When the presence of methane was reported in 2004 by three teams of scientists, the findings generated surprise and skepticism because only a few explanations seemed to be plausible.
One was geothermal chemical reactions involving water and heat in volcanoes or underground hot springs. But evidence for recent volcanism on Mars is scarce. Also, volcanoes would be expected to spew other gases like sulfur dioxide, and those are not plentiful in the planet’s atmosphere.
A second possibility is biological. On Earth, a class of bacteria known as methanogens breathes out methane as a waste product.