Life on early Mars may have met its demise because of climate change

Early Mars may have provided a favourable subsurface environment for microbial life that feeds on hydrogen and produces methane, says a model-based study published in Nature Astronomy.

The research led by Boris Sauterey of the University of Arizona dates back to the Noachian period, more than 3.7 billion years ago.

The environment of the earliest days of Mars, about 3.7 to 4.1 billion years ago, was likely favourable for allowing life to thrive, specifically, simple microbial organisms that consumed hydrogen and carbon dioxide and released methane. These types of microbes are common on Earth today and live in extreme environments, such as hydrothermal vents along the ocean floor.

This is the conclusion of a study recently published in Nature Astronomy by a team of scientists from several French and American research centers. In it, the hypothesis is launched that Mars could have had the ideal conditions to harbour life in the past, but that these could be altered by a change in its climate.

Logically, this does not mean that there has been a life similar to that here on Earth. There are no little green men. And, therefore, it does not mean that it was that life that caused climate change, as we are doing. The only thing we need to keep in mind is that changes in climate can drastically alter a planet's ability to support life. Precisely for this reason, we should not take such risks with ours.

A fleeting development

According to a press release, the then-existing large biosphere on Mars may have had a global cooling effect on the planet's climate, quickly ending the habitability of Mars. That reverse greenhouse effect would have killed the microbes, by progressively creating an environment that is increasingly cold and incompatible with life. 

However, scientists led by Boris Sauterey believe that it is possible to confirm his theory by identifying traces of this ancient life in three areas of Mars: Hellas Planitia, Isidis Planitia and Jezero Crater. 

The specialists modelled the interaction between the primitive environment of Mars and an ecosystem based on microorganisms that survive by consuming hydrogen and producing methane, considered one of the first forms of life on Earth. The simulations predict that the Martian crust was a viable location for this ecosystem, as long as the surface was not completely covered in ice. 

Although initially these conditions were met, the microbial ecosystem would have developed over time through mutual feedback with the planet's climate, cooling it globally. The drop in temperatures, from a tolerable range of 10 to 20 degrees Celsius to a frigid environment with heat signatures averaging -57 degrees Celsius, led to less habitable conditions closer to the surface. 

As a consequence of this, the microbes began to move progressively deeper into the Martian crust, causing their own demise as there was no internal source of energy available for their survival. One question that immediately arises, however, is why the same thing didn't happen on Earth. 

Differences with Earth

The model developed by the French scientists suggests that life thrived on Earth and quickly died out on Mars due to differences in the gases surrounding the two planets and their relative distances from the Sun. Because Mars is farther from the Sun than Earth depended directly on a profuse fog of greenhouse gases that managed to trap heat, mainly through carbon dioxide and hydrogen, in order to maintain temperatures favourable for life. 

As the ancient Martian microbes fed on hydrogen, a powerful greenhouse gas, and at the same time generated the methane, another greenhouse gas but with less impact than hydrogen, they slowly consumed the layer of heat that "protected" the planet . As a consequence of this reverse greenhouse effect, the low temperatures sentenced the failure of the possible development of complex life on Mars.

These kinds of planetary stories would not be strange or unusual: the researchers believe that life may not be innately self-sustaining in all the conducive planetary environments in which it emerges at some point, and may quickly die out. This would also explain why no form of life outside Earth has been verified so far, even on potentially habitable planets.

Climate change could end life on Mars

Before starting to break down the research of these scientists, it is important to note that this is a hypothesis. It is not at all certain that in the past there was life on Mars and that climate change would end it. Now, the conclusions of his study are interesting enough to continue delving into them.

According to their study, based on computer models, about 4 billion years ago, the atmosphere of Mars was much denser than it is today. In addition, the temperature would be lower than what the Earth currently has. But not as low as what is measured today on the red planet. It would be slightly above the freezing point of water, so it could be in a liquid state.

Its surface would be composed of porous regolith, saturated with brine, giving rise to an environment in which microbes could proliferate protected from cosmic radiation. Thus, it is calculated that the amount of biomass on Mars could have been equivalent to that of the primitive ocean on Earth.

Unfortunately, the models that best fit the data from Mars show that these microorganisms could consume mainly hydrogen and carbon dioxide as an energy source, producing methane in turn. And that methane would accumulate more and more in the environment, causing a thinning of the atmosphere and, in turn, a cooling of global temperatures that would prevent life from continuing to exist.

Discover traces of this early life

Spatial projections from the research indicate that lowland locations at low and mid-latitudes are good candidates for discovering traces of this early life at or near the surface.

In fact, they identify three locations: Hellas Planitia, Isidis Planitia, and Jezero Crater (where NASA's Perseverance rover is currently working) as the best places to look for signs of this early methanogenic life near the surface of Mars.

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