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Review
. 2018 Jun;18(6):630-662.
doi: 10.1089/ast.2017.1727. Epub 2018 May 10.

Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment

Victoria S Meadows  1   2 , Christopher T Reinhard  3   4 , Giada N Arney  2   5 , Mary N Parenteau  2   6 , Edward W Schwieterman  2   4   7   8   9 , Shawn D Domagal-Goldman  2   10 , Andrew P Lincowski  1   2 , Karl R Stapelfeldt  11 , Heike Rauer  12 , Shiladitya DasSarma  13   14 , Siddharth Hegde  15   16 , Norio Narita  17   18   19 , Russell Deitrick  1   2 , Jacob Lustig-Yaeger  1   2 , Timothy W Lyons  4   7 , Nicholas Siegler  11 , J Lee Grenfell  12
Affiliations
Review

Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment

Victoria S Meadows et al. Astrobiology. 2018 Jun.

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-depth, interdisciplinary example of O2 biosignature identification and observation, which serves as the prototype for the development of a general framework for biosignature assessment. Photosynthetically generated O2 is a potentially strong biosignature, and at high abundance, it was originally thought to be an unambiguous indicator for life. However, as a biosignature, O2 faces two major challenges: (1) it was only present at high abundance for a relatively short period of Earth's history and (2) we now know of several potential planetary mechanisms that can generate abundant O2 without life being present. Consequently, our ability to interpret both the presence and absence of O2 in an exoplanetary spectrum relies on understanding the environmental context. Here we examine the coevolution of life with the early Earth's environment 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. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. We review the most recent knowledge of false positives for O2, planetary processes that may generate abundant atmospheric O2 without a biosphere. We provide examples of how future photometric, spectroscopic, and time-dependent observations of O2 and other aspects of the planetary environment can be used to rule out false positives and thereby increase our confidence that any observed O2 is indeed a biosignature. These insights will guide and inform the development of future exoplanet characterization missions. Key Words: Biosignatures-Oxygenic photosynthesis-Exoplanets-Planetary atmospheres. Astrobiology 18, 630-662.

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Conflict of interest statement

No competing financial interests exist.

