Methane is therefore the most active greenhouse gas in planets in the outer zone?
I seem to recall there's a measurable greenhouse effect on Pluto, but I'd have to go back through the papers to remember what causes it.Edit:
Yes, caused by methane and carbon monoxide.
Something is producing methane on Titan and Mars.
Exactly what is a subject for controversy on both. In Titan's case, it's possible that the current abundance of methane is a recent and relatively short-lived (~ millions of years, rather than 10's or 100's of millions) phenomenon.