The bowels of the Earth began to actively exchange gases and liquids with the atmosphere and hydrosphere unexpectedly late, about 2.5 billion years ago. This speaks to the unusual nature of the cooling of the planet, say geologists in an article published in the journal Nature.
“Most people are unaware that there is a huge amount of water, various gases and other volatile substances in the bowels of the Earth. Their proportion is relatively low, but this is offset by the huge mass of the mantle. For this reason, the “breathing” of the planet, the exchange of gases between the lithosphere, atmosphere and hydrosphere, plays an important role in the existence and evolution of life,”says Rita Parai from the University of Washington in St. Louis (USA).
Circle of life
According to geologists, life exists on Earth and is absent on Venus due to the fact that the bowels of our planet do not stand still, but constantly “migrate” between its surface and the deep layers of the lithosphere. The movement of the continents, the gradual immersion of their rocks in the depths of the mantle and their subsequent "emergence" help the Earth "dump" excess heat and stabilize the climate.
This process, according to scientists, affects not only the climate, but also the composition of the atmosphere and oceans of the Earth. When the rocks of the continents sink deep into the mantle, they carry with them large quantities of sedimentary rocks containing various gases, water and other volatiles. They return to the surface along with volcanic eruptions, which often dramatically change the composition of air and water, and strongly affect life on Earth.
For example, recently geologists have discovered that the "surfacing" of the mantle in the vicinity of modern Norilsk led to the saturation of the atmosphere with a large amount of greenhouse gases and the "seeding" of the oceans with nutrients that accelerate the growth of microbes. Both of these events, which took place about 255 million years ago, served as a trigger for the Permian Extinction, the most serious cataclysm in the history of life on Earth.
Paray and her colleague Sujoy Mukhopadhyay from the University of California at Davis (USA) found out when such "light" planets started up by studying the oldest samples of the Earth's crust and mantle.
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As geologists explain, the bowels of the planet contain small amounts of noble gases that get there both along with the "sinking" crust, and arising from the decay of uranium, thorium and other radioactive elements.
Late start
Mukhopadhyay and Paray noted that the fractions of the isotopes of one of these gases, xenon, will be very different for rocks that are often in contact with water and the atmosphere, and for the primary matter of the Earth. For example, the primary mantle should contain a relatively large amount of xenon-129 and xenon-136, and the air and processed rocks of the crust should contain xenon-124 and xenon-128.
Guided by this idea, scientists analyzed several samples of meteorites, similar in composition to the primary matter of the Earth, as well as mantle rocks that left the interior of the planet relatively recently, and tried to calculate the time of launch of its "lungs".
These calculations showed that "atmospheric" xenon was virtually completely absent in the Earth's interior during the first two billion years of the planet's life. Such findings came as a big surprise to scientists.
On the one hand, this may mean that tectonic processes and the circulation of rocks in the lithosphere started unexpectedly late, only 2.5 billion years ago. This, according to Paray, is highly doubtful given the existing geological evidence. On the other hand, scientists do not exclude the possibility that xenon and other gases simply did not get into the mantle for the reason that the interior of the Earth in the first epochs of its life was much hotter than we think today.
This led to the fact that most of the gases left the crustal rocks even before they had time to sink into the deep layers of the mantle, which did not allow atmospheric xenon to "mix" with underground reserves of this gas and change their isotopic composition. About 2.5-2.4 million years ago, they cooled sharply, the reason for which remains to be seen.
Regardless of which of the theories is correct, both the one and the other interpretation of this discovery noticeably change our ideas about the appearance of the early Earth and the conditions in which the first living organisms arose, the authors of the article conclude.
