Scientific American: scientists have found out that life on Mars could destroy itselfLife on Mars, if it was there after all, may have destroyed itself, writes Scientific American.
And even if microorganisms can make the planet uninhabitable, intelligent beings will cope with this task faster, scientists warn.
Allison GaspariniAlthough we already know that Mars used to be a wetter, warmer and more habitable place than the dried-up desert it is today, researchers have yet to find convincing evidence that there was once life on this planet.
If there really was life there, then we face important questions: how did living beings influence the planet, and where can we find evidence of their existence? Data from a new study designed to answer these questions show that contrary to common sense, the biosphere of Mars – if it really existed – could have made a significant contribution to the transition of the planet to its current uninhabited state. The research data also makes it possible to identify some regions – including the Jezero crater, where NASA's Perseverance rover is currently operating – that are best suited for searching for traces of life. However, they also indicate that life on Mars could be the worst enemy for itself.
Applying climate models and terrain models to recreate the appearance of Mars as it was four billion years ago, French researchers concluded that at that time microorganisms may have existed safely just a few centimeters below the surface of the Red Planet, and they were protected from strong cosmic radiation by a layer of soil. However, this underground biosphere at some point, to its misfortune, began to go deeper, to which it was pushed by low temperatures, which became a consequence of its vital activity. The authors of the study, published in the journal Nature Astronomy, suggested that these hypothetical ancient microorganisms absorbed hydrogen and carbon from the atmosphere of Mars and produced methane. All these three substances act as heat-retaining greenhouse gases, that is, changes in the concentration of each of them can have a significant impact on the temperature on the surface of the planet. In this case, the decrease in the level of greenhouse gases produced by these hypothetical "methanogenic" microorganisms in the atmosphere of Mars provoked global cooling, as a result of which most of the planet was covered with ice, and it eventually turned into an uninhabited desert.
"In fact, we are arguing that life appearing on the planet in its specific form may turn out to be self–destructing," said Boris Sauterey, a researcher at the Sorbonne University in Paris and the lead author of the study. "It is this tendency to self–destruction that perhaps limits the possibilities of life to arise everywhere in the universe."
Gaia's Blessing or Medea's Curse
In 1965, the late chemist James Lovelock, then working at NASA's Jet Propulsion Laboratory, formulated a feasible strategy for detecting life on other planets. Lovelock and his fellow researchers argued that certain chemical compounds in the planet's atmosphere act as so-called biosignatures that indicate the presence of life. For example, on Earth, the coexistence of methane (produced by methanogens) and oxygen (produced by photosynthetic organisms) is a powerful biosignature. The thing is that in environmental conditions, both gases eliminate each other, so the constant presence of both indicates their continuous replenishment, which also occurs from biological sources. Lovelock's work formed the basis of the scientific search for alien life in other worlds, which continues today.
The idea that living organisms directly influenced the chemical composition of the Earth's atmosphere became the basis for what Lovelock called his "Gaia hypothesis", which he developed together with microbiologist Lynn Margulis in the 70s. The Gaia hypothesis, named after the ancient Greek goddess of the earth, states that life is a self–regulating system. Terrestrial organisms collectively interact with their environment in such a way that the habitability of their environment — in this case, the planet itself – is preserved. For example, an increase in global temperatures due to an excess of carbon dioxide in the atmosphere can also stimulate the growth of plants, which, in turn, take more greenhouse gases from the air, ultimately cooling the planet.
In 2009, paleontologist Peter Ward from the University of Washington expressed a less optimistic point of view. On a global scale, the scientist argued, life is more self-destructive than self-regulating, and eventually destroys itself. In contrast to the Gaia hypothesis, he named his idea after another character from Greek mythology – Medea, a mother who kills her own children. To argue for the "Medea hypothesis", Ward cited several episodes of mass extinction on Earth, which may indicate the self-destructive nature of life. On the eve of the Great Oxygen Catastrophe more than two billion years ago, photosynthetic cyanobacteria released a huge amount of oxygen into the Earth's atmosphere, which previously had practically no this highly active gas. This inevitably led to the extinction of the former hosts of the planet – methanogens and other "oxygen-free" organisms for which oxygen was poisonous. "Just look at the history of the Earth, and you will see periods when life turned out to be the worst enemy for itself," Ward said, commenting on the obvious connection between his Medea hypothesis and the study of Soteray and his colleagues. "I think the same story could have taken place on Mars."
