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Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H₂O Cross Sections

Ranjan, Sukrit and Schwieterman, Edward W. and Harman, Chester and Fateev, Alexander and Sousa-Silva, Clara and Seager, Sara and Hu, Renyu (2020) Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H₂O Cross Sections. Astrophysical Journal, 896 (2). Art. No. 148. ISSN 1538-4357.

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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. Photodissociation of H₂O 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Ranjan, Sukrit0000-0002-5147-9053
Schwieterman, Edward W.0000-0002-2949-2163
Harman, Chester0000-0003-2281-1990
Fateev, Alexander0000-0003-2863-2707
Sousa-Silva, Clara0000-0002-7853-6871
Seager, Sara0000-0002-6892-6948
Hu, Renyu0000-0003-2215-8485
Alternate Title:Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H2O Cross Sections
Additional Information:© 2020. The American Astronomical Society. Received 2020 March 4; revised 2020 May 12; accepted 2020 May 13; published 2020 June 23. We thank Iouli Gordon, Eamon Conway, Robert Hargreaves, Mike Wong, Kevin Zahnle, Timothy Lee, Shawn Domagal-Goldman, Mark Claire, David Catling, Nick Wogan, and Jim Kasting for helpful discussions and answers to questions. We thank an anonymous reviewer for critical feedback that improved this work. This work was supported in part by a grant from the Simons Foundation (SCOL grant 495062 to S.R.) and the Heising-Simons Foundation (51 Pegasi b Fellowship to C.S.S.). E.W.S. gratefully acknowledges support by NASA Exobiology grant 18-EXO18-0005 and NASA Astrobiology Program grants NNA15BB03A and 80NSSC18K0829. The work has received funding partly from the EMPIR program cofinanced by the Participating States and from the European Union's Horizon 2020 research and innovation program (Grant Number 16ENV08). This research has made use of NASA's Astrophysics Data System and the MPI-Mainz UV-VIS Spectral Atlas of Gaseous Molecules (Keller-Rudek et al. 2013).
Funding AgencyGrant Number
Simons Foundation495062
Heising-Simons Foundation51 Pegasi b Fellowship
European Research Council (ERC)16ENV08
Subject Keywords:Planetary theory ; Planetary atmospheres ; Exoplanet atmospheres ; Exoplanet atmospheric composition ; Extrasolar rocky planets ; Habitable planets ; Water vapor
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Planetary theory (1258); Planetary atmospheres (1244); Exoplanet atmospheres (487); Exoplanet atmospheric composition (2021); Extrasolar rocky planets (511); Habitable planets (695); Water vapor (1791)
Record Number:CaltechAUTHORS:20200625-075924553
Persistent URL:
Official Citation:Sukrit Ranjan et al 2020 ApJ 896 148
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:104032
Deposited By: George Porter
Deposited On:25 Jun 2020 16:13
Last Modified:25 Jun 2020 16:13

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