Primordial atmospheres

Some radioisotopes are continuously being produced by the bombardment of atoms on the surface of the earth or in its atmosphere with extraterrestrial particles or radiation. One of these is carbon-14, also known as radiocarbon, which is widely used for dating archaeological materials (see Textbox 55). Many radioisotopes that are not primordial or are not created by natural processes are now produced artificially using specialized equipment many of the "artificial" isotopes are of use for probing and analyzing materials. 

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). 

The prebiotic primeval soup, i.e., the oft-cited mixture of organic molecules in the primordial ocean, or in ponds which could have arisen in many ways, e.g., in the atmosphere or the hydrosphere the substances concerned could also have been delivered from outer space. 

Changing the initial conditions in the Urey-Miller experiment to favour a primordial atmosphere with CO2 as the primary carbon source does not produce such a rich mixture of prebiotic molecules and there is a significantly lower yield of amino acids an argument against an endogenous source of organic molecules. 

Considering the content of earth s contemporary waters and atmospheres, many questions arise as to the choice of essential elements at the time of life s origins 3.5 billion or more years ago. Certainly, sufficient quantities of the bulk elements were available in primordial oceans and at shorelines. However, the concentrations of essential trace metals in modern oceans may differ considerably from those found in prebiotic times. Iron s current approximate 10 mM... 

In addition to the transition phenomena mentioned so far in the present section, a variety of even larger scale processes might have operated during chemical evolution, namely, instabilities and bifurcations in the very atmospheric environment within which life emerged. As shown in the paper by Marcel Nicolet, the earth s atmosphere is the theater of a variety of complex chemical and transport phenomena. Moreover, as explained by Stanley L. Miller, the composition of the primordial atmosphere has certainly affected deeply the chemistry in the primitive oceans. Conversely, once life emerged the properties of the atmosphere changed radically, and this must have affected the further course of evolution. We refer to Prather et al.41 and North et al.42 for an account of present views on large scale transitions in the earth-atmosphere system. 

A large number of successful experimental studies which tried to work out plausible chemical scenarios for the origin of life have been conducted in the past (Mason, 1991). A sketch of a possible sequence of events in prebiotic evolution is shown in Figure 3. Most of the building blocks of present day biomolecules are available from different prebiotic sources, from extraterrestrial origins as well as from processes taking place in the primordial atmosphere or near hot vents in deep oceans. Condensation reactions and polymerization reactions formed non-in-structed polymers, for example random oligopeptides of the protenoid type (Fox... 

Since radiogenic He has a 3He/4He ratio less than the air value, the only serious candidate for an alternative to primordial 3He is production by decay of tritium (3H). Cosmic ray interactions in the atmosphere are a well-known source of 3He (Section 5.5), and some of this is channeled through tritium, which will enter surface water... 

---