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

<jats:title>Abstract</jats:title> <jats:p>Methane (CH<jats:sub>4</jats:sub>) is a primarily biogenic greenhouse gas. As such, it represents an essential biosignature to search for life on exoplanets. Atmospheric CH<jats:sub>4</jats:sub> abundance on Earth-like inhabited exoplanets is likely controlled by marine biogenic production and atmospheric photochemical consumption. Such interactions have been previously examined for the case of the early Earth where primitive marine ecosystems supplied CH<jats:sub>4</jats:sub> to the atmosphere, showing that the atmospheric CH<jats:sub>4</jats:sub> response to biogenic CH<jats:sub>4</jats:sub> flux variations is nonlinear, a critical property when assessing CH<jats:sub>4</jats:sub> reliability as a biosignature. However, the contributions of atmospheric photochemistry, metabolic reactions, or solar irradiance to this nonlinear response are not well understood. Using an atmospheric photochemical model and a marine microbial ecosystem model, we show that the production of hydroxyl radicals from water vapor photodissociation is a critical factor controlling the atmospheric CH<jats:sub>4</jats:sub> abundance. Consequently, atmospheric CH<jats:sub>4</jats:sub> partial pressure (<jats:italic>p</jats:italic>CH<jats:sub>4</jats:sub>) on inhabited Earth-like exoplanets orbiting Sun-like stars (F-, G-, and K-type stars) would be controlled primarily by stellar irradiance. Specifically, irradiance at wavelengths of approximately 200–210 nm is a major controlling factor for atmospheric <jats:italic>p</jats:italic>CH<jats:sub>4</jats:sub> when the carbon dioxide partial pressure is sufficiently high to absorb most stellar irradiance at 170–200 nm. Finally, we also demonstrated that inhabited exoplanets orbiting near the outer edge of K-type stars’ habitable zones are better suited for atmospheric <jats:italic>p</jats:italic>CH<jats:sub>4</jats:sub> buildup. Such properties will provide valuable support for future detection of life signatures.</jats:p>