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SPECULATION ON THE RELATIONSHIP BETWEEN ORGANIC AND ATMOSPHERIC EVOLUTION DANIEL L. GILBERT, Ph.D.* In recent years there have been several publications concerning the origin oflife (1-9), and it has been pointed out that there are probably several planets capable ofpossessing life today (10). This paper explores the relationships between the evolution oforganic and living forms and the evolution of planetary atmospheres. It attempts to describe the atmospheric conditions on a planet during its "pre-life" stage, possible "life" stage, and "post-life" stage. TABLE 1 Per Cent Atom Abundance ElementCosmos*Organisms f Environment f H ............ 86.6862.62.3 He ........... 13.18¦ CO2.......... - 94 X+02—*- XO2 (v represents other substances, which can be oxidized by O2.) * AFo = a temperature of 298° K.; calculated from Latimer (16). TABLE 4 Chemical Stability in Presence of Oxygen r AFo (Kcal)* 2O2+ CH4-* CO2+2 H2O .......... -196 3 02+4 NH3->2 N2+f3 H2O .......... -324 Most stable carbon compound is CO2. Most stable nitrogen comund is ?2. * AFo = a temperature of 298° K. ; calculated from Latimer (16). ducing universe. Nitrogen would remain in the form of N2, since it is more stable than NH3 in the presence of O2 (Table 4). The end result of the dehydrogenation ofa planet would be: (a) absence ofall H20;(£j) formation of N2; and (c) formation of CO2 (C and CO) and other oxides (XO2). Since the atmosphere of Venus contains very large amounts of carbon dioxide (14, 17), perhaps it is in this later stage. Eventually, the planet would be expected to lose its CO2 and N2 and be practically devoid ofan atmosphere, such as appears to be the situation for Mercury (14, 17). Living organisms possess metastable substances which are intermediates in this overallprocess. Thus, the "pre-life" stage refers to the planet during its strongly reducing atmosphere ofH2, H2O, CH4, and NH3; the "post60 Daniel L. Gilbert · Relationship between Organic and Atmospheric Evolution Perspectives in Biology and Medicine ¦ Autumn i960 life" stage refers to the planet during its oxidized atmosphere ofCO2 and N2 (or after loss ofboth gases); and the possible "life" stage would represent the transition between these two extremes. Although organisms can alter the kinetics ofthese processes, they cannot change the final results. II. Organic Evolution on Earth Figure ? illustrates oxidation states of C in the presence of H2. CH4 is the most stable. However, it does not take too much energy to form the various compounds from CH4. During the phase in which CH4 is transformed into CO2, various metastable compounds ofC should be expected to exist, such as C2H6 and CeHi2O6. Figure 2 illustrates oxidation states of OXIDATION STATES OF CARBON IN PRESENCE OF HYDROGEN nH¡, LIBERATED WITH CO2 -40 -50 L1S WOC-COOH ""PS-CiTtJU|™u— -INCREASING STATE OF OXIDATION — 1/0 '."bW.) + *M,0-»CO, Hlfíí. S'ïalr*-».) Fig. i.—The decrease in free energy for the various compounds according to the given reaction is plotted on the ordinate. Unless otherwise indicated, these values were obtained from Latimer for a temperature of2980K. (16). Since reactions are thermodynarnically possible only when there is a decrease in free energy, the lower the compound is on this free-energy plot, the more thermodynarnically stable it is. Example, illustrating the use ofthis figure: The value of AF0 for HCHO is 6.54 Kcal, so the AFo for the following reaction is —6.54 Kcal—HCHO + H2O —> CO2 + 2 H2. Likewise, the value ofAFo for CH3OH is —4.00 Kcal, so the AF0 ofthe following reaction is 4.00 Kcal—CH3OH + H2O —» CO2 + 3 H2. The difference between the AFo for these compounds represents the decrease in AFa for the reaction represented by the difference between the above type of reactions. Thus the difference between the AF0 for HCHO and CH3OH is 10.54 Kcal (e.g., 6.54 Kcal minus —4.00 Kcal), and so the AF0 ofthe following reaction is — 10.54 Kcal—HCHO + H2 —> CH3OH. 6l N in the presence ofH2. Again it is noticed that NH3 is the most stable, but that N2 is almost as stable. Considering some sample nitrogen-carbon compounds, it is also noticed again that NH3 and CH4 are most...


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