The Impact of Extended H₂O Cross-Sections on Temperate Anoxic Planet Atmospheres: Implications for Spectral Characterization of Habitable Worlds

JWST has created a new era of terrestrial exoplanet atmospheric characterization, and with it the possibility to detect potential biosignature gases like CH₄. Our interpretation of exoplanet atmospheric spectra, and the veracity of these interpretations, will be limited by our understanding of atmospheric processes and the accuracy of input modeling data. Molecular cross-sections are essential inputs to these models. The photochemistry of temperate planets depends on photolysis reactions whose rates are governed by the dissociation cross-sections of key molecules. H₂O is one such molecule; the photolysis of H₂O produces OH, a highly reactive and efficient sink for atmospheric trace gases. We investigate the photochemical effects of improved H₂O cross-sections on anoxic terrestrial planets as a function of host star spectral type (FGKM) and CH₄ surface flux. Our results show that updated H₂O cross-sections, extended to wavelengths $>$200 nm, substantially impact the predicted abundances of trace gases destroyed by OH. The differences for anoxic terrestrial planets orbiting Sun-like host stars are greatest, showing changes of up to three orders of magnitude in surface CO levels, and over an order of magnitude in surface CH₄ levels. These differences lead to observable changes in simulated planetary spectra, especially important in the context of future direct-imaging missions. In contrast, the atmospheres of planets orbiting M-dwarf stars are substantially less affected. Our results demonstrate a pressing need for refined dissociation cross-section data for H₂O, where uncertainties remain, and other key molecules, especially at mid-UV wavelengths $>$200 nm.