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Why is nitrogen the dominant gas in our atmosphere? What happened to the other gases from the early atmosphere?
Answer 1:

The early atmosphere was probably dominated by CO2 and H2O. This is the VERY EARLY atmosphere, when conditions at the surface exceeded the boiling point of water!

This early steam atmosphere condensed to form the oceans by about 4400 million years ago (the earth started forming about 4566 Million years ago). At that time the dominant gases were CO2and probably N2. Then, because CO2 is soluble in water and because erosion brings Ca ions into the early oceans, the Ca++ and the
CO3-- got together to form calcite CaCO3. Calcium carbonate (chalk!!) is very insoluble in water so by this reaction CO2 was slowly removed from the atmosphere and this left the N2 present.

Then about 2 billion years ago (2000 million years ago) free diatomic oxygen started to build up because of photosynthesis. If the nitrate salts were not so soluble in water we would have vast deposits of nitrate salts (like NaNO3) analogous to the vast deposits of chalk (calcium carbonate) locked up in the crust of the earth.

Your question is a good one and one that not too many geologists have worried about. Note that on planets with no oceans like Venus and Mars, the atmosphere is dominated by CO2 because there was no mechanism to remove (scrub) the CO2 from the atmosphere to the crust (i.e. fix the carbonate in a solid). However, N2 is present in the atmosphere of both Venus and Mars. Venus and Mars have about 2 to 3% N2 in their atmosphere.

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