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 vapour. Substantially warmer hot Jupiter exoplanets readily allow detections of H2O via transmission spectroscopy, but such signatures are often diminished by the presence of clouds made of other species. In contrast, highly scattering H2O 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 (Teff ~20m/s^2, fsed>~3, m ~180K display prominent H2O features embedded in the Rayleigh slope from 0.4-0.73um. High fsed enhances H2O features around 0.94um, and enables them to be detected at lower temperatures. High m results in dampened H2O absorption features, due to H2O vapour condensing to form bright optically thick clouds that dominate the continuum. We verify these trends via self-consistent modelling of the low gravity exoplanet HD 192310c, revealing that its reflection spectrum is expected to be dominated by H2O absorption from 0.4-0.73um for m<~10xsolar. 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.

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