Terrestrial atmosphere, hydrosphere, oxygen concentration, O2, pyrite pebbles, Earth's atmosphere, Great Oxidation, comets, asteroids, water vapor, volcanic degassing, Earth's formation, late heavy bombardment, isotopic studies, carbonaceous chondrites, extra-terrestrial input, Earth's mantle, primitive atmosphere, Earth's surface, water cycle, atmospheric oxygen, fluvial transport, oxidation reactions, Earth's history, Earth's water, solar system, unique planet, blue planet, Earth's early stages, formation of oceans, precipitation, clouds, volatile gases, intense volcanism, terrestrial environment, chemical conditions, Earth's mantle degassing, cometary impacts, asteroidal impacts, water sources, planetary formation, atmospheric evolution, geological history, Earth's uniqueness
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Abstract
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Abstract
The hydrosphere's origin is linked to Earth's formation and evolution, involving internal degassing and external inputs from comets and asteroids.
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[...] During the Earth's formation, molten rocks constituted a significant part of the Earth's mantle. As these rocks cooled, they released volatile gases, including water vapor, which accumulated in the primitive atmosphere. This degassing phenomenon occurred during periods of intense volcanism that marked the early history of the Earth. The water vapor released into the atmosphere condensed due to the planet's gradual cooling, forming clouds and then precipitation, which gave rise to the first oceans. This process may have contributed significantly to the formation of the hydrosphere. [...]
[...] Comets, composed mainly of ice, would have released water upon impact with the Earth, thus contributing to the enrichment of the hydrosphere. Furthermore, isotopic studies of certain types of carbonaceous asteroids (carbonaceous chondrites) have revealed a composition similar to that of terrestrial water, reinforcing the hypothesis of an extra-terrestrial input. Today, scientists agree that the terrestrial hydrosphere likely results from a combination of these two mechanisms. If internal degassing was able to form a significant portion of the Earth's water, external inputs have also played a significant role, enriching and complementing the initial water reserve. [...]
[...] Document 2 shows that the Earth's atmosphere was almost devoid of oxygen before about 2.2 billion years ago. The text specifies that rocks containing pyrite pebbles (FeS< sub >2)?) dating back 2.9 billion years have been found in South Africa. Pyrite, a very unstable mineral in the presence of O< sub >2?, is formed and preserved only in environments with low oxygen levels. Thus, before 2.2 billion years ago, the low concentration of O< sub >2? in the atmosphere allowed pyrite pebbles to form and be transported by rivers without undergoing alteration. [...]
[...] In fact, the increase in oxygen in the atmosphere has been reflected in the water of the rivers through gas exchanges, which has led to the rapid oxidation of pyrite. From then on, pyrite pebbles exposed to water containing oxygen have degraded, making it impossible to preserve them in the rivers after this period." The distribution of pyrite pebbles before 2.2 billion years ago, and then their disappearance after this period, is explained by the evolution of the Earth's atmosphere. The absence of O< sub >2? before this date allowed their transport and conservation, while the appearance of oxygen caused their oxidation and disappearance in fluvial environments. [...]
