Cows need water.
Methane is highest where water is.
Cows produce methane.
There are cows on Mars.
QED
...While I was much interested in Dejah Thoris' explanation of this wonderful adjunct to Martian warfare, I was more concerned by the immediate problem of their treatment of her. That they were keeping her away from me was not a matter for surprise, but that they should subject her to dangerous and arduous labor filled me with rage.
"Have they ever subjected you to cruelty and ignominy, Dejah Thoris?" I asked, feeling the hot blood of my fighting ancestors leap in my veins as I awaited her reply.
"Only in little ways, John Carter," she answered. "Nothing that can harm me outside my pride. They know that I am the daughter of ten thousand jeddaks, that I trace my ancestry straight back without a break to the builder of the first great waterway, and they, who do not even know their own mothers, are jealous of me. At heart they hate their horrid fates, and so wreak their poor spite on me who stand for everything they have not, and for all they most crave and never can attain. Let us pity them, my chieftain, for even though we die at their hands we can afford them pity, since we are greater than they and they know it."
Had I known the significance of those words "my chieftain," as applied by a red Martian woman to a man, I should have had the surprise of my life, but I did not know at that time, nor for many months thereafter. Yes, I still had much to learn upon Barsoom.
[A Princess of Mars, Edgar Rice Burroughs, Chapter 13 - Love-Making on Mars]