Primeval atmosphere

This primeval atmosphere was subjected to various energy sources, such as electrical discharge, solar radiation and heat from volcanoes these led to the formation of small organic molecules. 

The chemical composition of the Earth s interior determined the character (the oxidation state) of the primeval atmosphere. If metallic iron had collected in the Earth s core in the early phase of the accretion, the exhalations from the interior of the Earth would have consisted mainly of CO2 and H20, since the gas from the interior could only have come into contact with FeO and Fe203 silicates in the mantle. If, however, metallic iron had been distributed throughout the mantle, the iron and the FeO silicates would have had a reductive influence on the gases the gas exhaled into the atmosphere would then have consisted of CH4, H2 and NH3 (Whittet, 1997). 

The chemical composition of the primeval atmosphere is a central point of argument in the debate on the formation of life. This short remark made by M. Gaffey (1997) from Rensselaer Polytechnic Institute, Troy, New York, hit the nail on the head, and nothing has changed since ... 

Fig. 2.6 The main components of a typical weakly reducing primeval atmosphere as a function of the altitude above the Earth s surface. The mole fraction refers to the mixing ratio of the atmospheric mixture at an assumed surface pressure of one atmosphere. After Kasting (1993)... 

If the primeval atmosphere did not contain enough CO2 to maintain a greenhouse climate, the much lower solar irradiation at that time would have led to frozen oceans. But that would make almost all the assumed synthetic mechanisms for the formation of biomolecules impossible Bada et al. (1994) consider external help as a way out of this dilemma. They assume that the energy from meteor impacts (diameters up to around 100 km), converted into heat, would have sufficed to melt the oceanic ice. If such a process were to have occurred periodically, chemical evolution reactions (see Chap. 4) could have taken place in the ice-free periods and have led finally to biogenesis. 

As already mentioned, hydrogen cyanide is formed in simulation experiments using reducing primeval atmospheres. CN was discovered in interstellar space as early as 1940 by optical spectroscopy (Breuer, 1974), and later HCN itself (from measurements using millimetre wavelengths). Only a few years after the Miller-Urey experiments, Kotake et al. (1956) obtained HCN in good yields by reacting methane with ammonia over aluminium-silicate contacts ... 

The wavelengths at which most of the components of a primitive Earth atmosphere absorb lie, with few exceptions, under 200 nm. The exceptions include ammonia (< 230 nm), hydrogen sulphide (<260 nm) and ozone (180-300 nm). However, ozone was probably present in the primeval atmosphere only in trace amounts, since free oxygen was only available in extremely low concentrations. The young Earth thus had no protective ozone layer, so short-wavelength UV irradiation could readily penetrate the atmosphere. 

However, the question must always be asked as to whether these processes could have taken place on the primordial Earth in its archaic state. The answer requires considerable fundamental consideration. Strictly speaking, most of the experiments carried out on prebiotic chemistry cannot be carried out under prebiotic conditions , since we do not know exactly what these were. In spite of the large amount of work done, physical parameters such as temperature, composition and pressure of the primeval atmosphere, extent and results of asteroid impacts, the nature of the Earth s surface, the state of the primeval ocean etc. have not so far been established or even extrapolated. It is not even sure that this will be possible in the future. In spite of these difficulties, attempts are being made to define and study the synthetic possibilities, on the basis of the assumed scenario on the primeval Earth. Thus, for example, in the case of the SPREAD process, we can assume that the surface at which the reactions occur could not have been an SH-containing thiosepharose, but a mineral structure of similar activity which could have carried out the necessary functions just as well. The separation of the copy of the matrix could have been driven by a periodic temperature change (e.g., diurnal variation). For his models, H. Kuhn has assumed that similar periodic processes are the driving force for some prebiotic reactions (see Sect. 8.3). 

Were the first biomolecules formed in the primeval atmosphere or At hydrothermal vents in the depths of the primeval oceans or On the surface of the young Earth, at clay mineral surfaces or Via thioesters ... 

Then transformation of these intermediates in presence of carbonate, nitrite would let step 1 go on, ending up with the recent metallome, unlike with the RNA-world or under a primeval atmosphere containing substantial CO (Miller and Schlesinger 1984 Kasting 1993) which would break the sequence of... 

The formation of life on Earth is explained by the special characteristics of this planet. Among others, one can mention the distance between the Sun and the Earth,1 the dimension of the Earth and also, in relation with the parameters mentioned, the composition of the primeval atmosphere. After its formation the biosphere became an active partner in the control of environmental conditions that is, linkage was formed between the biosphere and the other media of the Earth. 

Today s atmosphere is the product of thousands of millions of years of evolution it has changed almost completely since the primeval atmosphere first formed. The evolution of the atmosphere is, of course, continuing. Although the rates of change... 

Although Miller has shown that the passage of an electric discharge through mixtures of methane, ammonia and water vapour (possible constituents of a primeval atmosphere), results in the production of some a-amino acids, these always occur as racemic mixtures of the l and d forms. In biological systems, both structural and functional proteins are built almost exclusively from L-amino acids. It is clearly easier to construct a well-ordered macromolecule from residues having the same stereochemistry, but the original selection of one-handed residue over the other is as yet unexplained. 

---