  Discoveries of terrestrial, Earth-sized exoplanets that lie within the
habitable zone (HZ) of their host stars continue to occur at increasing rates.
Transit spectroscopy can potentially enable the detection of molecular
signatures from such worlds, providing an indication of the presence of an
atmosphere and its chemical composition, including gases potentially indicative
of a biosphere. Such planets around nearby M-dwarf stars - such as TRAPPIST-1 -
provide relatively good signal, high signal/noise, and frequent transits for
follow-up spectroscopy. However, even with these advantages, transit
spectroscopy of terrestrial planets in the HZ of nearby M-stars will still be a
challenge. Herein, we examine the potential for future space observatories to
conduct such observations, using a Global Climate Model (GCM), a photochemical
model, and a radiative transfer suite to simulate modern-Earth-like atmospheric
boundary conditions on TRAPPIST-1e. The detectability of biosignatures on such
an atmosphere via transmission spectroscopy is modeled for JWST, LUVOIR, HabEx,
and Origins. We show that for any of these observatories, only CO2 would be
detectable at the 3 sigma level in transmission spectroscopy, when clouds are
included in our simulations. This is because the impacts of clouds on scale
height strongly limits the detectability of molecules in the atmosphere.
Synergies between space- and ground-based spectroscopy may be essential in
order to overcome these difficulties.
