Overview of the Special Issue “The Hadean World (Part II): Preparing a Site for the First Life“

Abstract

The special issue "The Hadean World (Part I) : The Birth of a Habitable Trinity Planet" focused on the Tandem model, which is a new planetary formation theory explaining the formation of a habitable trinity planet that enables the birth of life, and the ABEL model, which describes the formation process of the Earth. It went on to introduce the latest research on the origins of life. In the special issue Part II, we mainly focus on the birthplace of life, in order to unravel where and how primordial life emerged. Part II addresses the initial requisites for the birth of life and its early evolution, which will be discussed in the special issue Part III.It is assumed that the Hadean atmosphere had almost no free oxygen, and a large amount of carbon dioxide (a few bars) was present. Ueda and Sawaki (2019) quantitatively evaluate water-rock interactions under the Hadean surface environment with hydrothermal experiments. On the Hadean continent, komatiite was exposed ubiquitously and reacted with surface water to advance serpentinization. Komatiite is a volcanic rock, which mainly comprises olivine and is stable at temperatures above 1,000°C, while liquid water is stable between 0 and 100°C under a pressure of 1 bar. There-fore, when komatiite and liquid water exist simultaneously, serpentinization (olivine turn to serpentinite) proceeds as a non-equilibrium reaction. Serpentinization produces hydrogen due to the decomposition of Fe 2 SiO 4 (olivine) , which produces two hydrogenic atoms as H 2 through the chemical reaction Fe 2+ = Fe 0 + Fe 3+ to produce magnetite (FeO, Fe 2 O 3 ) . In the Hadean atmosphere, however, there should have been a large amount of carbon dioxide, which would have produced carbonate minerals through reactions with CO 2 by consuming Mg and Fe 2+ . Therefore, a reaction that converts Fe 2+ into Fe 3+ does not occur, thus inhibiting the production of hydrogen. This excellent example shows how chemical reactions under the Hadean environment are quite different from those in the present Earth's environment.It has been generally considered, since an experiment performed by Miller (1953) , that a strong input of external energy is necessary for the birthplace of life. In the case of life on the present Earth, the necessary external energy is provided by the Sun as solar energy. What was the energy source when life was born? Was Hadean solar a sufficient external source of energy for life? This problem has not been discussed quantitatively.