  Detection of water vapor in the atmosphere of temperate rocky exoplanets
would be a major milestone on the path towards characterization of exoplanet
habitability. Past modeling work has shown that cloud formation may prevent the
detection of water vapor on Earth-like planets with surface oceans using the
James Webb Space Telescope (JWST). Here we analyze the potential for
atmospheric detection of H2O on a different class of targets: arid planets.
Using transit spectrum simulations, we show that atmospheric H2O may be easier
to be detected on arid planets with cold-trapped ice deposits on the surface,
because such planets will not possess thick H2O cloud decks that limit the
transit depth of spectral features. However, additional factors such as band
overlap with CO2 and other gases, extinction by mineral dust, overlap of
stellar and planetary H2O lines, and the ultimate noise floor obtainable by
JWST still pose important challenges. For this reason, combination of space-
and ground-based spectroscopic observations will be essential for reliable
detection of H2O on rocky exoplanets in the future.
