For most species of life in the universe, oxygen can be a deadly poison. But, oddly enough, this can significantly simplify the search for such a life for astrobiologists. Imagine that you are getting into a time machine that can not only travel for billions of years, but also overcome light goals in outer space, all in order to find life in the universe. How would you start your search? Scientists' recommendations may surprise you.
At first, you might think that life may be like the familiar life on earth: grass, trees, frolicking animals at a watering hole under the blue sky and yellow sun. But this is the wrong line of thought. Astronomers censoring the planets of the Milky Way tend to believe that most of the life in the universe exists on worlds orbiting red dwarf stars, which are smaller but more numerous than stars like our Sun. In part because of this abundance, astronomers have to study them with great diligence. Take, for example, the red dwarf TRAPPIST-1, which is only 40 light years away. In 2017, astronomers discovered that at least seven Earth-like planets revolve around it. Many new observatories - led by a NASA star,with the James Webb Space Telescope - starting in 2019 and will be able to get to know the planets of the TRAPPIST-1 system, as well as many other planets near red dwarfs in search of life
Meanwhile, no one knows for sure what you will find when you visit one of these strange worlds in your space-time machine, but if the planet looks like Earth, chances are high that you will find microbes, and not an attractive megafauna. The study, published Jan.24 in Science Advances, demonstrates what this curious fact could mean for the search for aliens. One of the authors of the work, David Cutling, an atmospheric chemist at the University of Washington in Seattle, is peering into the history of our planet to develop a new recipe for the search for single-celled life on distant worlds in the near future.
Most of life on Earth today is microbial, and a careful reading of the planet's fossil and geochemical data shows that this has always been the case. Organisms such as animals and plants - and the oxygen these plants produce to breathe in animals - are relatively new phenomena that have emerged over the past half a billion years. Before that, out of four billion years of Earth's history, our planet spent the first two billion years in the role of a "muddy world" under the control of microbes feeding on methane, for which oxygen was not a life-giving gas, but a deadly poison. The development of photosynthetic cyanobacteria determined the fate of the next two billion years, and "methanogenic" microbes were driven into dark places where oxygen could not get - underground caves, deep swamps and other gloomy territories in which they still live. Cyanobacteria gradually greened our planet, slowly filled its atmosphere with oxygen and laid the foundation for the modern world. If you visited our planet in your time machine all these years, then nine times out of ten you would find only single-celled algae life, and you would also risk suffocating in oxygen-poor air.
This poses a challenge for scientists hoping to use the James Webb Telescope (rather than a time machine) to search for other worlds of life. Molecules in a planet's atmosphere can absorb transmitted light from stars, resulting in prints of light that astronomers can detect. The abundance of oxygen in the planet's atmosphere is one of the most obvious indicators of possible life, because it is not very easy to create it without biology. According to astrobiologists, this highly reactive gas can be a “biosignature” because in high concentrations it “goes out of balance” with the environment. Oxygen, as a rule, falls out of the air in the form of rust and other oxidations on metals, and does not stay in a gaseous state, so if there is a lot of it, something - perhaps photosynthesizing life - must constantly replenish it. But if you take our planet as an example, astrobiologists admit that oxygen may be the last thing they find - genetics says that complex photosynthesis as a process of producing oxygen was invented by cyanobacteria as an unusual evolutionary innovation that was found only once throughout the long history of the earth biosphere. Consequently, any hunter for life on other planets will see through the lens of a telescope, most likely, an oxygen-free planet. What other biosignatures can such a hunter look for?any hunter for life on other planets will see through the lens of a telescope, most likely an oxygen-free planet. What other biosignatures can such a hunter look for?any hunter for life on other planets will see through the lens of a telescope, most likely an oxygen-free planet. What other biosignatures can such a hunter look for?
Currently, the best way to find the answer is to go back to our time machine. Only this time it will be a virtual, computer model that plunges into the inaccessible depths of the anoxic past of the Earth (or the present alien world), exploring the possible chemistry of gases in the atmosphere and ocean that could take place. By using data from old rocks and other models to select the best assumptions about the chemistry of Earth's environment three billion years ago, a computer can see obvious imbalances - possible biosignatures. In fact, this is what Cutling did, working with Joshua Chrissansen-Totton and Stephanie Olson of the University of California, Riverside.