And in the spirit of the Gaia hypothesis, this event, which became absolutely catastrophic for oxygen-free life forms on Earth, served as a catalyst for the flourishing of other microorganisms: the influx of atmospheric oxygen played a key role in ensuring the biological diversity of our planet and in the emergence of multicellular ancestors of our modern biosphere. Thus, determining whether life follows the trajectory of Gaia or Medea is perhaps only a matter of point of view and requires a wider – interplanetary –viewing angle. But until scientists discover life on other planets, we can only rely on speculative comparisons obtained through theoretical research, such as the work of Soter.
A more thorough search for life on Mars
Kaveh Pahlevan, a researcher at the SETI Institute, claims that Soterey's study, in which he did not participate, "really expands our understanding of the impact of the biosphere on habitability." But he also noted that the study looked at the effect of only one type of metabolism on the planet. For example, it does not take into account the complexity of events such as the Great Oxygen Catastrophe, which occurred as a result of the conflicting influence of methanogens and cyanobacteria. Soterey acknowledges this possible disadvantage: "It can be assumed that a more complex, more diverse biosphere [on Mars] would not have such a negative impact on the habitability of the planet as the impact that only methanogens could have," he noted.
Nevertheless, this limitation of the researchers' conclusions may in itself point to one fundamental truth. The abundance of diverse microorganisms on ancient Earth – and the resulting evolutionary flexibility that allows recovery after catastrophic environmental changes – is probably the reason that the complex terrestrial biosphere managed to survive, while the supposedly simpler biosphere on Mars simply disappeared. According to Ward, the increase in diversity probably helped the biosphere to avoid the sad fate of the curse of Medea. "I sincerely believe that the only way out – the only way to preserve life on the planet when it appears there – is the development of intelligent life forms," Ward said. Only then, according to him, technological solutions may appear that will mitigate the "Medean" tendency of life to destroy its own habitat.
As part of their research, the scientists did not consider the possibility of the existence of modern methanogens hiding in the Martian bowels. Their probable presence could help explain the mysterious plumes of methane that scientists have repeatedly recorded in the atmosphere of the planet (although the cause of their appearance may be processes of inanimate nature).
As for ancient Mars, as part of the study, scientists noted those places on the planet where hypothetical microorganisms could live closer to the surface (that is, to be in the reach of sophisticated devices that allow detecting their traces). These "hot spots" coincide with those rare areas of Mars that could remain ice-free for a significant part of the planet's history, despite almost global glaciation as a result of global cooling. One of these places is the Jezero crater – the site of an ancient lake and a vast delta where fossils can remain. By a happy coincidence, it is there that NASA's Perseverance rover is currently working, which extracts materials potentially containing biosignatures for subsequent analysis in laboratories on Earth. However, it is unclear whether it will be possible to detect traces of ancient methanogens there. They may be buried under deep layers of sediments, through which the Perseverance rover will not be able to break through.
In addition to the Jezero crater, scientists name two even more promising sites where it may be possible to detect traces of ancient methanogens: the Plain of Hellas and the Plain of Isis. Such an increase in the number of likely targets indicates an increase in interest in the surface of Mars, which may lead to an expansion of the search for life on this planet. This is told by Victoria Orphan, a geologist from the California Institute of Technology, who did not participate in the study. According to her, Soteray's work is "a starting point that helps stimulate debate and think more deeply about future missions."
"However, of course, all these are just hypotheses, and therefore everything is ambiguous," Soterey admitted. "We can only say that, with some degree of probability, in this particular region of Mars, its crust was habitable." According to him, the fact that Mars was once inhabited does not mean that someone really lived on this planet.
Whether ancient methanogens lived on Mars or not, the results of a new study serve as a reminder that living organisms can create conditions for their own prosperity – or extinction. Even single-celled organisms have the ability to turn a completely habitable planet into an uninhabitable place. And, as Soterey added gloomily, "with modern technological means at their disposal, people are able to do it much faster."
