Exploring H₂O Prominence in Reflection Spectra of Cool Giant Planets

The H₂O abundance of a planetary atmosphere is a powerful indicator of formation conditions. Inferring H₂O in the solar system giant planets is challenging, due to condensation depleting the upper atmosphere of water vapor. Substantially warmer hot Jupiter exoplanets readily allow detections of H₂O via transmission spectroscopy, but such signatures are often diminished by the presence of clouds composed of other species. In contrast, highly scattering water clouds can brighten planets in reflected light, enhancing molecular signatures. Here, we present an extensive parameter space survey of the prominence of H₂O absorption features in reflection spectra of cool (T _eff < 400 K) giant exoplanetary atmospheres. The impact of effective temperature, gravity, metallicity, and sedimentation efficiency is explored. We find prominent H₂O features around 0.94 μm, 0.83 μm, and across a wide spectral region from 0.4 to 0.73 μm. The 0.94 μm feature is only detectable where high-altitude water clouds brighten the planet: T _eff ∼150 K, g ≳ 20 ms^-2, f _sed ≳ 3, m ≲ 10×solar. In contrast, planets with g ≲ 20 ms^-2 and T _eff ≳ 180 K display substantially prominent H₂O features embedded in the Rayleigh scattering slope from 0.4 to 0.73 μm over a wide parameter space. High f _sed enhances H₂O features around 0.94 μm, and enables these features to be detected at lower temperatures. High m results in dampened H₂O absorption features, due to water vapor condensing to form bright, optically thick clouds that dominate the continuum. We verify these trends via self-consistent modeling of the low-gravity exoplanet HD 192310c, revealing that its reflection spectrum is expected to be dominated by H₂O absorption from 0.4 to 0.73 μm for m ≲ 10×solar. Our results demonstrate that H₂O is manifestly detectable in reflected light spectra of cool giant planets only marginally warmer than Jupiter, providing an avenue to directly constrain the C/O and O/H ratios of a hitherto unexplored population of exoplanetary atmospheres.