[DoctorBeaver (OP)]

These planets that have liquid methane on them and violent storms in the atmosphere - why doesn't the lightning set the methane on fire? After all, methane is flammable, is it not?

[another_someone]

Methane it may have, but where is the oxidiser?

Very few chemicals will burn on their own (the few that do usually explode), they usually burn in reaction with something else.  On Earth, methane tends to burn in air (keep methane away from air, as you do in the depths of a gas well), and it is perfectly stable for millions of years.

[DoctorBeaver (OP)]

Ah, that explains it. Thank you.

[lyner]

I always have a problem with the process of 'inversion' which is supposed to have taken place a long time ago. The Earth's atmosphere was once Methane rich and it is now Oxygen rich. Over a long period, during the change,there must have been a mixture. Only one spark would have set it all off again. Why didn't this happen?

[DoctorBeaver (OP)]

I always have a problem with the process of 'inversion' which is supposed to have taken place a long time ago. The Earth's atmosphere was once Methane rich and it is now Oxygen rich. Over a long period, during the change,there must have been a mixture. Only one spark would have set it all off again. Why didn't this happen?

That's a damned good point

Could there be a way that nitrogen, or maybe even helium, came into it? I'm no chemist by any stretch of the imagination, so that's just wild speculation.

[another_someone]

I always have a problem with the process of 'inversion' which is supposed to have taken place a long time ago. The Earth's atmosphere was once Methane rich and it is now Oxygen rich. Over a long period, during the change,there must have been a mixture. Only one spark would have set it all off again. Why didn't this happen?

I doubt there was a time when the Earth had a balanced mix.

I would imagine that initially there was a lot of methane, and a little oxygen.  Not enough oxygen to create a big explosion, but still enough to oxidise a small amount of methane.  As more oxygen was created, it was consumed almost as fast as it was created, so never building up to dangerous levels, while at the same time methane was continually being used up, creating water and CO₂.  It would only be when almost all of the methane would have been consumed that the excess O₂ would start to build up in the atmosphere.

So, you would have had a two stage process.

The first stage would create a supply of O₂, but that would merely consume the methane, producing CO₂ and H₂O, and no free O₂.

Only the second stage would cease to create more CO₂ and H₂O, because all of the CH₄ had been consumed, and only then do you actually start building up free O₂ in the atmosphere.

[lyner]

Good explanation; I didn't think of a two-step solution.

[Soul Surfer]

It is generally thought that most of the oxygen in the atmosphere was locked up in carbon dioxide

[lyner]

What mechanism is it by which the enormous amounts of methane are found all over the Universe? Presumably it was formed before living organisms(?).

[Soul Surfer]

Lots of hydrogen quite a fair bit of finely divided carbon.  electromagnetic radiation gas giant planets, ligntning and lots of time  will form methane  because there's not all that much oxygen around to oxidise it into carbon dioxide  and the oxygen tends to form water with the hydrogen.

[lyner]

Yes, of course, there was always plenty of time!