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Journal ArticleDOI

Reflections on O2 as a Biosignature in Exoplanetary Atmospheres.

01 Oct 2017-Astrobiology (Mary Ann Liebert, Inc. 140 Huguenot Street, 3rd Floor New Rochelle, NY 10801 USA)-Vol. 17, Iss: 10, pp 1022-1052
TL;DR: Environmental factors for abiotic O2 have been identified and will improve the ability to choose optimal targets and measurements to guard against false positives, and thorough evaluation of potential biosignatures works to increase confidence in life detection.
Abstract: Oxygenic photosynthesis is Earth's dominant metabolism, having evolved to harvest the largest expected energy source at the surface of most terrestrial habitable zone planets. Using CO2 and H2O—molecules that are expected to be abundant and widespread on habitable terrestrial planets—oxygenic photosynthesis is plausible as a significant planetary process with a global impact. Photosynthetic O2 has long been considered particularly robust as a sign of life on a habitable exoplanet, due to the lack of known “false positives”—geological or photochemical processes that could also produce large quantities of stable O2. O2 has other advantages as a biosignature, including its high abundance and uniform distribution throughout the atmospheric column and its distinct, strong absorption in the visible and near-infrared. However, recent modeling work has shown that false positives for abundant oxygen or ozone could be produced by abiotic mechanisms, including photochemistry and atmospheric escape. Environm...
Citations
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Journal Article
TL;DR: In this paper, it is shown that Earth's climate has remained conducive to life for the past 3.5 billion years or more, despite a large increase in solar luminosity, probably because of previous higher concentrations of CO 2 and/or CH4.
Abstract: ■ Abstract Giant planets have now been discovered around other stars, and it is only a matter of time until Earth-sized planets are detected. Whether any of these planets are suitable for life depends on their volatile abundances, especially water, and on their climates. Only planets within the liquid-water habitable zone (HZ) can support life on their surfaces and, thus, can be analyzed remotely to determine whether they are inhabited. Fortunately, current models predict that HZs are relatively wide around main-sequence stars not too different from our sun. This conclusion is based on studies of how our own planet has evolved over time. Earth’s climate has remained conducive to life for the past 3.5 billion years or more, despite a large increase in solar luminosity, probably because of previous higher concentrations of CO 2 and/or CH4. Both these gases are involved in negative feedback loops that help to stabilize the climate. In addition to these topics, we also briefly discuss the rise of atmospheric O 2 and O3, along with their possible significance as indicators of life on other planets.

370 citations

Journal ArticleDOI
TL;DR: The coevolution of life with the early Earth's environment is examined to identify how the interplay of sources and sinks may have suppressed O2 release into the atmosphere for several billion years, producing a false negative for biologically generated O2.
Abstract: We describe how environmental context can help determine whether oxygen (O2) detected in extrasolar planetary observations is more likely to have a biological source. Here we provide an in...

241 citations


Cites background from "Reflections on O2 as a Biosignature..."

  • ...However, if these catalysts are not present, strong CO features in the presence of abundant O2 and CO2 could signal a false positive scenario (Schwieterman et al., 2016; Meadows, 2017; bottom panels of Figs....

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  • ...However, several plausible O2 false positive scenarios for planets in the habitable zone have since been identified (e.g., Wordsworth and Pierrehumbert, 2014; Luger and Barnes, 2015; Tian, 2015; Harman et al., 2015; see Meadows, 2017 for a more detailed review)....

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  • ...This increases its detectability even in the presence of clouds, or when using techniques such as transmission spectroscopy that do not probe deep into the planetary atmosphere (Meadows, 2017)....

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  • ...This cartoon summarizes the atmospheric mechanisms by which O2 could form abiotically at high abundance in a planetary atmosphere (Meadows, 2017)....

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  • ...…Reinhard et al., 2017), and false positives—planetary processes that generate large (modern Earth-like) abundances of O2 abiotically (e.g., Wordsworth and Pierrehumbert, 2014; Luger and Barnes, 2015; Gao et al., 2015; Harman et al., 2015; Tian, 2015; see Meadows, 2017 for a review), being possible....

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Journal ArticleDOI
TL;DR: In this article, the impact of active M dwarf stars on the atmospheric equilibrium and surface conditions of a habitable zone Earth-like planet is investigated, which is key to assessing M dwarf planet habitability.
Abstract: Understanding the impact of active M dwarf stars on the atmospheric equilibrium and surface conditions of a habitable zone Earth-like planet is key to assessing M dwarf planet habitability...

