Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H₂O 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. Photodissociation of H 2 O 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.

The 2020 release of the ExoMol database: Molecular line lists for exoplanet and other hot atmospheres

“…The new VUV section will provide temperature-dependent absorption cross sections. Recent results suggest the VUV absorption by H 2 O enhances the production of OH, which plays an important role in the atmospheric chemistry of exoplanetary atmospheres [254] . Currently, the VUV cross sections are provided for H 2 O, H 2 , CO 2 , SO 2 , NH 3 , H 2 CO and C 2 H 4 measured by Fateev's lab at the Danish Technical University (DTU) [255] and for CO 2 by Venot et al [256] .…”

Detectability of biosignatures on LHS 1140 b

“…In the current paper, we additionally introduced some minor updates compared to the photochemical model described in Wunderlich et al (2020). Recently, the water (H 2 O) cross section between 186-230 nm has been measured by Ranjan et al (2020) for a temperature of 292 K. We use this new cross section data in the current study. However, the weak NUV flux of M dwarfs suggests that the water photolysis is not affected significantly by the usage of the new measurements (see e.g.…”

Timescales for Prebiotic Photochemistry Under Realistic Surface Ultraviolet Conditions

“…For this work, we assume a 0.3 bar atmosphere composed of 33% N 2 , 33% CO, and 33% CO 2 with water at vapor pressure, the same as the >3.5 Ga atmosphere Rimmer et al ( 2019 ) used to explain the observed micrometeorite oxidation from the work of Tomkins et al ( 2016 ), and consistent with other early Earth model atmospheres invoked to explain the same data (Lehmer et al , 2020 ; Payne et al , 2020 ). We solve for the surface UV flux using the model of Rimmer & Helling ( 2016 ) and Rimmer and Rugheimer ( 2019 ) with the UV absorption and scattering cross sections from PHIDRATES (Huebner et al , 1992 ; Huebner and Mukherjee, 2015 ) for most species, with updated cross sections for CO 2 (Ityaksov et al , 2008 ), HCHO (Chen and Zhu, 2001 ), H 2 O (Ranjan et al , 2020 ), and CH 4 (Burkholder et al , 2020 ). Most of these cross sections were obtained from the Mainz database (Keller-Rudek et al , 2013 ).…”