How was the Earth's atmosphere formed?

What was the source of the gases that formed earth's original atmosphere?

• Answer:

  The gases were accreted along with all of the other constituents of the earth. material segregation occurs once the pressure-temperature-composition conditions are changed within the confines of the earth system (as the earth grows, a pressure and temperature gradient establishes itself within the interior), with volatiles (gas-forming substances) tending to migrate toward the lower densities found at and near the surface. The basic concept is that the atmosphere is simply a natural outcome of the physico-chemical segregation of materials from a more homogenous initial earth. the earth simply degasses its volatiles over time. this appears to still be occurring. If one adds to this idea that accretion is not a punctual event, but instead a long process, it leads to the idea that part of the atmosphere likely also comes from later arrival of less rocky (more volatile-enriched) materials in the later stages of accretion. Presumably the more volatile fractions are concentrated more towards the cooler periphery of the accretion disk and arrive later (take longer to get pulled into the proto-earth). From this comes the proposition that much of the water on earth derives from late cometary impacts. I am not convinced that such late impacts are required or sufficient to explain the evolution of the atmosphere and hydrosphere.