142 citations

Journal ArticleDOI
TL;DR: In this paper, a 1D terrestrial-planet climate model with line-by-line radiative transfer and mixing length convection (VPL Climate) coupled with a terrestrial photochemistry model was used to simulate environmental states for the TRAPPIST-1 planets.
Abstract: The TRAPPIST-1 planetary system provides an unprecedented opportunity to study terrestrial exoplanet evolution with the James Webb Space Telescope (JWST) and ground-based observatories. Since M dwarf planets likely experience extreme volatile loss, the TRAPPIST-1 planets may have highly-evolved, possibly uninhabitable atmospheres. We used a versatile, 1D terrestrial-planet climate model with line-by-line radiative transfer and mixing length convection (VPL Climate) coupled to a terrestrial photochemistry model to simulate environmental states for the TRAPPIST-1 planets. We present equilibrium climates with self-consistent atmospheric compositions, and observational discriminants of post-runaway, desiccated, 10-100 bar O2- and CO2-dominated atmospheres, including interior outgassing, as well as for water-rich compositions. Our simulations show a range of surface temperatures, most of which are not habitable, although an aqua-planet TRAPPIST-1 e could maintain a temperate surface given Earth-like geological outgassing and CO2. We find that a desiccated TRAPPIST-1 h may produce habitable surface temperatures beyond the maximum greenhouse distance. Potential observational discriminants for these atmospheres in transmission and emission spectra are influenced by photochemical processes and aerosol formation, and include collision-induced oxygen absorption (O2-O2), and O3, CO, SO2, H2O, and CH4 absorption features, with transit signals of up to 200 ppm. Our simulated transmission spectra are consistent with K2, HST, and Spitzer observations of the TRAPPIST-1 planets. For several terrestrial atmospheric compositions, we find that TRAPPIST-1 b is unlikely to produce aerosols. These results can inform JWST observation planning and data interpretation for the TRAPPIST-1 system and other M dwarf terrestrial planets.

137 citations

01 Dec 2011
TL;DR: In this article, a combination of Fe and Mo isotope systematics of Ca-Mg carbonates and shales from the 2.68 to 2.50 Ga Campbellrand-Malmani carbonate platform of the Kaapvaal Craton in South Africa was used to constrain free O2 levels in the photic zone of a Late Archean marine basin by the combined use of Fe-Mo isotope systems.
Abstract: Most geochemical proxies and models of atmospheric evolution suggest that the amount of free O2 in Earth’s atmosphere stayed below 10−5 present atmospheric level (PAL) until the Great Oxidation Event (GOE) that occurred between ∼2.2 and 2.4 Ga, at which time free O2 in the atmosphere increased to approximately 10−1 to 10−2 PAL. Although photosynthetically produced “O2 oases” have been proposed for the photic zone of the oceans prior to the GOE, it has been difficult to constrain absolute O2 concentrations and fluxes in such paleoenvironments. Here we constrain free O2 levels in the photic zone of a Late Archean marine basin by the combined use of Fe and Mo isotope systematics of Ca–Mg carbonates and shales from the 2.68 to 2.50 Ga Campbellrand–Malmani carbonate platform of the Kaapvaal Craton in South Africa. Correlated Fe and Mo isotope compositions require a key role for Fe oxide precipitation via oxidation of aqueous Fe(II) by photosynthetically-derived O2, followed by sorption of aqueous Mo to the newly formed Fe oxides. A dispersion/reaction model illustrates the effects of Fe oxide production and Mo sorption to Fe oxides, and suggests that a few to a few tens of μM free O2 was available in the photic zone of the Late Archean marine basin, consistent with some previous estimates. The coupling of Fe and Mo isotope systematics provides a unique view into the processes that occurred in the ancient shallow ocean after production of free O2 began, but prior to oxygenation of the deep ocean, or significant accumulation of free O2 in the atmosphere. These results require oxygenic photosynthesis to have evolved by at least 2.7 Ga and suggest that the Neoarchean ocean may have had a different oxygenation history than that of the atmosphere. The data also suggest that the extensive iron formation deposition that occurred during this time was unlikely to have been produced by anoxygenic photosynthetic Fe(II) oxidation. Finally, these data indicate that the ocean had significant amounts of O2 at least 150 Myr prior to previously proposed “whiffs” of O2 at the Archean to Proterozoic transition.

