  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
600mbar and 1mbar. 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 600-100mbar, strong
(5-sigma) 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 10mbar. A cloud/haze-layer at 1mbar would
prevent the detection of either molecule with the NIRSpec prism for observing
programs up to 50 transits (>200 hours of JWST time), the maximum considered.
