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It is widely anticipated that the James Webb Space Telescope (JWST) will betransformative for exoplanet studies. It has even been suggested that JWSTcould provide the first opportunity to search for biosignatures in an alienatmosphere using transmission spectroscopy. This claim is investigated,specifically for the proposed anoxic biosignature pair CH4-CO2. The mostfavourable known target is adopted (TRAPPIST-1e), with an assumed atmosphericcomposition similar to the Archean Earth. Compared to previous studies, a moresystematic investigation of the effect that cloud/haze-layers have on thedetectability of CH4 and CO2 is performed. In addition to a clear atmospherescenario, cloud/haze-layers are considered at eight pressure levels between600mbar and 1mbar. These pressures cover a plausible range for H2O cloud andphotochemical haze, based on observations of solar system atmospheres andphysical models of tidally-locked planets such as TRAPPIST-1e, although noassumptions regarding the cloud/haze-layer composition are made in this study.For the clear atmosphere and cloud/haze-layer pressures of 600-100mbar, strong(5-sigma) detections of both CH4 and CO2 are found to be possible withapproximately 5-10 co-added transits measured using the Near InfraredSpectrograph (NIRSpec) prism, assuming a dry stratosphere. However,approximately 30 co-added transits would be required to achieve the same resultif a cloud/haze-layer is present at 10mbar. A cloud/haze-layer at 1mbar wouldprevent the detection of either molecule with the NIRSpec prism for observingprograms up to 50 transits (>200 hours of JWST time), the maximum considered.

Detecting the proposed CH4-CO2 biosignature pair with the James Webb Space Telescope: TRAPPIST-1e and the effect of cloud/haze