121 citations

References
More filters
Journal ArticleDOI
01 Jan 1993-Icarus
TL;DR: The results suggest that mid-to-early K stars should be considered along with G stars as optimal candidates in the search for extraterrestrial life.

2,438 citations


"Reflections on O2 as a Biosignature..." refers background in this paper

  • ...…environmental resources required to fuel and drive photosynthesis are likely common on a terrestrial planet in the surface liquid water habitable zone—that region around a star in which an Earth-like planet could support liquid water on its surface (Kasting et al., 1993; Kopparapu et al., 2013)....

    [...]

  • ...In particular, the environmental resources required to fuel and drive photosynthesis are likely common on a terrestrial planet in the surface liquid water habitable zone—that region around a star in which an Earth-like planet could support liquid water on its surface (Kasting et al., 1993; Kopparapu et al., 2013)....

    [...]

Journal ArticleDOI
20 Feb 2014-Nature
TL;DR: The initial increase of O2 in the atmosphere, its delayed build-up in the ocean, its increase to near-modern levels in the sea and air two billion years later, and its cause-and-effect relationship with life are among the most compelling stories in Earth’s history.
Abstract: The rapid increase of carbon dioxide concentration in Earth’s modern atmosphere is a matter of major concern. But for the atmosphere of roughly two-and-half billion years ago, interest centres on a different gas: free oxygen (O2) spawned by early biological production. The initial increase of O2 in the atmosphere, its delayed build-up in the ocean, its increase to near-modern levels in the sea and air two billion years later, and its cause-and-effect relationship with life are among the most compelling stories in Earth’s history.

1,821 citations


"Reflections on O2 as a Biosignature..." refers background in this paper

  • ...…global accumulation of O2 in the atmosphere—likely mediated by burial and removal of organic carbon from Earth’s surface environment (Kasting, 2001; Lyons et al., 2014)— evidently occurred somewhat later, between 2.45 and 2.2 Ga (Farquhar et al., 2000; Bekker et al., 2004; Canfield, 2005), with…...

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  • ...This may have been the case on our planet, where the evolution of oxygenic photosynthesis may have predated the rise of atmospheric oxygen by hundreds of millions to billions of years (Catling et al., 2001; Catling and Kasting, 2007; Lyons et al., 2014; Planavsky et al., 2014a)....

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  • ...Geological evidence for oxidative weathering of continents suggests that O2 may have temporarily risen to appreciable levels in our atmosphere as far back as 2.5 Ga (Anbar et al., 2007; Kaufman et al., 2007; Buick, 2008; Lyons et al., 2014)....

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  • ...They argued that this is the lower bound for a biologically produced O2 concentration, although estimates of the Proterozoic O2 abundance vary widely (e.g., Kump, 2008; Lyons et al., 2014)....

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  • ...Yet the irreversible, global accumulation of O2 in the atmosphere—likely mediated by burial and removal of organic carbon from Earth’s surface environment (Kasting, 2001; Lyons et al., 2014)— evidently occurred somewhat later, between 2....

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Journal ArticleDOI
04 Aug 2000-Science
TL;DR: Mass-independent isotopic signatures in Precambrian rocks indicate that a change occurred in the sulfur cycle between 2090 and 2450 million years ago, implying that atmospheric oxygen partial pressures were low and that the roles of oxidative weathering and of microbial oxidation and reduction of sulfur were minimal.
Abstract: Mass-independent isotopic signatures for delta(33)S, delta(34)S, and delta(36)S from sulfide and sulfate in Precambrian rocks indicate that a change occurred in the sulfur cycle between 2090 and 2450 million years ago (Ma). Before 2450 Ma, the cycle was influenced by gas-phase atmospheric reactions. These atmospheric reactions also played a role in determining the oxidation state of sulfur, implying that atmospheric oxygen partial pressures were low and that the roles of oxidative weathering and of microbial oxidation and reduction of sulfur were minimal. Atmospheric fractionation processes should be considered in the use of sulfur isotopes to study the onset and consequences of microbial fractionation processes in Earth's early history.

1,556 citations


"Reflections on O2 as a Biosignature..." refers background in this paper

  • ...2 Ga (Farquhar et al., 2000; Bekker et al., 2004; Canfield, 2005), with recent sulfur isotope studies (Luo et al....

    [...]

  • ...…of organic carbon from Earth’s surface environment (Kasting, 2001; Lyons et al., 2014)— evidently occurred somewhat later, between 2.45 and 2.2 Ga (Farquhar et al., 2000; Bekker et al., 2004; Canfield, 2005), with recent sulfur isotope studies (Luo et al., 2016) placing it at 2.33 Ga. Recent…...

