The Earth's original atmosphere, the primary atmosphere, consisted mostly of hydrogen and helium, but the Earth is too small a planet to hold such light gases and they floated away into space. The Earth then accumulated a secondary atmosphere, mostly by volcanic out-gassing. Volcanic activity was much greater on the hotter primitive Earth. Volcanic gas consists mostly of steam (95%) and variable amounts of CO2, N2, SO2, H2S, HCl, B2O3, elemental sulfur, and smaller quantities of H2, CHt, SO3, NH3 and HF but no O2. Of all these, the concentration of CO2 was in the second highest amount (about 4%). In addition, water vapor reacted with primeval minerals such as nitrides to give ammonia, with carbides to give methane and with sulfides to give hydrogen sulfide. There was no free oxygen.
Our present atmosphere, the tertiary atmosphere, is of biological origin. The methane, ammonia, and other reduced gases have been consumed and the inert components (nitrogen, traces of argon, xenon etc.) have remained largely unchanged. Substantial amounts of oxygen have been produced by photosynthesis. This could not
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