Earth

Abstract

Accretion of the Earth and its companions from chondritic planetesimals appears to have been completed by about 4.55 Ga, 100 Ma after formation of the solar nebula, and according to Condie (1989, p. 29) 1 Ma before the Sun went through a T-Tauri stage and the resulting strong solar winds blew away the most volatile elements from the inner Solar System and the early atmospheres of the inner planets. In the three-dimensional model described by Boss (1990) compressional heating during nebula formation caused the gross depletion of volatiles on Earth relative to their solar abundances. The inner nebula may have experienced temperatures of 1500 K that were regulated by vaporization of iron grains. Remote sensing data indicate that planetesimals of the inner Asteroid Belt underwent post-accretionary heating during the first few million years of Solar System history, a heating that led to extensive melting and magmatic differentiation (Gaffey 1990). This heating, which may have been caused by electrical induction during the T-Tauri stage or by short-lived radioisotope activity, also may have affected the planetesimals in the inner Solar System whose accretion produced the rocky planets. Such planetesimal differentiation must have influenced the accretionary and post-accretionary geohistory of the rocky planets. Accretions from volatile-depleted planetesimals differentiated into metallic cores and silicate mantles and allowed the Earth’s core to form during accretion (Taylor and Norman 1990). Some investigators suggested that formation of the Earth may have been homogeneous, involving simultaneous condensation and accretion of compounds from a hot nebula as it cooled (Condie 1989, pp. 26–29). Others proposed an inhomogeneous model with an iron core and with the silicate mantle condensing sequentially, and still others believed that giant impacts influenced the evolution of the Earth, including the ejection of a primitive atmosphere, a source for melting the Earth, and a component for the mantle and core (Ahrens 1990; Melosh 1990; Newson and Sims 1991). Ahrens and O’Keefe (1989) also stated that possibly the core settled out at the same time that accretion occurred; thus there were some deviations from homogeneity during accretion of the planet. In a scenerio described by Benz and Cameron (1990), impact of the Earth by a giant planetesimal caused iron from the core of the impactor to penetrate the mantle and settle atop the Earth’s core where it was heated to several ten thousand degrees. Most of the mantle of the impactor and a considerable volume of the Earth’s mantle were ejected, but later some fell back onto the proto-earth. There was some heating of all parts of the interior of the proto-earth, and the surface layers reached temperatures of 16 000 K and were vaporized. It is this intense surface heating coupled with the presence of an orbiting disk of rock vapors and magmas that Benz and Cameron (1990) believed caused ejection of the early terrestrial atmosphere.