It can clearly be seen from Table 1 that the Earth's atmosphere consists mainly of nitrogen and oxygen. However, this composition, which makes the present life on our planet possible, cannot be considered "normal" in the solar system. Thus, even the nearest planets like Mars and Venus have a very different atmospheric composition.
The results of space research, especially the results obtained by the Soviet Venera and the American Mariner space-crafts, make it possible to estimate numerically these differences in the composition. Table 2 contains data on the global composition of the atmosphere of Venus, Earth and Mars according to Lovelock and Margulis (1974). The values given in the table are expressed in millibars partial pressure of the gas considered. It follows from the table that the pressure on the surface of Venus is high while it is low in the case of the Martian atmosphere. However, on both planets the atmosphere consists essentially of carbon dioxide, and oxygen is either missing or its pressure is very low. On the other hand, the Earth's atmosphere is composed of nitrogen and oxygen and the quantity of C02 is virtually negligible. In the fourth column of the table are values obtained by interpolation on the basis of the astronomical data of the Earth (e.g. distance from the Sun) and of the composition of the atmospheres of Venus and Mars. This interpolated atmosphere is called the estimated equilibrium atmosphere (Lovelock and Margulis, 1974). One can see that the real C02 partial pressure is a thousand times smaller while the real Oz pressure is nearly a thousand times greater than these values obtained by interpolation between Mars and Venus.
Another peculiarity of the Earth's atmosphere is the relatively high nitrogen pressure. Considering the physicochemical conditions on our planet, this fact is contradictory to chemical equilibrium considerations. Thus under our conditions (temperature, oxygen pressure, pH in ocean waters etc.) the stable state of nitrogen would be in nitrate compounds dissolved in ocean waters (Sillen, 1966).
It follows from this discussion that our atmosphere has many peculiar characteristics, on the one hand relative to the Venus and Mars or, on the other hand, considering it separately in the Earth-atmosphere system. The question therefore arises: how did this anomalous gas cover of the Earth form and what
Composition or atmosphere of Venus, Earth and Mars according to Lovelock and Margulis (1974)
Gas Venus Earth Mars (equi- .'
COj 90000 0.3 5 300 1.0x 10 3 N2 1 000 780 0.05 30 2.6x 10 02 0 210 0.1 0.3 7.0 xlO2
Note: The estimated equilibrium composition of the Earth's atmosphere is ako given. Values are the partial pressures in mb mechanism controls this strange gas mixture? Briefly, the biosphere played an important part in the formation and evolution of the atmosphere of our planet, and it is a major determining factor in the control of the present composition.
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.
Because of this active role, some authors (e.g. Lovelock and Margulis, 1974) refer directly to the homeostasis of the atmosphere. This homeostasis (that is, the capacity for control) was established and is regulated by the biosphere. Furthermore the atmosphere can be regarded as a contrivance in which each gas has its own function, it is considered a part of the biosphere.
The aim of this chapter is to discuss in some detail the formation and evolution of this anomalous atmosphere. The factors which governed the atmospheric composition during the formation of Earth and during geologic time will be presented briefly. Attention will be devoted to the formation of the present oxygen level and, generally speaking, to problems related to the interactions between the atmosphere and the biosphere.
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