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Finding liquid water on Mars in 2015 has reinforced the hope that signs of life are hiding somewhere on the red planet. But in practice, scientists have more or less ignored whether these specific bodies of water had anything to do with life today. The only way that water could exist in liquid form on such a cold planet, is if it was saturated with salt, which lowered the temperatures below the freezing point. Briny water is not considered an ideal living place for life.
But all this water may deserve another look. New results published in Nature Geoscience On Monday, these pools could contain more breathable oxygen than we had ever imagined, enough for life to exist on or near the surface.
"These are very good observations that open up new perspectives for life on Mars, especially those that were not possible on the Early Earth," says Lewis Ward, a Harvard University geobiologist. author of the new study.
It is thought that the Martian atmosphere contains only 0.145% oxygen, while about 21% swirls across the Earth. This number does not necessarily kill the dreams of Martians; After all, the first forms of life on Earth did not have access to free oxygen for the first billion years (the evolution of photosynthesis solved this for us). But oxygen allowing an organism to use much larger amounts of energy, the lack of free oxygen hinders any hope of finding a life at least a few notches above the body. minimum.
However, good conditions could allow large amounts of oxygen to dissolve in these Martian reserves, especially as oxygen dissolves better in the water at lower temperatures. . Unfortunately, "no one previously had really thought about the amount of oxygen that may be present in liquid water on Mars because we had no evidence that oxygen had been present. really played a big role, "says Ward. "This is the first attempt to really understand how much oxygen there could be today."
The investigation relies on the presence of manganese oxide on the Martian surface. On Earth, traces of oxides of manganese oxides occurred at about the onset of oxygen accumulation in the atmosphere, about 2.5 billion years ago. Unlike iron (which is responsible for the crimson appearance of the red planet), manganese is quite difficult to oxidize and the only way for us to find that the oxidation on Earth is either very slow, or with the help of biology.
But in 2014, the Curiosity rover discovered very concentrated manganese deposits on Mars, which prompted Ward and his colleagues to wonder if they contained near-watery environments containing the oxygen needed for it. 39; oxidation. Oxidation of manganese is quite closely (though certainly not exclusively) related to biological activity. The study could be thought of as a matryoshka doll made up of questions: first, how much oxygen do you need to oxidize manganese? So, is this concentration of oxygen able to exist in Martian waters? And finally, at the heart of the study: does this amount of oxygen increase the possibility that oxygen-breathing organisms exist on Mars?
"If there is enough oxygen to perform chemically useful work to oxidize manganese, it means there is enough work to do a biologically meaningful job," Ward says.
The new study provides encouraging answers to these questions. The team has developed models for six different salt concentrations, able to maintain liquid temperatures from -207 degrees Fahrenheit to about 80 degrees Fahrenheit, and to take into account various pressures on the planet. The models say that all this salty liquid is more than capable of capturing the pitiful amounts of oxygen dispersed over the Martian surface. In fact, the models suggest that today, there was more oxygen on Mars than on Earth at the beginning of photosynthesis, explains Ward. "This suggests that there might actually be enough [oxygen] that for aerobes that use oxygen to eat carbon to produce energy, "it turns out that there is a lot of oxygen in these brines to support bacteria and some sponges that do it. The results also suggest there is enough oxygen on the surface and below the surface of Mars to allow energy recovery from other sources, such as methane and iron.
Sponges are a particularly useful model for thinking about what might survive in these environments. "Sponges are one of the first species of animals to evolve," Ward says. They are simple filter filters that can survive on bacteria and survive in very simple ecosystems. Ward admits that "to be able to examine the potential of life on Mars and other planets, we'd better think exclusively in terms of microbes," but it's interesting to think of the attenuated oxygen requirements of sponges when we let's think about what we could do. evolve on Mars.
Ward and his team hope these results will be a first step in helping scientists develop a map of Mars regions that can withstand the highest oxygen levels, depending on temperature and atmospheric pressure, and overlay them. with another map illustrating the main hydrogen points of the planet. and methane that could be consumed for energy. "We could actually point to places on Mars that have the greatest potential for detection of biological activity" and potential biomass, he said. The study could also help us better understand the potential that other worlds like Europe might have for promoting life.
Of course, there is reason to be a little cautious about the implications of the study, since it is based on computer modeling and not on direct observations. It will be difficult to confirm the results – not only on Mars, but also on Earth, where scientists have not yet been able to conduct controlled laboratory experiments that correctly measure the solubility of oxygen in very cold brines . On Mars itself, it is necessary to be able to study the brines sufficiently in contact with the atmosphere to facilitate the gaseous exchanges and to allow the oxygen of the air to dissolve in the water.
Jonathan Toner, an astrobiologist at the University of Washington who did not participate in the study, said the paper did a good job in showing that the oxygen content of Martian brines may not be a factor limiting for life. discussing atmospheric interactions with brines, which "may also have implications beyond habitability," he says. It nevertheless emphasizes that a high rate of oxygen is only possible here with extremely high salinity levels, which is "very difficult" for life. Combined with average temperatures as low as -67 degrees Fahrenheit, "and you have conditions that are not known to support life on Earth. Life does not require that a factor, for example an oxygen supply, but a whole range of factors present at the same time. "If life exists on Mars today," it should adapt to extremely low nutrient reserves, low temperatures, and high salinity, above all that the planet has to offer. we know how to tolerate on the planet. "
Yet, few studies have so far suggested that we could dare to dream of such complex life forms on Mars. But every year, Mars proves more tempting than expected. Scientists charged with searching for life on the red planet have a lot to check on their task lists.
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