[Talisker]

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?

Excellent questions.

[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,