Impacts and evolution of early earth

Abstract

The problem of the first half-billion years of Earths existence is fundamental for all Earth sciences because it was in this period that the Earth and its envelopes were formed. During the last decades of the twentieth century joint efforts of scientists from different countries made it possible to develop a standard scenario of the origin of the Solar System and the planets that satisfied basic observational data accumulated by astrophysics and comparative planetology. The general description of this scenario, as well as solutions of related problems, can be found in Safronov and Vityazev (1985), Vityazev et al. 1990, Vityazev and Pechernikova (1991), and the multiauthor book, The Origin of the Earth and the Moon (2000). The use of a number of isotope systems (space- and geo-chronometers) provides a way of estimating the duration of the most essential stages of Earths formation. It was established that physical and chemical evolution of the preplanetary disk matter incorporated into growing planets began at the early stages of their formation. The basic exogenic source of energy and material for the growing planets was falling bodies, with sizes up to 1,000 km (Vityazev et al. 1990; Pechernikova and Vityazev 1996; Vityazev and Pechernikova 1996). The Earths core and mantle formation took place during the final stages of Earths growth (30-100 Ma after the Suns formation), actually simultaneously with the formation of the primitive crust, hydrosphere, and atmosphere. The impact events determined the influence of the formation of the Earths outer envelopes (Pechernikova and Vityazev, 1996; Vityazev and Pechernikova 1996). This chapter offers a brief overview of the modern understanding of the role of impacts during the first 500 Ma of the Earths evolution. The overview begins with a brief discussion of a recent modification in the theory of the Earths growth and a generalized formula of the mass accumulation rate. The it discusses spectra of mass and velocities of falling bodies and their composition and describes simulation of the frequency of falls of bodies with different sizes. Using the formula of the Earths growth rate and the estimate of the mass velocity spectrum of planetesimals, the rate of input of kinetic energy and the depth of mixing of the near-surface layers of growing planets are obtained. Then the interpretation of available scarce experimental data and observable relict signals from Hadean are presented, which for the first time enabled researchers to obtain the characteristics of compositions and conditions of the early upper geospheres, including the climate of the primary Earth. © 2008 Springer.