Formation Of The Earth And Atmosphere

The evolution of the atmosphere begins 4-5 billion years ago with the formation of the Earth, but there is good evidence that the present atmosphere is not directly related to the Earth's original atmosphere by a continuous line of evolution.

As the Earth formed from the stellar nebula from which the Solar System was created, its rocky core acquired an atmosphere by gravitationally attracting nebula gases. This was the Earth's first, or primary, atmosphere. However, the hydrogen and helium thus trapped are likely to have escaped into space as the young Sun sprang to life, so these lightweight, primordial-nebula gases are not found in today's atmosphere. However, heavy, chemically-inert primordial gases such as neon and argon can be found. Being heavy, these gases would not have been lost to space, and their inertness would prevent them being removed from the atmosphere by chemical reactions. Therefore, their presence in the contemporary atmosphere would signal a direct line of descent from that primordial nebula to the present day atmosphere.

However, isotopic analysis shows that the neon and argon in today's atmosphere are predominantly the result of nuclear decay, rather than the remnants of that early atmosphere. It seems that at some stage soon after formation, the Earth lost its primordial atmosphere of nebula gases, and acquired a secondary atmosphere, from which the present atmosphere evolved.

The young Earth was a violent place, with radioactive decay, heavy meteor bombardment, and frictional and gravitational forces, all acting together to heat up the rocky mass of the planet. Much of the rock was molten. Under these conditions, gases were released from the planet's hot rocky core. These gases, predominantly nitrogen, CO2, and water, had either been physically trapped within the solid Earth as it formed, or else were released by thermal decomposition of rocks and minerals. These are the gases that formed Earth's secondary atmosphere, the starting point for evolution towards the contemporary atmosphere.

In this earliest of epochs of geological history, the atmospheric CO2 probably reacted again to form carbonate rocks, and the water eventually condensed to form the oceans. This left an atmosphere consisting predominantly of nitrogen. Trace amounts of free oxygen would be able to form in this atmosphere, as sunlight split (photolysed) molecules of water and CO2 in the air. However, photolysis cannot result in particularly high concentrations of oxygen; a limit exists because the oxygen produced by pho tolysis absorbs light at the same frequency that CO2 and water photolyse. As the concentration of oxygen builds up, it blocks the light needed to further increase its concentration. Without photosynthesis (that is, without a biological process for oxygen production), there is a natural limit on the amount of oxygen in the atmosphere. The exact concentration depends on CO2 and water concentrations, and sunlight intensity, but no more than one-billionth of the present atmospheric level of oxygen existed before the emergence of life.

This limit to the pre-biological free oxygen concentration turns out to be critically important to the evolution of life, and, hence, to the evolution of the atmosphere. Had the atmosphere been appreciably oxidizing (containing significant levels of free oxygen), it would not have been possible for life's chemical precursors to accumulate on the planet's surface. The complex and fragile molecules of life, slowly forming and beginning to organize into proto-living systems in stagnant ponds and lakes, would have rapidly oxidized, and thus, destroyed, had the pre-biotic atmosphere been too oxidizing.

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