Atmospheric Chemistry of Venus-Like Exoplanets

Schaefer, Laura; Fegley, B.

doi:10.1088/0004-637x/729/1/6

Cited by 57 publications

(27 citation statements)

References 46 publications

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“…Disequilibrium by itself is not an unequivocal indicator of life, since it can also be caused by abiotic processes such as photochemistry or geothermally driven surface chemistry. In particular, photochemistry can produce substantial amounts of O 2 and O 3 , as found in the Earth's stratosphere as well as on Venus and Mars, and as can be expected in Venus-like exoplanets (Segura, 2007;Montessin et al, 2011;Schaefer and Fegley, 2011). These forms of chemical disequilibrium, however, result directly through the interaction of radiation and atmospheric chemistry and do not involve exchange fluxes between the surface and the atmosphere.…”

Section: Chemical Disequilibrium and Lifementioning

confidence: 92%

Quantifying drivers of chemical disequilibrium: theory and application to methane in the Earth's atmosphere

2013

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It has long been observed that Earth’s atmosphere is uniquely far from its thermochemical equilibrium state in terms of its chemical composition. Studying this state of disequilibrium is important both for understanding the role that life plays in the Earth system, and for its potential role in the detection of life on exoplanets. Here we present a methodology for assessing the strength of the biogeochemical cycling processes that drive disequilibrium in planetary atmospheres. We apply it to the simultaneous presence of CH4 and O2 in Earth’s atmosphere, which has long been suggested as a sign of life that could be detected remotely. Using a simplified model, we identify that the most important property to quantify is not the distance from equilibrium, but the power required to drive it. A weak driving force can maintain a high degree of disequilibrium if the residence times of the compounds involved are long; but if the disequilibrium is high and the kinetics fast, we can conclude that the disequilibrium must be driven by a substantial source of energy. Applying this to Earth’s atmosphere, we show that the biotically generated portion of the power required to maintain the methane– oxygen disequilibrium is around 0.67TW, although the uncertainty in this figure is about 10% due to uncertainty in the global CH4 production. Compared to the chemical energy generated by the biota by photosynthesis, 0.67TW represents only a very small fraction and, perhaps surprisingly, is of a comparable magnitude to abiotically driven geochemical processes at the Earth’s surface. We discuss the implications of this new approach, both in terms of enhancing our understanding of the Earth system, and in terms of its impact on the possible detection of distant photosynthetic biospheres

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Section: Chemical Disequilibrium and Lifementioning

confidence: 92%

Quantifying drivers of chemical disequilibrium: theory and application to methane in the Earth's atmosphere

2013

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“…is faster. Consequently, CO 2 on its own is unstable to conversion to CO and O 2 (Schaefer & Fegley 2011 On Mars, such OH-driven catalytic cycles stabilize the CO 2 atmosphere against conversion to CO and O 2 (McElroy & Donahue 1972;Parkinson & Hunten 1972;Krasnopolsky 2011). These catalytic cycles are diverse but unified in requiring OH to proceed (Harman et al 2018).…”

Section: Photochemistry Of Co 2 -Rich Atmospheresmentioning

confidence: 99%

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

Ranjan

Schwieterman

Harman

et al. 2020

ApJ

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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.

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“…Planets orbiting well interior (Abe et al, 2011;Zsom et al, 2013) or exterior (Pierrehumbert and Gaidos, 2011) to an Earth-like planet's habitable zone boundaries (Kopparapu et al, 2013) must be considered. The planet diversity also could well extend to the surface sources and sinks of gases, especially the redox state of the planetary surfacea wide variety of habitable planets have been hypothesized in this regard, from water worlds (Kuchner, 2003;Léger et al, 2004), to planets with hydrogen-rich atmospheres (Pierrehumbert and Gaidos, 2011;Seager et al, 2013a), to Venuslike worlds (Schaefer and Fegley, 2011) and planets with increased volcanism (Kaltenegger and Sasselov, 2010;Hu et al, 2013). The extreme and far UV radiation that drives atmospheric photochemistry will vary depending on host star type and age (Guinan et al, 2003;Shkolnik and Barman, 2014).…”

Section: A Brief Background To Biosignature Gasesmentioning

confidence: 99%

Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry

2016

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Thousands of exoplanets are known to orbit nearby stars. Plans for the next generation of space-based and ground-based telescopes are fueling the anticipation that a precious few habitable planets can be identified in the coming decade. Even more highly anticipated is the chance to find signs of life on these habitable planets by way of biosignature gases. But which gases should we search for? Although a few biosignature gases are prominent in Earth's atmospheric spectrum (O 2 , CH 4 , N 2 O), others have been considered as being produced at or able to accumulate to higher levels on exo-Earths (e.g., dimethyl sulfide and CH 3 Cl). Life on Earth produces thousands of different gases (although most in very small quantities). Some might be produced and/or accumulate in an exo-Earth atmosphere to high levels, depending on the exo-Earth ecology and surface and atmospheric chemistry.To maximize our chances of recognizing biosignature gases, we promote the concept that all stable and potentially volatile molecules should initially be considered as viable biosignature gases. We present a new approach to the subject of biosignature gases by systematically constructing lists of volatile molecules in different categories. An exhaustive list up to six non-H atoms is presented, totaling about 14,000 molecules. About 2500 of these are CNOPSH compounds. An approach for extending the list to larger molecules is described. We further show that about one-fourth of CNOPSH molecules (again, up to N = 6 non-H atoms) are known to be produced by life on Earth. The list can be used to study classes of chemicals that might be potential biosignature gases, considering their accumulation and possible false positives on exoplanets with atmospheres and surface environments different from Earth's. The list can also be used for terrestrial biochemistry applications, some examples of which are provided. We provide an online community usage database to serve as a registry for volatile molecules including biogenic compounds.

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Atmospheric Chemistry of Venus-Like Exoplanets

Cited by 57 publications

References 46 publications

Quantifying drivers of chemical disequilibrium: theory and application to methane in the Earth's atmosphere

Quantifying drivers of chemical disequilibrium: theory and application to methane in the Earth's atmosphere

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

Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry

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Atmospheric Chemistry of Venus-Like Exoplanets

Cited by 57 publications

References 46 publications

Quantifying drivers of chemical disequilibrium: theory and application to methane in the Earth's atmosphere

Quantifying drivers of chemical disequilibrium: theory and application to methane in the Earth's atmosphere

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

Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry

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