[alexander_busek]

O2 would be heavier than CH4. So (1) what keeps it in the upper atmosphere rather than the lower; (2) why wouldn’t the methane burn at the first spark?

1) Nothing "kept" the oxygen there - possibly, it was being continually formed and destroyed, i.e., released and then recombined in chemical compounds with other elements - think "ozone layer" (O3 is 50% heavier than O2, and yet the ozone layer is full of it.)

2) To burn, there would have had to have been sufficient oxygen present in the same layer as the methane; and the overall density would have had to have been sufficient (i.e.: a perfect mixture of oxygen and methane will still not combust if it is too thin).

Regards,

[HiTech RedNeck]

O3 is formed out of O2. What would you have O2 formed out of? Windborne rootless plants?

[Talisker]

1. Could also just be a less dense layer of oxygen, floating over the methane. Molecular weight assumes individual interactions, charged layers could act uniformly.

2. At the boundary of oxygen and methane you could get ignition - a thin layer of fire in the sky all around the world. What a sight that would be!