Atmosphere gas composition before life origin in earth?

Answer 1

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Initially, Earth may have had a thin atmosphere of hydrogen and helium. (These are the two most abundant elements in the universe and would have been very abundant in the nebula from which our solar system formed.) However, this atmosphere (if it ever existed) would have been lost early on as Earth does not have sufficient gravity to hold onto these elements in the heat of our location in the solar system.

The next atmosphere would have been created by the release of gases for Earth's interior. We can reasonably assume that these gases would have had about the same composition as the gases that still come out of volcanoes today. That being the cast, this atmosphere would have been 70% water vapor (H2O), 15% carbon dioxide (CO2), 5% nitrogen (N2), and 5% sulfur dioxide (SO2) with the remaining 5 percent being chlorine (Cl2), fluorine (F2), and other trace gasses.

As the Earth cooled and the atmosphere got thicker, eventually the water vapor would have condensed to form clouds and it would have started to rain. This would have eventually removed the vast majority of the water vapor from the air and created the oceans. This would have left us with an atmosphere that was mostly CO2 and N2.

CO2 dissolves fairly easily in water creating a weak acid (carbonic acid) that reacts fairly well with a number of minerals. The net result of this reaction was that most of the CO2 was removed from the atmosphere and stored away as carbonate rocks. (No, it was not photosynthetic life that was primarily responsible for removing CO2 from the atmosphere.) While this was going on, photosynthetic life was also evolving and began to use CO2, H2O, and sunlight to produce carbohydrates with O2 being released as a byproduct. So, these two processes working together removed almost all of the CO2 in the atmosphere while leaving the N2 behind and releasing O2. This, in a nutshell, eventually got us to the atmosphere that we have today.