Thermal Evolution of the Earth During the First Billion Years

Christophe Sotin

There is good evidence that life occurred on Earth during the first billion years of its history. Modelling the dynamics of the Earth at this period of time is critical to understand the conditions of the emergence of life. These conditions are the result of the coupling between the inner and outer envelopes of the Earth. Several processes such as volcanism, magnetic field and plate tectonics originate in the Earth's deep layers. They control the physical and chemical conditions of the outer layers (atmosphere, hydrosphere, and crust) where life appeared and developed. The goal of this chapter is to describe these internal processes and to present models for Earth's evolution. After a descriptive summary of our current knowledge of the Earth's deep interior, this chapter explains the mechanisms of heat transfer to the surface by sub-solidus thermal convection, a process that drives the Earth's surface dynamics (volcanism and plate tectonics). The last part of this chapter addresses the Earth's magnetic field and how it prevents atmospheric escape and preserves the present atmosphere. Throughout this chapter, references to conditions existing on Earth-like planets are given to illustrate how the knowledge of these planets contributes to a better understanding of the history of our own planet.

5.1 Internal Structure and Dynamics of the Earth 5.1.1 Description of the Different Layers

Seismological data acquired during almost a century provide a radial description (1D) of the physical properties of the Earth (density, elastic constants, pressure). The model, known as PREM (Preliminary Reference Earth Model) was published in 1981 (Dziewonski and Anderson, 1981). It gives a more accurate description of the Earth's internal structure than the model proposed at the beginning of the 20th century (Lehmann, 1936 and review by Bolt, 1982), which determined the depth of the major discontinuities thanks to the reflection of seismic waves on these interfaces. The 'solid' Earth is divided into three main layers with distinct chemical compositions: the core, the mantle and the crust (Fig. 5.1).

M. Gargaud et al. (Ed.): Lectures in Atrobiology, Vol. I, pp. 165—193 © Springer-Verlag Berlin Heidelberg 2005

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