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Methane (CH4) is a primarily biogenic greenhouse gas. As such, it representsan essential biosignature to search for life on exoplanets. Atmospheric CH4abundance on Earth-like inhabited exoplanets is likely controlled by marinebiogenic production and atmospheric photochemical consumption. Suchinteractions have been previously examined for the case of the early Earthwhere primitive marine ecosystems supplied CH4 to the atmosphere, showing thatthe atmospheric CH4 response to biogenic CH4 flux variations is nonlinear, acritical property when assessing CH4 reliability as a biosignature. However,the contributions of atmospheric photochemistry, metabolic reactions, or solarirradiance to this nonlinear response are not well understood. Using anatmospheric photochemical model and a marine microbial ecosystem model, we showthat production of hydroxyl radicals from water vapor photodissociation is acritical factor controlling the atmospheric CH4 abundance. Consequently,atmospheric CH4 partial pressure (pCH4) on inhabited Earth-like exoplanetsorbiting Sun-like stars (F-, G-, and K-type stars) would be controlledprimarily by stellar irradiance. Specifically, irradiance at wavelengths ofapproximately 200-210 nm is a major controlling factor for atmospheric pCH4when the carbon dioxide partial pressure is sufficiently high to absorb moststellar irradiance at 170-200 nm. Finally, we also demonstrated that inhabitedexoplanets orbiting near the outer edge of K-type stars' habitable zones arebetter suited for atmospheric pCH4 buildup. Such properties will valuablysupport future detection of life signatures.

Controls of Atmospheric Methane on Early Earth and Inhabited Earth-like Terrestrial Exoplanets