginning of Mars, until it spearheaded the climate change that caused it to disappear

Researchers from the Arizona Department of Ecology and Evolutionary Biology simulated the conditions hypothetical life forms would have encountered on Mars 4 billion years ago, when liquid water was likely present in abundance on the red planet. Credit: ESO / M. Kornmesser

If there ever was life on Mars – and that’s a huge “if” – the planet’s childhood conditions most likely would have supported it, according to a study by researchers at the University of Arizona.

Dry and extremely cold, with a muted atmosphere, it is extremely unlikely that today’s Mars could support any life on the surface. But 4 billion years ago, Earth’s smallest red neighbor may have been far more hospitable, according to the study, published in Astronomy of nature.

Most Mars experts agree that the planet started out with a much denser atmosphere than it is today. Rich in carbon dioxide and hydrogen, it would likely have created a temperate climate that allowed water to flow and, perhaps, microbial life to thrive, according to Regis Ferrière, a professor in the Arizona Department of Ecology and Evolutionary Biology and one of the two. senior authors on the paper.

The authors are not claiming that life existed on the first Mars, but if that were the case, Ferrière said, “our study shows that the subsoil, the first Mars most likely would have been habitable for methanogenic microbes.”

Such microbes, which make their living by converting chemical energy from their environment and releasing methane as a waste product, are known to exist in extreme habitats on Earth, such as hydrothermal vents along the crevices of the ocean floor. There, they support entire ecosystems adapted to overwhelming water pressure, near-freezing temperatures and total darkness.

The research team tested a hypothetical scenario of an emerging Martian ecosystem using state-of-the-art models of the crust, atmosphere and climate of Mars, along with an ecological model of a community of Earth-like microbes that metabolize carbon dioxide and hydrogen. .

On Earth, most hydrogen is bound in water and is not frequently encountered alone, except in isolated environments such as hydrothermal vents. Its abundance in the Martian atmosphere, however, could have provided a large supply of energy for methanogenic microbes some 4 billion years ago, at a time when conditions would have been more favorable for life, the authors suggest. The first Mars would have been very different from what it is today, Ferrière said, tending towards heat and humidity rather than cold and dryness, thanks to the large concentrations of hydrogen and carbon dioxide, both strong greenhouse gases that trap the heat in the atmosphere.

“We think Mars may have been a little colder than Earth at the time, but not as cold as it is now, with average temperatures most likely above the freezing point of water,” he said. “While present-day Mars has been described as a dust-covered ice cube, we imagine the first Mars as a rocky planet with a porous crust, soaked in liquid water that likely formed lakes and rivers, possibly even seas or oceans.”

That water would have been extremely salty, he added, according to spectroscopic measurements of exposed rocks on the Martian surface.

To simulate the conditions that early life forms would encounter on Mars, the researchers applied models that predict surface and crust temperatures for a given atmospheric composition. They then combined this data with an ecosystem model they developed to predict whether biological populations would be able to survive in their local environment and how they would affect it over time.

Life may have thrived at the beginning of Mars, until it spearheaded the climate change that caused it to disappear

The study revealed that while ancient Martian life may have initially thrived, it would have made the planet’s surface ice-covered and uninhabitable, under the influence of consumed hydrogen and methane released into the atmosphere. Credits: Boris Sauterey and Regis Ferrière

“Once our model was produced, we put it to work in the Martian crust, figuratively speaking,” said the paper’s first author, Boris Sauterey, a former postdoctoral fellow in Ferrière’s group who is now a postdoctoral fellow. doctorate at the Sorbonne Université in Paris. “This allowed us to evaluate how plausible an underground Martian biosphere would be. And if such a biosphere existed, how it would change the chemistry of the Martian crust and how these processes in the crust would affect the chemical composition of the atmosphere.”

“Our goal was to model the Martian crust with its mix of rock and salt water, let gases from the atmosphere diffuse into the ground and see if methanogens could live with it,” said Ferrière, who has a joint appointment. at Paris Sciences & Lettres University in Paris. “And the answer is, in general, yes, these microbes could have made a living in the crust of the planet.”

The researchers then decided to answer an intriguing question: If life thrived underground, how deep would one have to go to find it? The Martian atmosphere would provide the chemical energy that organisms would need to thrive, Sauterey explained, in this case hydrogen and carbon dioxide.

“The problem is that even on early Mars it was still very cold on the surface, so the microbes would have to go deeper into the crust to find habitable temperatures,” he said. “The question is how deep does biology have to go to find the right compromise between temperature and the availability of molecules from the atmosphere they needed to grow? We found that the microbial communities in our models would be happier in the few hundred meters. . “

By modifying their model to take into account how the processes occurring above and below the soil affect each other, they were able to predict the climate feedback of the change in atmospheric composition caused by the biological activity of these microbes. In a surprising twist, the study revealed that while ancient Martian life may have initially thrived, its chemical feedback in the atmosphere would have triggered a global cooling of the planet, eventually making its surface uninhabitable and driving life forever. deeper underground, and perhaps to extinction.

“According to our results, the atmosphere of Mars would have been completely changed by biological activity very quickly, within a few tens or hundreds of thousands of years,” Sauterey said. “By removing the hydrogen from the atmosphere, the microbes would have drastically cooled the planet’s climate.”

Soon the surface of Mars would become glacial as a result of biological activity. In other words, climate change driven by Martian life may have helped make the planet’s surface uninhabitable very soon.

“The problem these microbes would have faced is that the atmosphere of Mars has practically disappeared, completely thinned, so their energy source would have vanished and they would have had to find an alternative source of energy,” Sauterey said. “Also, the temperature would have dropped significantly and they would have had to go much deeper into the crust. For now, it’s very difficult to say how long Mars would have been habitable.”

Future Mars exploration missions may provide answers, but the challenges will remain, according to the authors. For example, while they identified Hellas Planitia, a vast plain carved out by the impact of a large comet or asteroid very early in the history of Mars, as a particularly promising site for looking for evidence of past life, the topography of the place generates some of the Le more violent dust storms than Mars, which could make the area too risky to be explored by a self-contained rover.

However, once humans begin exploring Mars, such sites could be shortlisted for future missions to the planet, Sauterey said. For now, the team focuses its research on modern Mars. NASA’s Curiosity rover and the European Space Agency’s Mars Express satellite have detected elevated levels of methane in the atmosphere, and although such spikes may result from processes other than microbial activity, they allow for the interesting possibility that life forms such as methanogens may have survived in isolated pockets on Mars, deep underground, an oasis of alien life in an otherwise hostile world.

Underground microbes may have swarmed ancient Mars

More information:
Boris Sauterey et al, Early habitability of Mars and global cooling by H2-based methanogens, Astronomy of nature (2022). DOI: 10.1038 / s41550-022-01786-w

Provided by the University of Arizona

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