2, 7 billion years ago, when living organisms had just begun to master photosynthesis and fill the air with oxygen, atmospheric pressure was no more than a quarter of the current one. At the surface of the Earth, it was as it remains today only at an altitude of about 10 km.
The work, which University of Washington professor David Catling and his colleagues report in the journal Nature Geoscience, casts doubt on all previous ideas about the Earth's atmosphere in the Neoarchean era, between 2.5 and 2.8 billion years ago. We are accustomed to seeing the air of that time as dense, thick and foggy - such a picture was born from the famous "paradox of the young weak Sun".
Indeed, astronomers point out that our star had not yet reached full strength at that time and its luminosity was about 30% lower than today. On the other hand, reliable geological data show that vast oceans of liquid water existed on the planet, and the temperature was even higher than today. It is believed that for this the Earth should have been strongly "heated" by some additional factor, and the main candidate for this role is precisely the dense atmosphere that can create a greenhouse effect.
There was practically no oxygen in it, the content of carbon dioxide remained low, and nitrogen dominated. For such an atmosphere to cause the necessary heating of the planet, it needs to be dense: calculations show that its pressure should have been 1, 6-2, 4 times higher than the current one. However, the findings made by David Catling's team unexpectedly refute these notions.
Researchers have studied the lava rocks of the Pilbara Craton, an ancient geological platform in the west of present-day Australia, frozen for about 2.7 billion years. The state of air bubbles in this fossilized foam is in no way consistent with previous calculations: according to estimates by Catling et al., The atmospheric pressure at that time was noticeable - 23% - less than the current one. The authors admit that the accuracy of this estimate is not too high, however, in principle, they do not doubt the result: with a probability of 0.95, this value lies in the range of 0–0.5 of the modern atmospheric pressure.
The lack of nitrogen in the atmosphere of that era could be associated with the activity of living organisms. The mechanisms of nitrogen fixation, which appeared in some microorganisms about 3.2 billion years ago, allowed them to actively trap nitrogen molecules directly from the air, convert them into ammonium salts and use them to build proteins and nucleic acids. The innovation provided these microbes with long-term prosperity, but also led to a drop in atmospheric pressure.
Scientists believe that only later, when nitrogen fixation was supplemented by photosynthesis, and oxygen began to appear in the atmosphere, ammonium was able to quickly oxidize and return to the air in the form of molecular nitrogen, and the cycle of matter between the atmosphere and the earth's crust, the ocean and living beings was finally established on the planet.