The planet came into being some time after the formation of the Sun approximately 4.55 billion years ago (Ga), most likely from the rapid accretion of material within the solar nebula, and the moon-forming impact likely occurred shortly thereafter. Several key markers can be placed along Earth’s evolutionary path constraining the state of the environment during different epochs. The atmospheric composition has clearly not been constant over time, and the state of the atmosphere in the first billion years of Earth’s existence has significant implications for the origin and evolution of life. The current balance of gases in the Earth’s atmosphere is heavily driven by the biosphere, as evidenced by the large abundance of O 2 outside of thermodynamic equilibrium. Earth’s current atmosphere has a surface pressure of 1 bar, and is composed of approximately 78% nitrogen (N 2) and 21% oxygen (O 2), with small amounts (< 1%) of other species including argon (Ar), water (H 2O), and carbon dioxide (CO 2). Planet Earth has gone through a lengthy evolution, from its initial formation to its present state as the only known harbor of life in the universe. INTRODUCTION TO “EARLY EARTH” AND ITS ATMOSPHERE This review intends to bridge current knowledge of the range of possible atmospheric conditions in the prebiotic environment and pathways for synthesis under such conditions by examining the possible products of organic chemistry in the early atmosphere.ġ. The results highlighted here show that a variety of products can be formed in mildly reducing or even neutral atmospheres, demonstrating that contributions of atmospheric synthesis to the organic inventory on early Earth should not be discounted. Since such a haze layer – if formed – would serve as a reservoir for organic molecules, the chemical composition of the aerosol should be closely examined. Intertwined with the discussion of atmospheric synthesis is the consideration of an organic haze layer, which has been suggested as a possible ultraviolet shield on the anoxic early Earth. This work provides a survey of the range of chemical products that can be produced given a set of atmospheric conditions, with a particular focus on recent reports. Abiotic synthesis of organic molecules is more productive in reduced atmospheres, yet the primitive Earth may not have been as reducing as earlier workers assumed, and recent research has reflected this shift in thinking. Experimental and theoretical studies over the past half century have shown that atmospheric synthesis can yield molecules such as amino acids and nucleobases, but these processes are very sensitive to gas composition and energy source. Earth’s atmospheric composition at the time of the origin of life is not known, but it has often been suggested that chemical transformation of reactive species in the atmosphere was a significant source of prebiotic organic molecules.
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