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The focus of my question pertains to the natural chemical and biological mechanisms which evolved the atmosphere on Earth as we have known it up to modern times (excluding human activity), and how it might inform expectations around the occurrence of other "Earth-like" planets yet to be discovered.

Is the quantity of nitrogen stable? If so, where did it come from and why isn't it continuing to increase? Why is it molecular N2 and not, for example, ammonia as we see on other planets or large moons in our solar system? I suppose this question could be extended to include Venus - why it has its supply of nitrogen (and why it is several times that of Earth). One theory on the emergence of life on Earth states that amino acids formed from a "chemical soup" which included ammonia, which can only be found today in trace amounts.

What about the quantity of oxygen? Oxygen is one of the most abundant elements on Earth, most of it in the rocks of the crust and mantle. Again, excluding any effects of human activity, and aside from the obvious effects of life, what affects the quantity of oxygen in the atmosphere that brought it to and/or maintains it at modern levels?

Why are the partial pressures of these principle gases at the values as we currently know them and not some other values? Excluding the effects of human activity, is Earth's atmosphere continuing to evolve? If so, in what way, and if not, is it because of some sort of equilibrium, and if that is so, what are the processes involved?

Hypothetically: if we found another rocky planet of approximately Earth mass/surface gravity orbiting a host star similar to our sun at a distance that would provide about the same amount of heat, would it be likely to have a similar amount of atmospheric nitrogen as Earth, and, again hypothetically, if life got started there, would it arrive at a similar proportion of atmospheric oxygen in the atmosphere? If so, why, and if not, why not?

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  • $\begingroup$ Oxygen was released from carbon dioxide by plants doing photosynthesis. But some oxygen was bound by oxidizing dissolved iron into iron ore, see Oxygen Revolution $\endgroup$ – Uwe Nov 5 '17 at 16:48
  • $\begingroup$ Ammonia is a weak base and reacts with acids to form salts. Ammonia is found in small quantities in rainwater, it reacts with the carbon dioxide dissolved in the rain. If the soil is acidic, ammonia reacts with the acids. Ammonia is used by plants. $\endgroup$ – Uwe Nov 5 '17 at 17:34
  • $\begingroup$ @uwe, I'm looking beyond photosynthesis and "the great rust" to the question of how the totals we have came to be part of Earth in the first place, and why they are free O2 and N2 in the atmosphere and not chemically bound in some other form. $\endgroup$ – Anthony X Nov 5 '17 at 17:36
  • $\begingroup$ But there is free oxygen in air due to photosynthesis. There is free nitrogen because it does react only at high temperature or pressure. $\endgroup$ – Uwe Nov 5 '17 at 17:59
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    $\begingroup$ That's quite a number of questions, for most of them we don't have definitive answers. But some cross-referencing to astro might help: astronomy.stackexchange.com/questions/18740/… and references therein. Answering your question would include to go from stellar nucleosynthesis, to planet formation, to chemical evolution and atmospheric escape, so it is VERY broad. That's why I prefer linking a reference. $\endgroup$ – AtmosphericPrisonEscape Nov 6 '17 at 17:20

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