  Transit spectroscopy of terrestrial planets around nearby M dwarfs is a
primary goal of space missions in coming decades. 3-D climate modeling has
shown that slow-synchronous rotating terrestrial planets may develop thick
clouds at the substellar point, increasing the albedo. For M dwarfs with Teff >
3000 K, such planets at the inner habitable zone (IHZ) have been shown to
retain moist greenhouse conditions, with enhanced stratospheric water vapor
(fH2O > 1E-3) and low Earth-like surface temperatures. However, M dwarfs also
possess strong UV activity, which may effectively photolyze stratospheric H2O.
Prior modeling efforts have not included the impact of high stellar UV activity
on the H2O. Here, we employ a 1-D photochemical model with varied stellar UV,
to assess whether H2O destruction driven by high stellar UV would affect its
detectability in transmission spectroscopy. Temperature and water vapor
profiles are taken from published 3-D climate model simulations for an IHZ
Earth-sized planet around a 3300 K M dwarf with an N2-H2O atmosphere; they
serve as self-consistent input profiles for the 1-D model. We explore
additional chemical complexity within the 1-D model by introducing other
species into the atmosphere. We find that as long as the atmosphere is
well-mixed up to 1 mbar, UV activity appears to not impact detectability of H2O
in the transmission spectrum. The strongest H2O features occur in the JWST MIRI
instrument wavelength range and are comparable to the estimated systematic
noise floor of ~50 ppm.

Earth and Planetary Astrophysics