The Primitive Atmosphere

From the viewpoint of a model of prebiotic chemical evolution and of the primitive atmosphere of the Earth,174175 photosynthetic reactions of C02 were also examined, and formaldehyde with various nitrogen-containing products was obtained. 

There was no free oxygen or nitrogen in the primitive atmosphere. How and why did the composition of air change ... 

Among activated forms of amino acids, mixed anhydrides with inorganic phosphate or phosphate esters require a special discussion because they are universally involved in peptide biosynthesis through the ribosomal and non-ribosomal pathways. These mixed anhydrides have stimulated studies in prebiotic chemistry very early in the history of this field. Amino acyl adenylates 18c have been shown to polymerize in solution [159,160] and in the presence of clays [139]. However, their participation as major activated amino acid species to the prebiotic formation of peptides from amino acids is unlikely for at least two reasons. Firstly, amino acid adenylates that have a significant lifetime in aqueous solution become very unstable as soon as either CO2 or bicarbonate is present at millimolar concentration [137]. Lacey and coworkers [161] were therefore conduced to consider that CO2 was absent in the primitive atmosphere for aminoacyl adenylate to have a sufficient lifetime and then to allow for the emergence of the modern process of amino acid activation and of the translation apparatus. But this proposition is unlikely, as shown by the analysis of geological records in favor of CO2 contents in the atmosphere higher than present levels [128]. It is also in contradiction with most studies of the evolution of the atmosphere of telluric planets [30,32], Secondly, there is no prebiotic pathway available for adenylate formation and ATP proved to be inefficient in this reaction [162]. 

It seems therefore that the primitive atmosphere did change considerably in the first billion years, and from a highly reducing state, dominated by methane and ammonia, slowly turned into a slightly... 

If components of the primitive atmosphere to be integrated into ligands do not act as ligands at all, step 1 will be realized in any case conversely, however, substantial amounts of CO (E CL) = -fO.99 V) in that atmosphere would have impeded step 1 exactly since it is readily used in prebiotic reactions (e.g. Miller and... 

Studies of the reactions of many atmospherically important atomic and free radical species were described in Section 9 this Section deals primarily with important molecular species. A brief review of the progress achieved recently in the field of atmospheric chemistry has been provided by Cox, " with emphasis on the reactions of O3 and important H-, N-, C-, halogen-, and S-containing species. Waynehas reviewed extraterrestrial atmospheric photochemistry and Strobel " has reviewed the photochemistries of the atmospheres of Jupiter, Saturn, and Titan. Kaye and Strobeldescribed a 1-dimensional photochemical model of PHj chemistry in the atmosphere of Saturn. A study of the photochemical reactions of H2O and CO in the Earth s primitive atmosphere has been presented by Bar-Nun and Chang. " They concluded that even if the primitive atmosphere initially contained no H2 and contained carbon only in the form of CO and CO2, photochemical processes would have enriched the environment with a variety of organic compounds. 

The view is held that the origin of iife in the universe ca. 3.5 billion years ago is attributable to the formation of radicals amino acids and nucleotides may have been formed from simple reduced components of the primitive atmosphere, assuming free radicals were generated through solar radiation (D. Harmann, 1986). 

The concentration of sulfur vapor in the primitive atmosphere would have been limited only by its satnration vapor pressnre, which depends strongly on the temperature. As it has been shown above that the temperatnre was abont 85 °C, sulfur vapor could have been presented in sufficient concentrations to shield the Earth s surface from harmful solar UV radiation. Thus, a warm, C02-rich ancient atmosphere may have been favorable to early life. 

Walker, J. C. G., 1978 Oxygen and hydrogen in the primitive atmosphere. Pure and Appl. Geophvs. 116,222-231. 

In order to estimate the abundance of hydrogen in the primitive atmosphere, we assume steady-state conditions and set the escape flux equal to that resulting from thermal degassing ... 

Figure 12-2 shows the course of hydrogen partial pressure and mixing ratio in the primitive atmosphere during the first 3 billion yr after Earth s formation. The critical parameter is Kz. We have here adopted the value appropriate for present conditions at the homopause, Kz = 102 m2/s, which... 

Fig. 12-2. Partial pressure (in bar, solid line) and mixing ratio (dashed line) of hydrogen in the primitive atmosphere on Earth according to Eqs. (12-7) and (12-8). K2 = 102m2/s, H = 9 km, A = 3 x KT4. The eddy diffusion coefficient corresponds to present conditions at the homopause. If K2 in the primitive atmosphere had been smaller by a certain factor, the hydrogen partial pressure would have been greater by the same factor and vice versa.

Hydrogen is discussed here in some detail, because Miller and Urey (1959) suggested that its presence in the primitive atmosphere led to the formation of methane on account of the reaction... 

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