Four billion years ago, the solar system was still young. The process of formation of the planets that constitute it came to an end, and the bombardment of asteroids to which they were subjected was attenuated.
Our planet then became habitable and then, some time later (a few tens or hundreds of millions of years), inhabited. Earth’s early biosphere was very different then than it is today. Life had not yet invented photosynthesis, its main source of energy today.
These primordial microbes – common ancestors of all current life forms on Earth – must therefore survive in the oceans using another source of energy: the consumption of chemical species released by the interior of the planet at the level of hydrothermal systems and volcanoes. , which accumulated as gases in the atmosphere.
Microorganisms called “hydrogenotrophic methanogens”, one of the oldest forms of life in our biosphere, particularly benefited from the atmospheric composition of the time. These microorganisms feed on COtwo (carbon dioxide) and Htwo (dihydrogen), then abundant in the atmosphere (the Htwo represented 0.01% to 0.1% of the atmospheric composition, compared to about 0.00005% today), and thus collected enough energy to colonize the surface of the oceans.
In return, they released large amounts of methane (from which methanogens get their name) into the atmosphere. This powerful greenhouse gas built up there and warmed the climate, at a time when a dimmer sun than today alone could not necessarily maintain temperate conditions on the planet’s surface.
The appearance of life on Earth has thus been able to participate, through methanogens, in the consolidation of the habitability of our planet and in the establishment of favorable conditions for the evolution and complexity of the terrestrial biosphere during the billions of years years that followed.
If this is the most likely scenario for the early evolution of Earth’s habitability, what about the other planets in the solar system? Take the example of our neighbor, the red planet. As we explore Mars, it seems increasingly certain that environmental conditions similar to those that allowed methanogens to abound in planet Earth’s oceans were developing at the same time on the Red Planet, or more specifically beneath its surface.
A Martian microbial life could have found, in the first four kilometers of the porous crust of Mars, a refuge with the rigorous conditions of the surface (in particular with the harmful UV radiation), favorable temperatures and compatible with the presence of liquid water, and a potentially abundant energy source in the form of atmospheric gases that diffuse into the crust.
Therefore, it is quite natural that our research group asked the following question: what happened on Earth could also have happened on the red planet?
A portrait of Mars does
four billion years
To answer this question, we paired three models. Our results have just been published in the scientific journal Nature Astronomy. The first allows us to predict how volcanism on the surface of Mars, the internal chemistry of its atmosphere and the escape of certain chemical species into space determine the pressure and composition of the atmosphere. These characteristics then determine the climate.
The second model describes the physicochemical characteristics of the porous crust of Mars: temperature, chemical composition, presence of liquid water. These are determined in part by surface conditions (surface temperature, atmospheric composition) and in part by internal features of the planet (internal thermal gradient, degree of crustal porosity).
Therefore, these first two models allow us to simulate the surface and subterranean environment of a young Mars. However, many uncertainties remain regarding the main characteristics of this environment (intensity of volcanism at the time, thermal gradient of the crust).
To remedy this problem, we explored in the model a large number of possibilities of what these features might have been, thereby generating a set of scenarios of what Mars might have looked like four billion years ago.
Boris SautereyFprovided by the author|
The third and final model describes the biology of hypothetical Martian methanogenic microorganisms. It is based on the assumption that the latter would have been similar to terrestrial methanogens, at least from the point of view of their energy needs. It allows us to assess the habitability of our microbes, relative to subterranean environmental conditions on Mars, in each of the environmental scenarios generated by the two previous models.
If these are habitable, the model assesses how many of these microorganisms might have survived below the surface of Mars and, together with the crustal and surface models, the influence of this subterranean microbial biosphere on the chemical composition of the crust, on the atmosphere. and about the weather. By establishing the link between the microscopic scale of the biology of methanogenic microbes and the global scale of the Martian climate, the coupling of these three models allows the simulation of the behavior of a Martian planetary ecosystem.
A very probable underground life
Various geological clues indicate that liquid water circulated on the surface of Mars four billion years ago, forming rivers, lakes, and even oceans. Therefore, the climate on Mars was more temperate than it is today. To explain such a climate, our surface model estimates that Mars’ atmosphere was dense (almost as thick as Earth’s atmosphere today) and particularly rich in COtwo and Htwoeven more than the Earth’s atmosphere was at the time.
This atmospheric context particularly rich in COtwo in fact it would have conferred to the Htwo atmosphere the characteristics of a particularly potent greenhouse gas, more potent than it would have been, under the same conditions, CH4. In other words, 1% of Htwo in the atmosphere then warmed the climate of Mars more than 1% CH4.
In some of the scenarios produced by our model, this greenhouse effect is not enough to produce the climatic conditions necessary to maintain liquid water on the surface of Mars: the red planet is then covered with ice. While workable temperatures exist deep in the crust, it remains uninhabitable: locked in by surface ice, COtwo and the Htwo Atmospheric gases, an essential energy source for methanogenic life, cannot penetrate the crust.
However, in most of our scenarios, the presence of liquid water on the surface of Mars is possible at least in the warmer regions. In these regions, COtwo and the Htwo atmospheric elements can penetrate the crust. Our biological model then predicts that in all these scenarios, the methanogenic microorganisms would have found viable temperatures and had access to a sufficient energy source for their survival in the first hundred meters of the crust.
To summarize, although we currently have no objective evidence of past or present life on Mars, it is very likely that the Martian crust hosted an underground biosphere composed of methanogenic microorganisms four billion years ago.
brutally cold weather
Could these hypothetical Martian methanogens, like their counterparts on Earth, have warmed their planet’s climate? Our story becomes less optimistic here. An underground biosphere based on methanogenesis would have profoundly altered the atmosphere of Mars, consuming the vast majority of its Htwo and releasing a significant amount of CH4.
Now, as we have seen, the Htwo is, in the context of the early atmosphere of Mars, a more potent greenhouse gas than CH4. While the appearance of methanogenesis on Earth participated in the establishment of a favorable climate, thus consolidating terrestrial habitability, a Martian methanogenic life would, by consuming most of the Htwo Mars atmosphere suddenly cooled the climate by several tens of degrees and participated in the expansion of the ice sheet.
In regions still free of surface ice, our microorganisms would also likely have had to go much deeper into the crust to find viable temperatures, thus moving away from their atmospheric power source. Mars, therefore, would have become, under the action of life, much less welcoming than it was initially.
Tendency to self-destruct
In the 1970s, James Lovelock and Lynn Margulis developed the Gaia hypothesis, according to which the Earth’s habitability would be maintained through harmonious and mutual self-regulation of the terrestrial biosphere and planet Earth. We, the human species, were an unfortunate exception to this.
This concept gave rise to the idea of the “Gaian bottleneck”: perhaps it is not the necessary conditions for life that are missing in the universe but the capacity for life, once it has arisen, to maintain the habitability of its planetary environment. long-term.
What our study suggests is even more pessimistic. As the example of methanogenesis on Mars shows, even the simplest life can, under certain conditions, actively compromise the habitability of its planetary environment. Is it possible, then, that this tendency to self-destruction limits the abundance of life in the universe?
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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