The possible existence of habitable exoplanets has occupied the minds of mankind for more than a hundred years. However, until now, we have not been able to find a single habitable planet, and in the meantime, new studies again and again continue to present facts that reduce the chances of other planets for possible habitability. One of the most important factors of potential habitability is the presence of oxygen in the planet's atmosphere suitable for breathing. But how can oxygen accumulate in the atmospheres of exoplanets?
To answer this question, it is worth taking a closer look at the evolution of the Earth's atmosphere. According to modern concepts, the increase in the concentration of oxygen in the planet's atmosphere occurred in three stages, starting with the event known as the "oxygen catastrophe" (2.4 billion years ago), followed by similar events in the Neoproterozoic (800 million years ago) and Paleozoic (400 million years ago), after which the atmosphere reached its current oxygen level of 21 percent.
Such an abrupt enrichment of the Earth's atmosphere with oxygen could be associated, for example, with the spread of photosynthetic terrestrial plants or with giant volcanic eruptions. Thus, this scenario links the increase in oxygen concentration to random events. If a certain species of photosynthetic plants hadn’t suddenly come out onto land or specific volcanoes had not erupted, oxygen might not have appeared in the atmosphere.
In contrast to this point of view, a new numerical model built by scientists led by Lewis J. Alcott shows that, given the cycles of carbon, oxygen and phosphorus on Earth, the increase in oxygen concentration can be explained by the internal dynamics of the planet and does not require “miraculous »Events.
For example, studying the phosphorus cycle helped the team determine its relationship to oxygen in the atmosphere. Phosphorus plays an important role in the life of oceanic algae. The amount of phosphorus in the ocean today determines the amount of oxygen in the atmosphere - and the same situation took place earlier in the history of the Earth, according to the authors of the work.
Plants in the ocean require phosphorus for photosynthesis, but high phosphate levels increase oxygen consumption at great depths as a result of the eutrophication process. When photosynthetic organisms die, they decompose, also consuming oxygen from the water. When the oxygen level drops, additional phosphorus is released from the material deposited on the bottom. As a result of this feedback loop, oxygen quickly disappears. This means that the oxygen level may have dropped rapidly in the Earth's atmosphere, but in fact dropped slowly due to another process associated with the Earth's mantle.
Throughout the history of the Earth, volcanic activity took place, which gradually decreased due to the cooling of the mantle, and, according to the computer model of Olcott and his colleagues, it was this factor that could determine the gradual accumulation of oxygen in the Earth's atmosphere.
The research is published in the journal Science.