Life may have thrived on early Mars until it fueled the climate change that caused its demise

If there had ever been life on Mars — and that’s a huge if — conditions during the planet’s infancy would most likely have supported it, according to a study led by University of Arizona researchers.

Dry and extremely cold, with a weak atmosphere, it is extremely unlikely that any form of surface life would survive on present-day Mars. But 4 billion years ago, Earth’s smaller, red neighbor may have been much more hospitable, according to the study published in natural astronomy.

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

The authors do not argue that life existed on early Mars, but if it did, Ferrière said, “our study shows that subterranean early Mars was very likely to have been habitable for methanogenic microbes.”

Such microbes, which earn their living by converting chemical energy from their environment and releasing methane as a waste product, are known to occur in extreme habitats on Earth, such as in hydrothermal vents along cracks in the sea floor. There they support entire ecosystems adapted to crushing 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 Martian crust, atmosphere and climate in conjunction with an ecological model of a community of terrestrial microbes that metabolize carbon dioxide and hydrogen.

On Earth, most hydrogen is bound in water and is not commonly found alone except in isolated environments such as hydrothermal vents. However, its abundance in the Martian atmosphere could have provided abundant energy for methanogenic microbes about 4 billion years ago, at a time when conditions would have been more favorable for life, the authors suggest. Early Mars would have been very different than it is today, Ferrière said, trending warm and wet rather than cold and dry thanks to large concentrations of hydrogen and carbon dioxide — both potent greenhouse gases that trap heat in the atmosphere.

“We think Mars may have been slightly cooler than Earth at the time, but not nearly as cold as it is today, with average temperatures most likely being above the freezing point of water,” he said. “While current Mars has been described as a dust-covered ice cube, we envision early Mars as a rocky planet with a porous crust saturated with liquid water that likely formed lakes and rivers, perhaps even seas or oceans.”

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

To simulate the conditions that early life forms would have encountered on Mars, the researchers used models that predict surface and crustal temperatures for a given atmospheric composition. They then combined this data with an ecosystem model they developed to predict whether biological populations could have survived in their local environment and how they would have affected it over time.

“Once we had our model, we deployed it to the crust of Mars – 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 the Sorbonne Université in Paris. “This allowed us to assess how plausible an underground Mars biosphere would be. And if such a biosphere existed, how it would have changed 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 salt water, let gases from the atmosphere diffuse into the ground and see if methanogens could live with it,” says Ferrière, who has a joint appointment at University of Paris Sciences & Lettres in Paris. “And the answer, in general, is yes, these microbes could have made a living in the Earth’s crust.”

The researchers then set out to answer an intriguing question: If life thrived underground, how deep would you have to go to find it? The Martian atmosphere would have provided the chemical energy the organisms needed to thrive, Sauterey explained — in this case, hydrogen and carbon dioxide.

“The problem is that even on early Mars, the surface was still very cold, so microbes had to go deeper into the crust to find comfortable 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 that they needed to grow? We found that the microbial communities in our models would have been happiest in the top few hundred feet.”

By modifying their model to account for how processes occurring above and below ground interact, 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 revealed that while ancient Martian life may have initially thrived, its chemical feedback into the atmosphere would have triggered global cooling of the planet, ultimately rendering its surface uninhabitable and driving life deeper and deeper underground would have, and possibly to extinction.

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

The surface of early Mars would soon have become glacial as a result of biological activity. In other words, the climate change caused by life on Mars may have contributed to the planet’s surface becoming uninhabitable very early on.

“The problem these microbes would then have faced was that the Martian atmosphere was basically gone, completely diluted, so their energy source would have gone and they would have had to find an alternative energy source,” Sauterey said. “Also, the temperature would have dropped significantly and they would have had to penetrate much deeper into the crust. At the moment it is very difficult to say how long Mars would have remained habitable.”

Future Mars exploration missions could provide answers, but challenges remain, the authors say. For example, while they identified Hellas Planitia, a vast plain formed very early in Mars’ history by the impact of a large comet or asteroid, as a particularly promising place to look for evidence of past life, the site’s topography produces some of the fiercest dust storms on Mars, which could make the area too risky to explore by an autonomous rover.

However, once humans start exploring Mars, such sites could return to the shortlist for future missions to the planet, Sauterey said. For now, the team is focusing 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 while such spikes could result from processes other than microbial activity, they raise the intriguing possibility that life forms such as methanogens may have survived in isolated environments Islands on Mars, deep underground – oases of extraterrestrial life in an otherwise hostile world.