Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H$_2$O Cross-Sections

Sukrit Ranjan; Edward W. Schwieterman; Chester Harman; Alexander Fateev; Clara Sousa-Silva; Sara Seager; Renyu Hu

doi:10.3847/1538-4357/ab9363

paper

Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H $_{2}$ O Cross-Sections

arXiv:2004.04185 · doi:10.3847/1538-4357/ab9363

Abstract

We present a study of the photochemistry of abiotic habitable planets with anoxic CO $_{2}$ -N $_{2}$ atmospheres. Such worlds are representative of early Earth, Mars and Venus, and analogous exoplanets. H $_{2}$ O 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 H $_{2}$ O 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 H $_{2}$ O 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 O $_{2}$ , suppressing both by orders of magnitude relative to past predictions and eliminating the low-outgassing "false positive" scenario for O $_{2}$ as a biosignature around solar-type stars. Enhanced [OH] increases rainout of reductants to the surface, relevant to prebiotic chemistry, and may also suppress CH $_{4}$ and H $_{2}$ ; the latter depends on whether burial of reductants is inhibited on the underlying planet, as is argued for abiotic worlds. While we focus on CO $_{2}$ -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 H $_{2}$ O 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.

Manuscript (this version) accepted to ApJ. Cross-section data available at https://github.com/sukritranjan/ranjanschwietermanharman2020. Feedback continues to be solicited

Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H $_{2}$ O Cross-Sections

Abstract

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Cited by in corpus (33)

Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H2​O Cross-Sections

Abstract

References in corpus (21)

Cited by in corpus (33)

Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H $_{2}$ O Cross-Sections