Oomo2 m

range, photochemical reactions [2.1] and [2.2] are "shadowed" by the end-product of the process. The equilibrium Oz concentration formed in this way is calculated to be 10"3 PAL(Berkner and Marshall, 1965 and 1967). Walker(1978) recently argued that this equilibrium value is even smaller if the recombination of the products of photolysis is also taken into account.

Thus it should be stated: in spite of the fact that some free oxygen could be formed in the pre-biologic atmosphere by the photodissociation of water vapour, its present level cannot be explained by this process alone.

Among the secondary gaseous constituents, C02 reacted with silicates in the solid Earth (see later), while an important part of the HzO condensed. However, the inert N2, which comprises only 1% of volcanic gases, accumulated in the atmosphere (Rasool and de Bergh, 1970) and became the major component of the gaseous envelope of our planet. It can be seen from Table 2 that the abundance of nitrogen in the Earth's atmosphere is approximately equal to that found in the gaseous covering of Venus. This fact can be explained by the similar size of the two planets since similar size suggests a similar volcanic activity in the past. If this explanation is correct the relatively smaller N2 abundance of the Martian atmosphere connotes lesser volcanic activity on this smaller planet.

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