The early atmosphere would have been similar to the sun which consisting mainly hydrogen ( H2) and Helium (He) Because these were the main gases in the dusty, gassy, disc around the sun From which the planets are formed. However, this atmosphere lost quickly because of two reasons: -

1.  The earth gravity was not strong enough to prevent such light gases from escaping into space. I.e.; The earth and its atmosphere were very hot And the molecules of the hydrogen and helium move really fast Especially when they are warm and they move so fast that eventually they escape earth gravity and drifted it into space.

2.  It appeared that around 30 million years, after the earth formation. it was stuck by a large object ( As big as the size of Mars), This result the origin of moon and loose of earths early hydrogen and helium gases.

The earth’s second atmosphere came earth itself, The surface of the earth during this time Was extremely hot with numerous volcanoes.

This was a time when the earth was under near constant bombardment By many objects with varying sizes.

Slowly the earth started to cool down and second atmosphere begin to form. a new atmosphere was established by outgassing of volcanoes ( Which is also called as Degassing process).

The mixture of gases was released in degassing process like water vapor( 80%), hydrogen H2 (10%), Nitrogen ( Few%), and some amount of Ammonia (NH3).

** In the second atmosphere of earth virtually No oxygen present thus, No ozone layer was there. UV radiation flooded the earth’s surface.

Much of carbon dioxide(co2) Dissolve in the oceans and eventually a simple form of bacteria develop that could live on the energy from the sun. When they digest, they produce oxygen as a waste product. Thus, oxygen (02) Build up in the atmosphere, while the carbon dioxide CO2 level continue to drop. Meanwhile, the ammonia molecules were broken apart by sunlight leaving hydrogen and nitrogen in the atmosphere.

Now, the hydrogen being the lightest element goes to the top of the atmosphere and much of it eventually drifted into the space.

This is how earth’s third atmosphere came into existence.

Now, the main composition of atmosphere is like that: -

Nitrogen   ( N2) – 78%

Oxygen   (02) – 20%

Argon – 0.93%

Carbon dioxide  (co2) – 0.03%

     _ FINISH _

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