Formation of the Moon and the Earth from a Common Supraplanetary Gas—Dust Cloud (Lecture Presented at the XIX All-Russia Symposium on Isotope Geochemistry on November 16, 2010)

摘要

A hypothesis is proposed on the formation of the Earth and the Moon from a large-scale gas—dust cloud, the size of which is limited by the Hill radius, i.e., approximately one million kilometers. The compression of the supraplanetary gas-dust cloud resulted in an adiabatic temperature increase in its interior parts and evaporation of volatiles, including iron, from the surface of particles. At a certain stage, within 50— 70 Ma after solar system formation, the supraplanetary gas-dust disk was fragmented, the Moon was separated, and the Earth embryo was formed. The remaining part of the gas-dust material was accreted mainly to the Earth. During this process, the gas dominated by primordial hydrogen was squeezed out of the disk. Vapor was removed together with hydrogen from the interparticle space. The hydrodynamic lifting resulted in the loss of volatiles, including Rb, Xe, and Pb, which is reflected in the Rb-Sr, Xe-I-Pu, and U-Pb isotopic systems. The gas—dust accretion was accomplished within 110—130 Ma (most likely, -120 Ma) after the beginning of solar system formation. Since then, the hydrodynamic lifting and volatile loss have ceased, and the history of the Earth as a condensed body has started.