The Impact of Extended H_2O Cross-Sections on Temperate Anoxic Planet Atmospheres: Implications for Spectral Characterization of Habitable Worlds
arxiv(2024)
摘要
JWST has created a new era of terrestrial exoplanet atmospheric
characterization, and with it the possibility to detect potential biosignature
gases like CH_4. 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. H_2O is one such
molecule; the photolysis of H_2O produces OH, a highly reactive and
efficient sink for atmospheric trace gases. We investigate the photochemical
effects of improved H_2O cross-sections on anoxic terrestrial planets as a
function of host star spectral type (FGKM) and CH_4 surface flux. Our
results show that updated H_2O 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
CH_4 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 H_2O, where uncertainties
remain, and other key molecules, especially at mid-UV wavelengths >200 nm.
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