Their "time machine" is a numerical approximation of a huge volume of air trapped in a large transparent box with an open ocean at the base of the box; the computer simply calculates how the gases in the box will react and mix over time. Ultimately, the interacting gases use up all the "free energy" in the box and reach equilibrium - when the reaction requires additional energy from the outside, as if the soda was exhausted. By comparing a cocktail of exhausted gases to the revitalized mixture originally locked in the box, scientists can calculate exactly how and when the world's atmosphere was in equilibrium. This approach could reproduce the most obvious example of atmospheric imbalance that our planet has - the presence of oxygen and traces of methane. Simple chemistry showsthat these gases should not coexist for a long time, but they coexist on Earth, which makes it clear that something on our planet breathes and lives. But for an ancient Earth without oxygen, the model would exhibit completely different behavior.
"Our research provides an answer" to the question of how to find anoxic life on an Earth-like planet, Cutling says. Most of life is simple - like microbes - and most planets have not yet reached the stage of oxygen-rich atmospheres. The combination of relatively abundant carbon dioxide and methane (in the absence of carbon monoxide) is the biosignature of such a world.
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Chrissansen-Totton explains in more detail: “The presence of methane and carbon dioxide at the same time is unusual, because carbon dioxide is the most oxidized state of carbon, and methane (consisting of a carbon atom bonded to four hydrogen atoms) is the opposite. It is very difficult to produce these two extreme forms of oxidation in the atmosphere at the same time in the absence of life. A solid planet with an ocean and more than 0.1% methane in the atmosphere should be considered a potentially habitable planet, scientists say. And if atmospheric methane reaches a level of 1% or more, then in this case the planet will not be “potentially”, but “most likely” habitable.
Jim Casting, an atmospheric chemist at the University of Pennsylvania, says these results are "on the right track," although "the idea that methane might be a biosignature in an anoxide atmosphere is relatively old."
In addition, Cutling and his co-authors figured out how their methane signature should manifest itself and how to distinguish it from non-living sources. According to their model, methane in the atmosphere of an anoxic planet of the terrestrial type should usually react with carbon dioxide, which is still in the air, mix with nitrogen and water vapor, and rain down as a heavy compound. Further calculations showed that no abiotic (that is, non-living) sources of methane on a solid planet will be able to produce enough gas to interfere with this process - be it volcanic gas pollution, chemical reactions in deep-sea vents, and even asteroid falls. Only a living population of methane-eating bacteria can explain the gas. More importantly, even if abiotic sources provide enough methane,they will almost inevitably produce a lot of carbon monoxide, a gas that is poisonous to animals but loved by many microbes. Together, methane and carbon dioxide, in the absence of carbon monoxide, on a solid planet with an ocean could well be interpreted as a sign of oxygen-independent life.
This is good news for astronomers. The James Webb Telescope will struggle to directly detect the presence of oxygen on any potentially habitable planet it sees on its mission. Just as your eyes can distinguish visible light, but cannot see radio or X-rays, Webb's vision is tuned to the infrared spectrum - a part of the spectrum that is ideal for studying ancient stars and galaxies, but does not cope well with oxygen absorption lines, where they are scattered and rare … Some scientists fear that the search for life will have to be postponed until other, more capable telescopes are available. But while Webb cannot see oxygen with ease, his infrared eyes can perfectly see signs of oxygen-free life. The telescope is capable of simultaneously detecting methane,carbon dioxide and carbon monoxide in the atmospheres of some planets near red dwarf stars. For example, in the TRAPPIST-1 system.
Yet Webb is unlikely to master the most important part of Cutling's criteria - determining the relative amount of each gas - and cannot understand, for example, whether volcanoes or farting microbes produce methane on a given planet. It is unlikely that Webb will find an anoxide biosphere on any planet under a red sun.
Another thing is important. Life is more important to seek than oxygen.
Ilya Khel
