Papers › Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H₂O Cross-Sections
Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H₂O Cross-Sections
Sukrit Ranjan, Edward W. Schwieterman, Chester Harman, Alexander Fateev, Clara Sousa-Silva, Sara Seager, Renyu Hu
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We present a study of the photochemistry of abiotic habitable planets with anoxic CO₂-N₂ atmospheres. Such worlds are representative of early Earth, Mars and Venus, and analogous exoplanets. H₂O photodissociation controls the atmospheric photochemistry of these worlds through production of reactive OH, which dominates the removal of atmospheric trace gases. The near-UV (NUV; >200 nm) absorption cross-sections of H₂O play an outsized role in OH production; these cross-sections were heretofore unmeasured at habitable temperatures (<373 K). We present the first measurements of NUV H₂O absorption at 292 K, and show it to absorb orders of magnitude more than previously assumed. To explore the implications of these new cross-sections, we employ a photochemical model; we first intercompare it with two others and resolve past literature disagreement. The enhanced OH production due to these higher cross-sections leads to efficient recombination of CO and O₂, suppressing both by orders of magnitude relative to past predictions and eliminating the low-outgassing "false positive" scenario for O₂ as a biosignature around solar-type stars. Enhanced [OH] increases rainout of reductants to the surface, relevant to prebiotic chemistry, and may also suppress CH₄ and H₂; the latter depends on whether burial of reductants is inhibited on the underlying planet, as is argued for abiotic worlds. While we focus on CO₂-rich worlds, our results are relevant to anoxic planets in general. Overall, our work advances the state-of-the-art of photochemical models by providing crucial new H₂O cross-sections and resolving past disagreement in the literature, and suggests that detection of spectrally active trace gases like CO in rocky exoplanet atmospheres may be more challenging than previously considered.
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