[DannyTN]

Thanks for the explanation.

So where does methane in comets come from?

If volcanism generates methane, that implies, that the mantle has methane. Wouldn't it be logical to assume that there is a slow seepage of methane from the mantle up through the ground?

[SedVictaCatoni]

Here, you might find this article useful on the topic.

[Liberal Classic]

Comets are composed of frozen water, ammonia, and methane, left over from the formation of the solar system. They are created through natural chemical reactions. Other such chemicals that occur naturally include carbon dioxide, sulfur dioxide, nitrous oxide, (laughing gas) potassium chloride, sodium chloride (salt), acetylene, and formaldehyde, methanol. I once read about a nebula which was comprised of 200 proof ethanol alcohol!