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

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

References in corpus (21)

• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth
• Was Venus the First Habitable World of our Solar System?
• The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST
• Abiotic oxygen-dominated atmospheres on terrestrial habitable zone planets
• Abiotic Ozone and Oxygen in Atmospheres Similar to Prebiotic Earth
• Carbon monoxide and water vapor in the atmosphere of the non-transiting exoplanet HD 179949 b
• Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life
• HAZMAT I: The Evolution of Far-UV and Near-UV Emission from Early M Stars
• The Habitable Exoplanet Observatory (HabEx) Mission Concept Study Final Report
• UV Surface Environment of Earth-like Planets Orbiting FGKM Stars Through Geological Evolution
• Validation of Twelve Small Kepler Transiting Planets in the Habitable Zone
• Hydrogen Cyanide in Nitrogen-Rich Atmospheres of Rocky Exoplanets
• The LUVOIR Mission Concept Study Final Report
• Constraints on the Early Terrestrial Surface UV Environment Relevant to Prebiotic Chemistry
• Nitrogen Oxide Concentrations in Natural Waters on Early Earth
• Testing the Early Mars H2-CO2 Greenhouse Hypothesis with a 1-D Photochemical Model
• Origins Space Telescope Mission Concept Study Report
• O2- and CO-Rich Atmospheres for Potentially Habitable Environments on TRAPPIST-1 Planets
• Rethinking CO Antibiosignatures in the Search for Life Beyond the Solar System
• Stability of Nitrogen in Planetary Atmospheres in Contact with Liquid Water

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