The presence of life brings many unexpected consequences.
On Earth, for instance, when cyanobacteria spread widely in ancient oceans more than two billion years ago, their production of increasingly large amounts of oxygen killed off much of the other anaerobic life present at the day because oxygen is a toxin, unless an organism finds ways to adapt. One of the first global ices followed because of the changed chemistry of the atmosphere.
Now a group of researchers at the University of Arizona has modeled a similar dynamic that could have potentially taken place on early Mars.
As the group reports in the journal Nature Astronomy, their work has found that if microbial life was present on a wetter and warmer ancient Mars — as some now think that it potentially was — then it would almost certainly have lived below the surface. The rock record shows that the atmosphere would then have consisted largely of carbon dioxide and hydrogen, which would have warmed the planet with a greenhouse effect.
By using a model that takes 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 revealed that while ancient Martian life may have initially prospered, its chemical feedback to the atmosphere would have kicked off a global cooling of the planet by the methanogen’s use of the atmospheric hydrogen for energy and the production of methane as a byproduct.
That replacement of hydrogen with methane ultimately would render its surface uninhabitable and drive life deeper and deeper underground, and possibly to extinction.
“According to our results, Mars’ atmosphere would have been completely changed by biological activity very rapidly, within a few tens or hundreds of thousands of years,” said Boris Sauterey, a former postdoctoral student at the University of Arizona who is now a fellow at Sorbonne Université in Paris. .
“By removing hydrogen from the atmosphere, microbes would have dramatically cooled down the planet’s climate.”
Rich in carbon dioxide and hydrogen, the Martian atmosphere would have likely created a temperate climate that allowed water to flow and possibly allowed microbial life to thrive, according to University of Arizona professor and co-author Regis Ferrière,
Early Mars would have been very different from what it is today, Ferrière said, trending toward warm and wet rather than cold and dry, thanks to large concentrations of hydrogen and carbon dioxide – both strong greenhouse gases that trap heat in the atmosphere.
“We think Mars may have been a little cooler than Earth at the time, but not nearly as cold as it is now, with average temperatures hovering most likely above the freezing point of water,” he said. “While current Mars has been described as an ice cube covered in dust, we imagine early Mars as a rocky planet with a porous crust, soaked in liquid water that likely formed lakes and rivers, perhaps even seas or oceans.”
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.
“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 the University of Arizona and Paris Sciences & Lettres University.
“And the answer is, generally speaking, yes, these microbes could have made a living in the planet’s crust.”
The authors are not claiming that they know that methanogenic life existed on early Mars. But if it did, Ferrière said, that early Mars underground environment — especially in areas rich in salty water — is where they would be found.
This kind of methane-producing anaerobic microbe live on Earth by converting chemical energy from their environment and releasing methane as a waste product.
They are known to exist in extreme habitats 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.
On Earth, most hydrogen has been tied up in water and generally not encountered on its own, other than in isolated environments such as hydrothermal vents. That’s quite different from the hydrogen that existed in the early Martian atmosphere.
Methanogens can derive energy from hydrogen and so it would have provided an ample “food” source for methanogenic microbes about 4 billion years ago, at a time when conditions for habitability would have been more conducive to life.
Ironically, the methane produced by the microbes is itself a greenhouse gas, and is considered a powerful one on Earth today.
But Sauterey told me that it is not as strong a greenhouse chemical as atmospheric hydrogen.
“Martian methanogens, if they populated the crust of Mars, would have therefore (pulled) a very potent warming gas from the atmosphere, hydrogen, and released a less potent one instead, methane,” he said. “The net effect on the climate would have been a cooling one.”
The result would be that the planet would gradually and then dramatically cool without the hydrogen to keep greenhouse gases in the atmosphere. And then the longer term net effect would have been extinction, unless some saving adaptation took place.
The microbes could, for instance, have descended deeper into the Martian regolith and found enough warmth and similar sources of energy. We know that kind of adaptation is possible because it has happened so often on Earth.