Most of the water on Earth can be “cometary” origin

Isotopic analysis of gases in the “tail” of comet Churyumov-Gerasimenko showed that most of the reserves of xenon and other noble gases, as well as water, could get to the Earth together with comets, scientists say in an article published in the journal Science.

“The unusual isotopic composition of xenon in the matter of comet Churyumov-Gerasimenko allows us to assess how much these objects were” invested “in the formation of the earth’s atmosphere, they could collide with either the embryo of the Earth, or fall on it in the Gadejsky period, 4.5-3 , 8 billion years ago.This scenario also explains why the fall of comets had almost no effect on the proportion of heavy hydrogen in the water of the world’s ocean, “said Kathrin Altwegg from the University of Bern in Switzerland and her colleagues.

The Riddle of Noble Elements

Today, scientists believe that the Earth formed in the “warm” part of the protoplanetary disk, where organic molecules and water could not exist because the ultraviolet radiation of the light broke the water and the “bricks of life” into parts and prevented them from uniting. For this reason, scientists believe that water was brought to Earth from the cold distant approaches of the solar system, and organic matter either appeared already on the surface of the planet, or was brought along with moisture.

The role of these “water carriers” and suppliers of substances, according to Altvegg, claims two classes of small celestial bodies – asteroids and comets, equally bombarded the surface of the young Earth about 3.8 billion years ago.

Over the past two years, scientists, thanks to the Rosetta mission and ground-based telescopes, have provided plenty of evidence that comets could lead in this race. For example, it turned out that on their surface there are alcohol, sugars and a number of other organic compounds, from which the real “life blocks”, of which proteins and DNA are composed, can form.

Altvegg and her team found further evidence that the comets were actively involved in the formation of the Earth, studying the data that the COSINA spectrometer installed on board the Rosetta probe collected during the flights through the “tail” and during the approach to the surface of the comet Churyumov- Gerasimenko at the end of May 2016.

The Ancient Sun

As the scientists say, these spans pointed to an unexpected thing: in the bowels of the comet there were unusually few heavy isotopes of xenon and quite a lot of xenon-129, a light isotope whose atoms appeared in the first instants of the life of the solar system as a result of the decomposition of unstable iodine-129 in a gas-dust “cocoon” “The sun.

Its half-life is only 15.6 million years, so that all the reserves of iodine-129 should disappear from the solar system in the first 100 million years of its life, leaving a unique trace in the form of xenon-129 atoms in the atmospheres of all its planets.

His share in the Earth’s atmosphere, as noted by Altvegg and her colleagues, does not coincide with the similar value for asteroids-chondrites, from which the planets of the solar system were “stuck”. Their matter contains little xenon-129 and much more xenon-134 and xenon-136 than in terrestrial air. Scientists for a long time could not find an explanation for this anomaly.

The answer to this riddle was found in the gas “tail” of the comet Churyumov-Gerasimenko – its subsoil contains 40-60% less heavy xenon isotopes than the matter of chondrites and solar wind, constantly replenishing the reserves of this gas in the Earth’s atmosphere. As calculations of scientists show, approximately 23% of terrestrial xenon fell into the atmosphere of our planet together with comets, and the remaining 77% – with solar wind and asteroids.

This discovery, as Altvegg and her colleagues note, makes it possible to clarify some details of the role of the comet in the formation of the Earth. Such shares of xenon isotopes in their interior, for example, suggest that the ice grains in comets were formed even before the birth of the solar system, several million years before the central part of the gas-dust “cocoon” where the Sun was born began to warm up.

This makes it possible to explain some oddities in the chemical composition of the Churyumov-Gerasimenko comet, such as the presence of free oxygen and sulfur in it, and also explains why the proportions of heavy and normal hydrogen in the oceans of the Earth do not coincide with the values ​​characteristic of comets. In addition, data on the isotopic composition of xenon are in good agreement with measurements on the proportions of other noble gases, argon and krypton, which also testify to the “cometary” origin of water and air on the Earth.

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