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

It is widely anticipated that the James Webb Space Telescope (JWST) will be transformative for exoplanet studies. It has even been suggested that JWST could provide the first opportunity to search for biosignatures in an alien atmosphere using transmission spectroscopy. This claim is investigated, specifically for the proposed anoxic biosignature pair CH4–CO2. The most favourable known target is adopted (TRAPPIST-1e), with an assumed atmospheric composition similar to the Archean Earth. Compared to previous studies, a more systematic investigation of the effect that cloud/haze layers have on the detectability of CH4 and CO2 is performed. In addition to a clear atmosphere scenario, cloud/haze layers are considered at eight pressure levels between 600 and 1 mbar. These pressures cover a plausible range for H2O cloud and photochemical haze, based on observations of solar system atmospheres and physical models of tidally locked planets such as TRAPPIST-1e, although no assumptions regarding the cloud/haze-layer composition are made in this study. For the clear atmosphere and cloud/haze-layer pressures of 600–100 mbar, strong (5σ) detections of both CH4 and CO2 are found to be possible with approximately 5–10 co-added transits measured using the Near Infrared Spectrograph (NIRSpec) prism, assuming a dry stratosphere. However, approximately 30 co-added transits would be required to achieve the same result if a cloud/haze layer is present at 10 mbar. A cloud/haze layer at 1 mbar would prevent the detection of either molecule with the NIRSpec prism for observing programmes up to 50 transits (>200 h of JWST time), the maximum considered.