Did life on Mars cause its own extinction?
We might not yet have proof of life on early Mars. But some discoveries have provided tantalizing clues that Mars was quite capable of supporting primitive, microscopic life. On October 10, 2022, researchers announced a new study suggesting that – if primitive organisms did once live on Mars – there might have been a familiar-sounding twist to their evolution. They might initially have flourished, but then ended up causing their own extinction.
According to this study, life on Mars might have brought about its own end by changing the composition of the atmosphere and making it too cold. Researchers from the University of Arizona and the Institut de Biologie de l’Ecole Normale Supérieure (IBENS) in France published their findings in the peer-reviewed journal Nature Astronomy on October 10, 2022.
Life on Mars?
The new study supports previous ones, that early Mars would have been more hospitable for microorganisms than it is today. Scientists believe the planet once had a much thicker atmosphere than it does now. That atmosphere would have allowed more temperate conditions and liquid water on the surface. The subsurface, however, may have been an even better environment for microbes to thrive in.
According to the new paper, those organisms may have been similar to methanogenic microbes (methanogens) on Earth. Those types of microbes release methane as a waste product. Regis Ferrière in the University of Arizona’s Department of Ecology and Evolutionary Biology is one of the two lead authors of the new paper. He said:
Our study shows that underground, early Mars would very likely have been habitable to methanogenic microbes.
A Martian ecosystem
Could such an ecosystem have existed on Mars? To test this, the researchers used state-of-the-art models of Mars’ crust, atmosphere and climate. They also incorporated an ecological model of a community of Earth-like microbes metabolizing carbon dioxide and hydrogen. (There is still debate over whether Mars’ early atmosphere was mostly carbon dioxide or hydrogen. Another new study just recently covered by EarthSky provides evidence that it was hydrogen). If there was enough hydrogen, the researchers say, it could have supplied energy for methanogenic microbes about 4 billion years ago. This would also support the hypothesis that Mars was warmer and wetter at the time. Ferrière said:
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. 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.
Temperature and survivability
Specifically, the researchers wanted to know if microbes would have been able to survive the Martian environment at the time. They used computer models to predict temperatures in the atmosphere and crust. They did this for various theoretical atmospheric compositions. Then, they combined that data with an ecosystem model. They used that model to predict whether microbial populations could have survived. Moreover, the model assessed how the microorganisms would have affected their local environment. The paper’s other lead author, Boris Sauterey, from the Institut de Biologie de l’Ecole Normale Supérieure (IBENS) in Paris and the University of Arizona, said:
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 results support 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. And the answer is, generally speaking, yes, these microbes could have made a living in the planet’s crust.
An underground biosphere
The results support the plausibility of a Martian biosphere, most likely underground, in the crust. So, how deep would it have been? A few feet? Deeper? Although warmer in general than today, it was still fairly cold on the surface. As Sauterey noted:
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. 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.
As the paper describes:
We find that subsurface habitability was very likely, and limited mainly by the extent of surface ice coverage. Biomass productivity could have been as high as in early Earth’s ocean. However, the predicted atmospheric composition shift caused by methanogenesis would have triggered a global cooling event, ending potential early warm conditions, compromising surface habitability and forcing the biosphere deep into the Martian crust.
Did life on Mars cause its own demise?
As on Earth, life on Mars, even if below the surface, would have affected its environment, including the atmosphere. Ironically, that life could have thrived initially, but then become a casualty of its own success. The researchers’ results suggest that it would have altered the chemical properties of the atmosphere in a way that would have caused a global cooling. That drop in temperature would have forced the microbes to move even deeper underground to where it was warmer. Ultimately, however, the whole biosphere may have been wiped out and driven to extinction. Sauterey said:
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. By removing hydrogen from the atmosphere, microbes would have dramatically cooled down the planet’s climate.
The Martian surface would have become uninhabitable and even the near subsurface much less habitable. Sauterey added:
The problem these microbes would have then faced is that Mars’ atmosphere basically disappeared, completely thinned, so their energy source would have vanished and they would have had to find an alternate source of energy. In addition to that, the temperature would have dropped significantly, and they would have had to go much deeper into the crust. For the moment, it is very difficult to say how long Mars would have remained habitable.
Bottom line: Life on Mars may have initially thrived early on, but then ultimately caused its own extinction. The atmosphere might have became too cold after microbes altered its composition.