The bowels of the Earth began to actively exchange gases and liquids with the atmosphere and the hydrosphere unexpectedly late, about 2.5 billion years ago. This indicates the unusual nature of the planet’s cooling, say geologists in an article published in the journal Nature.
“Most people do not suspect that there is a huge amount of water, various gases and other volatile substances in the depths of the Earth, their proportion is relatively low, but this is compensated by the huge mass of the mantle.” Therefore, the planet’s “breathing”, gas exchange between the lithosphere, atmosphere and hydrosphere , plays an important role in the existence and evolution of life, “says Rita Parai of 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 into the depths of the mantle and their subsequent “ascent” 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 Earth’s atmosphere and oceans. When the continental rocks sink deep into the mantle, they carry with them large quantities of sedimentary rocks containing various gases, water and other volatile substances. They return to the surface together with eruptions of volcanoes, which often dramatically changes the composition of air and water, and greatly affects terrestrial life.
For example, recently geologists have discovered that the “surfacing” of the mantle in the vicinity of modern Norilsk has led to saturation of the atmosphere with a large amount of greenhouse gases and “sowing” of the oceans with nutrients that accelerate the growth of microbes. Both of these events, which occurred approximately 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 in Davis, USA, found out when such “light” planets were launched, studying the oldest samples of the rocks of the Earth’s crust and mantle.
As explained by geologists, the bowels of the planet contain small amounts of noble gases that fall there both along with the “sinking” crust, and those resulting from the decay of uranium, thorium and other radioactive elements.
Mukhopadhyay and Parai noticed that the proportion of the isotopes of one of these gases, xenon, will vary greatly 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 relatively a lot of xenon-129 and xenon-136, and air and processed crustal rocks – 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 rocks of the mantle that had left the bowels of the planet relatively recently, and tried to calculate the time of launching its “lungs.”
These calculations showed that the “atmospheric” xenon was virtually completely absent in the bowels of the Earth during the first two billion years of the planet’s life. Such conclusions were a big surprise for scientists.
On the one hand, this may mean that tectonic processes and the cycle of rocks in the lithosphere started unexpectedly late, only 2.5 billion years ago. This, according to Parai, is extremely doubtful given the existing geological evidence. On the other hand, scientists do not exclude the fact that xenon and other gases simply did not fall into the mantle because the depths of the Earth were much hotter in the early days of her life than we believe today.
This led to the fact that most of the gases left the crust before they could sink into the deep layers of the mantle, which did not allow the atmospheric xenon to “mix” with the underground reserves of this gas and change their isotopic composition. Approximately 2.5-2.4 million years ago, they cooled sharply, the cause of what remains to be determined.
Regardless of which of the theories is true, both interpretations of this discovery significantly change our ideas about the appearance of the early Earth and the conditions in which the first living organisms arose, the authors conclude.