Secondary constituents

After the dissipation of cosmic gases (approximately 4 x 109 years ago) several gaseous materials were liberated from the solid Earth. The substances formed in this way are termed secondary constituents. These atmospheric components were due either to volcanic, desorption and thermal processes or to chemical reactions.

At an early age the iron in the upper part of the mantle was in a reduced state (as evidenced by very old iron minerals; see Junge, 1966). The presence of reduced iron means that the gases and vapours given off by the solid mantle were also reduced; otherwise they would have oxidized the iron. Probably, the secondary atmosphere consisted mainly of methane, ammonia and water vapour.2

With the decrease of the abundance of hydrogen, the composition of the volatile materials approached that of the present time. This means that they were in a more oxidized state than was the case earlier—COz, N2, H20. The temperature at the Earth's surface was around -10 to -15°C (Rasool and De Bergh, 1970), considering the Sun-Earth distance and the surface albedo. With the increase in the quantity of secondary constituents, the atmosphere also became a controlling factor of the temperature, since C02 and water vapour absorb the infrared radiation emitted by the Earth's surface to a significant degree. Hence the temperature began to rise. When its value reached 0 °C and the vapour pressure of water reached 6.1 mb condensation processes took place leading to the accumulation of ocean waters.

At this time in the Earth's history the carbon dioxide abundance was higher than its present value since this gas accumulated in the absence of a biosphere. Rutten (1966) speculated that the atmospheric C02 level was 10 times PAL (present atmospheric level) about 3 x 109 years ago. On the other hand the presence of HzO in the atmosphere led, by photochemical dissociation, to the formation of free radicals and molecular oxygen. An estimate of the importance of these reactions is necessary to give some idea of the oxygen level in pre-biospheric times. The photodissociation of water vapour can be represented as follows (Suess, 1966):

Two atomic oxygens formed in this way can combine to yield molecular oxygen:

where M is a neutral third body. It was demonstrated by Nicolet and Bates (1950) that the photolysis of water is induced by solar radiation with wavelengths shorter than 0,195 )xm. Since oxygen strongly absorbs the short-wave radiation in the same

2 The laboratory experiments of Miller (1953) proved Oparin's hypothesis, according to which the organic compounds necessary for the formation of the life were produced in a reducing atmosphere.

0 0

Post a comment