Recent Mars missions indicate the red planet's atmosphere contains methane gas.
Methane also exists here on Earth, where it's closely tracked as a greenhouse gas that's contributing to climate change. The nice folks at the EPA have a spiffy chart that tracks the sources and quantities of methane released into the atmosphere each year.
If we eliminate human activities from the list, it appears most methane is produced by something called "enteric fermentation in ruminant livestock." (Enteric fermentation is the politically correct definition of burps and farts.)
Mars has cows!