Mars may have been home to methane-producing microbial life billions of years ago, according to computer simulations of early Martian geology.
While it has long been suggested that Mars could have supported simple organisms in its younger days, the viability of this hypothesis has never been quantified, say the French researchers who developed the simulations. Until now.
Their research is published in Nature Astronomy.
While the solar system was still quite young, billions of years ago, Mars was being bombarded by a huge number of meteorites and asteroids. There is also evidence to suggest that the Martian surface at least periodically supported liquid water.
This early period in Mars’s geological history is known as the Noachian after the ancient patriarch, Noah. A precise age for the Noachian is unknown, but it probably spanned 4.1 billion to 3.7 billion years ago. It is roughly equivalent, geologically, to Earth’s Hadean 4.6-4 billion years ago.
“During the Noachian, Mars’ crust may have provided a favourable environment for microbial life,” the authors of the recent study write. They add that the top layer of Mars’s surface would have made a good home for early life, sheltering them from ultraviolet and cosmic radiation.
Noachian Mars would have been a suitable habitat for hydrogenotrophic methanogens – simple microbial organisms that consumed hydrogen and carbon dioxide, and produced methane as waste. On Earth, hydrogenic methanogenesis was among the earliest metabolisms to emerge.
The scientists used a state-of-the-art model to see the effect of methanogenic hydrogenotrophy on the early Martian system. They combined a photochemical climate model (looking at the influence of radiation on the chemicals in the Martian atmosphere) and a model of the early Martian crust. They could then analyse atmospheric composition, climate, thermal properties of the crust, and gas exchange between the crust and atmosphere.
“We find that subsurface habitability was very likely, and limited mainly by the extent of surface ice coverage,” the authors write. “Biomass productivity could have been as high as in the 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,” the authors say.
Hydrogenotrophic methanogen effects on Earth’s early climate of were recently analysed. Comparing this study to the modelling of early Mars shows similarities and differences.
“On the one hand, models predict very likely habitability to hydrogenotrophic methanogens on both young planets, with similar biomass production,” the authors write. “On the other hand, climate feedbacks work in opposite directions.”
While on Earth hydrogenotrophic methanogens may have helped maintain temperate conditions, they would have cooled the early Martian surface by 33-45°C. This is because on Earth, the nitrogen-rich atmosphere would have seen increased methane production led to a greenhouse effect. On Mars, however, H2 actually has a stronger greenhouse effect than the methane that would have been produced by its consumption.
The authors say that the best evidence supporting their model’s predictions would be the discovery on present-day Mars of methanogenic life descended from hypothetical microbes modelled. But, as the Martian atmosphere has thinned in the intervening billions of years, life on Mars would have had to shift its energy source from H2 to feeding off thermal energy and chemical reactions like ionisation deeper below the planet’s surface.
But the work could also help inform the search for the fossilised remains of Noachian microbes. That 3 to 4-billion-year-old fossils have been found and analysed on Earth suggests the same may be true on Mars.
For such searches in the future, the authors suggest three sites – Hellas Planitia, Isidis Planitia and Jezero Crater – as the best places to look for signs of early methanogenic life near the surface of Mars.
Handy, then, that the Perseverance rover, tasked with looking for signs of ancient life on Mars, is already making tracks on the Jezero Crater.
Cosmos, 12 October 2022