The Chondritic Uniform Reservoir (CHUR) is a scientific model in astrophysics and geochemistry for the mean chemical composition of the part of the Solar Nebula from which, during the formation of the Solar System, chondrites formed. This hypothetical chemical reservoir is thought to have been similar in composition to the current photosphere of the Sun.
When the Sun formed from its protostar, around 4.56 billion years ago, the solar wind caused outward dispersal of gas particles from the central part of the Solar Nebula. In this way, most lighter volatiles (e.g. hydrogen, helium, oxygen, carbon dioxide), that had not yet condensed in the inner, warmer regions of the nebula, were lost. This fractionation process caused the terrestrial planets and asteroid belt to become relatively enriched in heavy elements, with respect to the composition of the Sun and the Jovian gas planets.
Certain type of meteorites, CI-chondrites, have chemical compositions that are almost identical to the solar photosphere, except for the abundances of volatiles. Because the Sun contains 99.86% of the mass of the Solar System, chondrites are considered proxies for the composition of the early solar nebula (with the exception of volatile loss), and are therefore representative of the material from which the terrestrial planets, including the Earth, were formed.
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