  A number of transiting, potentially habitable Earth-sized exoplanets have
recently been detected around several nearby M dwarf stars. These worlds
represent important targets for atmospheric characterization for the upcoming
NASA James Webb Space Telescope. Given that available time for exoplanet
characterization will be limited, it is critically important to first
understand the capabilities and limitations of JWST when attempting to detect
atmospheric constituents for potentially Earth-like worlds orbiting cool stars.
Here, we explore coupled climate-chemistry atmospheric models for Earth-like
planets orbiting a grid of M dwarf hosts. Using a newly-developed and validated
JWST instrument model - the JWST Exoplanet Transit Simulator (JETS) - we
investigate the detectability of key biosignature and habitability indicator
gaseous species for a variety of relevant instruments and observing modes.
Spectrally-resolved detection scenarios as well as cases where the spectral
impact of a given species is integrated across the entire range of an
instrument/mode are considered and serve to highlight the importance of
considering information gained over an entire observable spectral range. When
considering the entire spectral coverage of an instrument/mode, detections of
methane, carbon dioxide, oxygen and water at signal-to-noise ratio 5 could be
achieved with observations of several tens of transits (or less) for cloud-free
Earth-like worlds orbiting mid- to late-type M dwarfs at system distances of up
to 10-15 pc. When compared to previous results, requisite exposure times for
gas species detection depend on approaches to quantifying the spectral impact
of the species as well as underlying photochemical model assumptions. Thus,
constraints on atmospheric abundances, even if just upper limits, by JWST have
the potential to further our understanding of terrestrial atmospheric
chemistry.
