According to current knowledge, 3.7 billion years ago conditions for microbial life were quite favorable on Mars. An international team of researchers has for the first time quantitatively investigated whether and how microbes were able to develop under the conditions of the time. Their result: Similar to Earth, bacteria that fed on hydrogen and produced methane could also have dominated on Mars. But at the same time, the bacteria worsened their living conditions on Mars: The planet cooled down by about 40 degrees due to the methane, the scientists report in the journal "Nature Astronomy".
"On Earth, hydrogenotrophic methanogens - i.e. hydrogen-consuming, methane-producing protozoa - were among the very first forms of life," explain Boris Sauterey from the University of Arizona and his colleagues. While these bacteria were mainly active in the oceans on Earth, the porous crust on Mars offered them an ideal habitat. "There they were protected from ultraviolet and cosmic radiation," say the scientists, "and the water there provided them with the hydrogen for their metabolism."
The model developed by Sauterey and his colleagues shows that the methanogens on Mars could have produced a biomass similar to that of their counterparts in the Earth's oceans. However, with very different consequences. While the methane produced by the bacteria as a climate gas stabilized the temperature of the atmosphere on Earth, it led to a cooling of 33 to 45 degrees on Mars. As a result, the microorganisms on the red planet could not spread on the surface, but had to migrate down into ever deeper layers of rock.
The different reactions of the planetary climate to the methanogenic bacteria may appear paradoxical at first, but can be explained by the different composition of the atmospheres of the two young planets, the researchers explain. "In a carbon dioxide-dominated atmosphere like on Mars, hydrogen produces a stronger greenhouse effect than methane," say Sauterey and his colleagues. The decomposition of hydrogen and the production of methane therefore lead to cooling.
However, even then nitrogen was dominant in the Earth's atmosphere. The effect is exactly the opposite. The researchers emphasize that this difference shows how important climatic feedbacks are for the development of life on a planet - and that they can work in both directions. "Feedbacks between life and the environment can threaten habitability on a planetary scale," say the scientists.
The cooling effect may have been even stronger on Mars because the model does not account for the effects of possible surface icing. This requires improved climatic models for the red planet, according to Sauterey and his colleagues. However, the researchers also provide indications of where traces of the early methanogenic bacteria are most likely to be found on Mars at not too great a depth in the soil: in the Hellas and Isidis lowlands and in the Jezero crater. Because these regions have probably cooled less and remained free of ice on the surface.