When did life end on Mars?
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If life ever existed on Mars, it’s a huge assumption, but the conditions of Mars’ childhood most likely supported it, according to a study led by researchers at the University of Arizona.
It is dry, very cold, and has a weak atmosphere, so little terrestrial life can survive on Mars today. But four billion years ago, Earth’s little red neighbors may have been much friendlier, according to a study published in Nature Astronomy.
Most Mars experts agree that Mars began with a much denser atmosphere than it does today. According to Regis Ferrière, a professor in the Department of Ecology and Evolutionary Biology at the University of Arizona and one of his two senior authors on the paper, it is rich in carbon dioxide and hydrogen, enabling water flow and It probably created a temperate climate that allowed microbial life to thrive. paper.
The authors are not arguing that life existed on early Mars, but if it did, Ferrière said, “our study shows that underground, early Mars would very likely have been habitable to methanogenic microbes.”
Such microbes, which make a 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 fissures in the ocean floor. There, they support entire ecosystems adapted to crushing water pressures, near-freezing temperatures and total darkness.
The research team tested a hypothetical scenario of an emerging Martian ecosystem by using state-of-the-art models of Mars’ crust, atmosphere and climate, coupled with an ecological model of a community of Earthlike microbes metabolizing carbon dioxide and hydrogen.
On Earth, most hydrogen is tied up in water and not frequently encountered on its own, other than in isolated environments such as hydrothermal vents. Its abundance in the Martian atmosphere, however, could have provided an ample supply of energy for methanogenic microbes about 4 billion years ago, at a time when conditions would have been more conducive to life, the authors suggest. Early Mars was very different from what it is today, with high concentrations of hydrogen and carbon dioxide, powerful greenhouse gases that trap heat in the atmosphere, making it warm and wet rather than cold and dry, Ferrière said. had become
“At the time, Mars was probably slightly colder than Earth, but not as cold as it is today, with average temperatures probably above the freezing point of water,” he said. “Modern Mars has been described as dust-covered ice cubes, but early Mars was a porous, liquid-water-saturated planet that may have formed lakes and rivers, and even seas and oceans. It is imagined as a rocky planet with a crust.”
Spectroscopic measurements of rocks exposed on the surface of Mars show that the water was very salty, he added.
To simulate the conditions early lifeforms would have encountered on Mars, the researchers applied models that predict the temperatures at the surface and in the crust for a given atmospheric composition. They then combined those data with an ecosystem model that they developed to predict whether biological populations would have been able to survive in their local environment and how they would have affected it over time.
“Once we had produced our model, 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 at Sorbonne Université in Paris. “This allowed us to evaluate how plausible a Martian underground biosphere would be. And if such a biosphere existed, how it would have modified the chemistry of the Martian crust, and how these processes in the crust would have affected the chemical composition of the atmosphere.”
“Our goal was to make a model of the Martian crust with its mix of rock and salty water, let gases from the atmosphere diffuse into the ground, and see whether methanogens could live with that,” said Ferrière, who holds a joint appointment at Paris Sciences & Lettres University in Paris. “And the answer is, generally speaking, yes, these microbes could have made a living in the planet’s crust.”
The researchers then set out to answer an intriguing question: If life thrived underground, how deep would one have had to go to find it? The Martian atmosphere would have provided the chemical energy that the organisms would have needed 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 microbes would have had to go deeper into the crust to find habitable temperatures,” he said. “The question is how deep does the biology need to go to find the right compromise between temperature and availability of molecules from the atmosphere they needed to grow? We found that the microbial communities in our models would have been happiest in the upper few hundreds of meters.”
By modifying their model to take into account how processes occurring above and below ground influence each other, they were able to predict the climatic feedback of the change in atmospheric composition caused by the biological activity of these microbes. In a surprising twist, the study suggests that while life may have first thrived on ancient Mars, its chemical feedback to the atmosphere caused a global cooling of the planet, ultimately leading to its surface. uninhabitable, revealing that the life growing deeper and deeper within has been driven underground. , and may become extinct.
“According to our results, biological activity would have completely changed the Martian atmosphere very rapidly, within tens of thousands or hundreds of thousands of years,” he said. “By removing hydrogen from the atmosphere, microbes would have dramatically cooled the Earth’s climate.”
Early Martian surfaces would have quickly become glaciers as a result of biological activity. In other words, Martian life-induced climate change may have contributed to the Martian surface becoming uninhabitable very early on.
“The problem that these microbes would have faced was that the atmosphere on Mars was basically lost and completely diluted, so they lost their energy source and had to find alternative energy sources,” says Sauterey. said Mr. “Additionally, the temperature would have dropped so much that we would have had to penetrate deeper into the crust. At this time, it is very difficult to pinpoint how long Mars has been habitable.”
According to the authors, future Mars missions may provide answers, but challenges remain. For example, we have identified the Hellas Plain, a vast plain formed by collisions of large comets and asteroids very early in Mars’ history, as a particularly promising place to look for evidence of past life, although the site’s topography produces the most intense. Dust storms on Mars may make this area too risky for robotic rovers to explore.
But once humans begin exploring Mars, such sites could return to the shortlist for future missions to the planet, Sauterey said. For now, the team is focused on current Mars research. NASA’s Curiosity rover and the European Space Agency’s Mars His Express satellite have detected rising concentrations of methane in the atmosphere. Such spikes may result from processes other than microbial activity, but raise the intriguing possibility that life forms such as methanogens may have survived in isolated environments. Environment Islands of Mars, deep underground – extraterrestrial oasis in an otherwise hostile world.
More information: Boris Sauterey et al, Early Mars habitability and global cooling by H2-based methanogens, Nature Astronomy (2022). DOI: 10.1038/s41550-022-01786-w
Journal information: Nature Astronomy
Provided by University of Arizona