MARTIAN SURFACE. “Radiolysis-powered ecosystems can use these chemicals for metabolic processes. The possibility of methane, N2 and traces of O2 being by-products of such an ecosystem cannot be ruled out and could possibly explain the presence of methane that cannot yet be explained by standard physics and chemistry models.”
https://royalsocietypublishing.org/doi/10.1098/rsif.2016.0459