    [...]

Journal ArticleDOI
TL;DR: In this article, an updated 1D radiative-convective, cloud-free climate model is used to obtain new estimates for HZ widths around F, G, K, and M stars.
Abstract: Identifying terrestrial planets in the habitable zones (HZs) of other stars is one of the primary goals of ongoing radial velocity (RV) and transit exoplanet surveys and proposed future space missions. Most current estimates of the boundaries of the HZ are based on one-dimensional (1D), cloud-free, climate model calculations by Kasting et?al. However, this model used band models that were based on older HITRAN and HITEMP line-by-line databases. The inner edge of the HZ in the Kasting et?al. model was determined by loss of water, and the outer edge was determined by the maximum greenhouse provided by a CO2 atmosphere. A conservative estimate for the width of the HZ from this model in our solar system is 0.95-1.67?AU. Here an updated 1D radiative-convective, cloud-free climate model is used to obtain new estimates for HZ widths around F, G, K, and M stars. New H2O and CO2 absorption coefficients, derived from the HITRAN 2008 and HITEMP 2010 line-by-line databases, are important improvements to the climate model. According to the new model, the water-loss (inner HZ) and maximum greenhouse (outer HZ) limits for our solar system are at 0.99 and 1.70?AU, respectively, suggesting that the present Earth lies near the inner edge. Additional calculations are performed for stars with effective temperatures between 2600 and 7200?K, and the results are presented in parametric form, making them easy to apply to actual stars. The new model indicates that, near the inner edge of the HZ, there is no clear distinction between runaway greenhouse and water-loss limits for stars with T eff 5000?K, which has implications for ongoing planet searches around K and M stars. To assess the potential habitability of extrasolar terrestrial planets, we propose using stellar flux incident on a planet rather than equilibrium temperature. This removes the dependence on planetary (Bond) albedo, which varies depending on the host star's spectral type. We suggest that conservative estimates of the HZ (water-loss and maximum greenhouse limits) should be used for current RV surveys and Kepler mission to obtain a lower limit on ??, so that future flagship missions like TPF-C and Darwin are not undersized. Our model does not include the radiative effects of clouds; thus, the actual HZ boundaries may extend further in both directions than the estimates just given.

1,526 citations


"Reflections on O2 as a Biosignature..." refers background in this paper

  • ...…environmental resources required to fuel and drive photosynthesis are likely common on a terrestrial planet in the surface liquid water habitable zone—that region around a star in which an Earth-like planet could support liquid water on its surface (Kasting et al., 1993; Kopparapu et al., 2013)....

    [...]

  • ...In particular, the environmental resources required to fuel and drive photosynthesis are likely common on a terrestrial planet in the surface liquid water habitable zone—that region around a star in which an Earth-like planet could support liquid water on its surface (Kasting et al., 1993; Kopparapu et al., 2013)....

    [...]

  • ...Such a planet will be outside or very close to the conservative habitable zone boundaries for its mass and atmospheric characteristics (Kopparapu et al., 2013, 2014) and, if showing the abundant O2 of a recent runaway or a hot Neptune, will also exhibit strong stratospheric H2O, but no O3 in either…...

    [...]

Journal ArticleDOI
23 Feb 2017-Nature
TL;DR: The observations reveal that at least seven planets with sizes and masses similar to those of Earth revolve around TRAPPIST-1, and the six inner planets form a near-resonant chain, such that their orbital periods are near-ratios of small integers.
Abstract: One aim of modern astronomy is to detect temperate, Earth-like exoplanets that are well suited for atmospheric characterization. Recently, three Earth-sized planets were detected that transit (that is, pass in front of) a star with a mass just eight per cent that of the Sun, located 12 parsecs away. The transiting configuration of these planets, combined with the Jupiter-like size of their host star—named TRAPPIST-1—makes possible in-depth studies of their atmospheric properties with present-day and future astronomical facilities. Here we report the results of a photometric monitoring campaign of that star from the ground and space. Our observations reveal that at least seven planets with sizes and masses similar to those of Earth revolve around TRAPPIST-1. The six inner planets form a near-resonant chain, such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.1 and 12.35 days) are near-ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inwards. Moreover, the seven planets have equilibrium temperatures low enough to make possible the presence of liquid water on their surfaces.

1,476 citations

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