(ORDO NEWS) — Scientists who study the history of the formation of the Earth are continually looking for any evidence of past events. They can serve, for example, the most insignificant differences in the isotopic composition of rocks and gas envelopes of different planets, as well as other celestial bodies.
In the new study, the noble gas krypton became such “evidence” – the most reliable explanation for the observed ratio of its isotopes in the Earth’s interior can only be the bombardment of our planet with large bodies that arrived from the outer regions of the solar system about 4.5 billion years ago.
Scientific work with new data on the isotopic composition of krypton contained in the earth’s magma was published in the journal Nature. It was prepared by the staff of the University of California at Davis with the participation of colleagues from the Woods Hole Oceanographic Institute and the University of Oregon. The team was led by Sandrine Péron, now at ETH Zurich, and the study culminated in her postdoctoral studies in the United States led by Professor Sujoy Mukhopadhyay.
It was the labors of the mentor Peron that made it possible to carry out the measurements necessary to make the discovery. In the laboratory of Mukhopadhyaya, a method was previously developed for determining the isotopic composition of ultra-low concentrations of noble gases in the composition of minerals.
Using a pressure chamber, scientists extracted a mixture of ancient gases from lava samples, isolated only noble gases from them, and then separated krypton from argon and xenon. The obtained practically pure krypton could already be subjected to spectral analysis in order to reveal the desired ratio of the isotopes Kr-78 and Kr-80.
The samples collected by the Peron team are very ancient, they are more than 4.4 billion years old. The gases that were present on the Earth during that period of its formation were literally trapped in molten rocks. Over time, these “time capsules” sank very deep into the bowels.
The only places you can get them are volcanic hotspots in the Galapagos and Iceland. It is there that lava is poured onto the surface of the planet, which is carried in a powerful flow into the mantle from practically the outer boundaries of the earth’s core. When the melt solidifies, some of the gases do not have time to escape and are trapped in microscopic bubbles among the glassy matrix of the volcanic rock.
Each such bubble contains very small amounts of krypton: a few hundreds of millions of atoms. That is why, in order to determine the isotopic composition of this gas in ancient rocks, a complex procedure for “cleaning and enriching” the sample was needed.
At all stages of the process, perhaps the main challenge for the researchers was avoiding air pollution of the samples. From the moment of sampling to sending the purified ancient krypton to the spectrometer. Only in this way could scientists confidently compare the isotopic composition of the current atmosphere and ancient gas mixtures. A difficult requirement was met, and this allowed the researchers to draw interesting conclusions.
The results of the analyzes show that in ancient rocks and today’s atmospheric air, the ratio of the isotopes Kr-78 and Kr-80 is strikingly different. Moreover, the samples of gases from lava are almost identical to those found in carbon-rich meteorites from the outer borders of the solar system. This means that the Earth, already in the early stages of its formation, received a fair amount of volatile chemical elements and compounds by bombardment with such celestial bodies.
This conclusion strongly contradicts the currently dominant hypothesis of the origin of volatile elements on our planet. In scientific circles, the version is considered the most reasonable, according to which the Earth received most of them after the formation of the Moon as a result of a colossal impact event (collision with Theia). Note that, although this theory is well substantiated, there are also alternative ones, which often explain some empirical data much better.
Moreover, the results of the work of Peron and colleagues lead to even more interesting conclusions. There were definitely several sources of volatile elements and compounds on Earth, no less than three or four. It is well proven that our planet received neon from the Sun.
Naturally, meteorites of various classes over the past 4.4 billion years have brought them a lot too. The likely collision that formed the moon four and a half billion years ago should have carried away many of the light components of Earth’s chemistry. So after it, something returned the vital matter back to us. But why Kr-86 in the mantle is slightly less than in other isotopes than in meteorites from the borders of the solar system is still unclear. Perhaps there was (or is) some other source of light “grades” of this noble gas, which we have not yet found.
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