Earth Accretion

Abstract

Accretion of the Earth is described in terms of the infall of volatile-, silicate-, sulfide-, and iron-bearing planetesimals. The shock pressures induced in the constituents of the planetesimals upon impact with the Earth determine their fate and the oxidation state associated with reaction of the coaccreting protoatmosphere. For planetesimal infall velocities of <0.9 km/sec, the accreting Earth has a radius of ≾1400 km, and the impacting planetesimals retain their full complement of volatiles, e.g., H_2O, CO_2, SO_2 , NH_3, CH_4, and noble gases. This portion of the initially accreted Earth is the present source of the mantle-derived ^3He. The present rate of ^3He flux from the Earth's mantle is fully consistent with the existence of a planetesimal-derived primitive undifferentiated reservoir of an equivalent mass as a sphere of 1400 to 2000 km radius. Upon the onset of impact vaporization, at shock pressures of ~19 GPa, the water driven from the planetesimals by impact is able to react with metallic iron. This oxidation results in an increase in the iron silicate budget of the Earth. The composition of the mantle indicates that a maximum of 0.12 to 0.22 of the Earth's mantle accreted under oxidizing conditions. As more water is released by impact, the protoatmosphere inhibited infrared radiative cooling of the surface. As a consequence, impact energy is trapped under the protoatmosphere and the surface temperature rises to the melting point of wet silicates (~1400 K); surface temperature then becomes buffered by the melting reaction. Composition of the protoatmosphere is controlled by temperature and partial solubility in molten silicate. Iron silicates are reduced to metallic iron. Iron, both thermally and shock heated, and iron sulfide form the Earth's core. As accretion continues, the largest planetesimals impacting the Earth increase in diameter from several kilometers to several hundred kilometers and possibly to thousands of kilometers. We examine the constraints on impact-induced loss of the entire protoatmosphere as a result of giant impact of large planetesimals. A ~2000-km-diameter, 10^(37)-erg impactor is found to be sufficient to completely eject into space the protoatmosphere of the Earth. Impact of a larger 3000-km-diameter object is calculated to expel the protoatmosphere to speeds of >15 km/sec.

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