At least in theory, around four billion years ago, Mars offered certain microbes favorable living conditions, according to a modeling study: Protozoa could have lived from the conversion of hydrogen and carbon dioxide to methane. Due to the special characteristics of the Martian atmosphere, however, this process would have led to a significant cooling of the climate, according to the simulations. If they existed, the Martian microbes themselves could have helped transform their habitat into an increasingly hostile world, say the scientists.
Extreme drought, bitter cold and thin air: Mars today seems to offer hardly any opportunities for living beings as we know them. However, as is now evident from numerous indications, this was not always the case: It is assumed that around four billion years ago our neighboring planet still had a comparatively dense atmosphere with a high content of carbon dioxide and hydrogen. Presumably, the greenhouse effect of these gases made a temperate climate possible at the time. There are various indications that this formed liquid water that saturated the planet's porous crust and filled large bodies of water. However, as Mars continued to evolve, it lost its temperate conditions and evolved into the frigid desert planet it is today. But scientists largely agree that young Mars provided the basic conditions that, in the case of Earth, led to the emergence of the first microbial life forms.
Hydrogen-based life possible
One of the most exciting questions in astrobiology is whether life on Mars really once existed – or perhaps even still exists in refuges. So far, however, there are no solid indications. But you can think about probabilities and run through what-if scenarios, say the scientists led by Boris Sauterey from the University of Arizona in Tucson. Specifically, they looked at what kind of microbes might once have found good living conditions on young Mars and how their theoretical existence would have affected the evolution of the planet.
As the researchers explain, the most likely hypothetical Martian microbes are organisms whose livelihood is based on the conversion of hydrogen and carbon dioxide into methane. This is known as an ancient concept from terrestrial microbes of the archaea group. On Earth, hydrogen is rarely found uncombined except in isolated environments such as rock or hydrothermal vents. However, its abundance in the Martian atmosphere could have provided a widely available energy source for methanogenic microbes about four billion years ago, the scientists explain. The gases could therefore have diffused in sufficient quantities into the Martian soil, which at the time also offered life-friendly humidity and temperature conditions.
The researchers used model simulations to explore the plausibility of this hypothetical scenario as part of their study. In doing so, they linked assumptions about the characteristics of the Martian crust as well as the atmosphere and the climate in the time from about four billion years ago in a complex way. The model simulations also incorporated biological information about terrestrial microbes that metabolize hydrogen and carbon dioxide into methane. As the team reports, the simulations basically confirmed that the Martian crust could have represented a suitable habitat for a corresponding ecosystem at the time. According to the results, the hypothetical Martian microbes could have even formed significant amounts of biomass.
Microbial climate change
The scientists then modified their model simulations to investigate the consequences of the activity of the hypothetical microbes. It was shown that the conversion of atmospheric hydrogen into methane would have caused global cooling and increasing icing of the planet. As the researchers explain, this effect, which does not apply to the nitrogen-rich Earth's atmosphere, is linked to the CO2-dominated gas envelope of Mars: With this composition, hydrogen ultimately develops a stronger greenhouse effect than methane. According to the model simulations, this connection could have made itself felt in the case of Mars: "By removing hydrogen from the atmosphere, the microbes would have drastically cooled the planet's climate," says Sauterey.
That is, intense biological activity would have caused climate change that would have helped make the planet's surface hostile to life at a very early stage. Thus, the microbes would have robbed themselves of their livelihood. “Temperatures would have dropped significantly, requiring them to go much deeper into the crust. Another problem these microbes would then have faced is that the Martian atmosphere has basically disappeared. So their energy source would have dried up and they would have had to find an alternative energy source. It's difficult to say how long Mars could have provided them with life opportunities," Sauterey said.
All of this is hypothetical, of course—to what extent these scenarios actually played out on Mars remains unclear. However, intensive research into our neighboring planet could possibly one day provide clues as to whether methane-producing microbes really did once exist there. It is even possible that they still exist in some subterranean refuge. But as the researchers emphasize, their study at least clarifies something fundamental: "The study illustrates the possibility that feedback between organisms and the environment can endanger the livability of a planet," the scientists write.
Source: University of Arizona, professional article: Nature Astronomy, doi: 10.1038/s41550-022-01786-w