Figures

<b>FIG. 1.</b>
FIG. 1.
Overview of the major processes controlling atmospheric O2 and CH4 levels (left panels). Red circles/arrows denote net biogenic sources (e.g., after accounting for biological consumption), gray ovals/arrows denote potential abiotic sources, and blue squares/arrows denote net sink fluxes. Schematic depiction of the evolution of atmospheric O2 and CH4 throughout Earth's history (right panels). Using results from Reinhard et al. (2017), shaded boxes show ranges based on geochemical proxy or model reconstructions, whereas red curves show possible temporal trajectories through time. (Credit: Chris Reinhard)
<b>FIG. 2.</b>
FIG. 2.
Potential false positive mechanisms for O2. This cartoon summarizes the atmospheric mechanisms by which O2 could form abiotically at high abundance in a planetary atmosphere (Meadows, 2017). The extreme left panel is Earth, the four panels to the right show the different mechanisms and their observational discriminants. Circled molecules, if detected, would help reveal a false positive mechanism, a lack of detection of the “forbidden” molecules in the bottom shaded bar would also help to reveal the false positive mechanism. For example, the presence of CO and CO2, and the absence of CH4, is a strong indicator for a photochemical source of O2 from the photolysis of CO2 on a habitable CO2-rich M dwarf planet (Figure credit: Ron Hasler).
<b>FIG. 3.</b>
FIG. 3.
Reflectance spectrum of the Proterozoic Earth assuming 0.1% PAL O2 (Planavsky et al., 2014b), generated using the Atmos coupled climate-photochemical model (Arney et al., 2016). Note the absence of strong O2 features, but its photochemical byproduct O3 produces a relatively strong feature in the UV from 0.2 to 0.3 μm. This figure was generated using the LUVOIR simulator available at https://asd.gsfc.nasa.gov/luvoir and described in Robinson et al., (2016). LUVOIR, Large UltraViolet Optical Infrared Surveyor. (Credit: G. Arney)
<b>FIG. 4.</b>
FIG. 4.
The integration time as a function of wavelength required to obtain an S/N = 10 for the modern Earth orbiting a star at 10 pc for a 15 m LUVOIR-class telescope. The spectrum is roughly centered on the O2 0.76 μm A-band. Spectral resolution = 150 in the visible region. S/N = 10 can be obtained in 10 s of hours at most wavelengths. This figure was generated using the LUVOIR simulator available at https://asd.gsfc.nasa.gov/luvoir and described in Robinson et al. (2016) (credit: G. Arney).
<b>FIG. 5.</b>
FIG. 5.
The reflectance spectrum obtainable in 30 h by a 15 m space-based telescope observing modern Earth orbiting a Sun-like star at 10 pc. The gray bars denote the noise level, which increases significantly at wavelengths longward of 1.8 μm due to thermal radiation from the telescope, which is assumed to be heated to 270 K. This figure was generated using the LUVOIR simulator available at https://asd.gsfc.nasa.gov/luvoir and described in Robinson et al. (2016) (credit: G. Arney).
<b>FIG. 6.</b>
FIG. 6.
Transit transmission spectra of potential planetary environments with different O2 abundances for planet orbiting the M5.5V star Proxima Centauri (Meadows et al., 2018). Illustrating spectral features that can help distinguish photosynthetic from abiotically generated O2 in a planetary atmosphere. From top to bottom: self-consistent Earth-like atmosphere with 50% cloud cover (21% O2); 10 bar abiotic O2 (95% O2) atmosphere produced by early ocean loss with ocean remaining (purple) and desiccated (orange); 1 bar desiccated CO2/CO/O2 atmosphere that has reached a kinetic–photochemical equilibrium between the photolysis rate of CO2 and kinetics-limited recombination (15% O2). Effective atmospheric radius in kilometers is on the left y axes and transit depth is shown on the right y axes. The photosynthetic source for O2 in the Earth-like case is made more likely by the presence of O2/O3, water, and methane. High O2 cases with and without water are distinguished by the presence of O4, and the behavior of the 0.5–0.7 μm Chappuis band that is sensitive to tropospheric O3, which is more abundant in the desiccated case. The desiccated chemical equilibrium atmosphere is easily distinguished by its high levels of CO.
<b>FIG. 7.</b>
FIG. 7.
Synthetic transmission spectrum of high O2 atmosphere with NIR O4 features at 1.06 and 1.27 μm. The model atmosphere is a hypothetical 100 bar O2 atmosphere left behind by massive H escape during premain sequence evolution (Luger and Barnes, 2015). Data and error bars (1σ) for simulated JWST-NIRISS (left) and JWST-NIRSpec (right) were calculated with the noise model of Deming et al. (2009) assuming 65 h integrations (10 transits of an Earth-size planet around GJ876) and photon-limited noise. Figure adapted from Schwieterman et al. (2016). JWST, James Webb Space Telescope; NIR, near-infrared.
<b>FIG. 8.</b>
FIG. 8.
Reflected light spectra of potential Proxima Centauri b climates from top to bottom: self-consistent Earth-like atmosphere with 50% cloud cover, 10 bar abiotic O2 atmosphere with ocean, 10 bar desiccated O2 atmosphere, 1 bar desiccated CO2/CO/O2 atmosphere in kinetic–photochemical equilibrium (see Meadows et al., 2018). The O2 content in these atmospheres is produced from very different mechanisms: Earth O2 is biological, the 10 bar abiotic O2 atmospheres result from the super-luminous premain sequence evolution of the planet's M dwarf host star, and the last atmosphere is a 1 bar CO2 atmosphere that has <1 ppm hydrogen, resulting in a slow CO2 recombination timescale compared with its photolysis rate and Earth-like levels of O2. However, because hydrogen is also required to destroy ozone (O3), this atmosphere exhibits more O3 and a stronger Chappuis O3 band at ∼0.6 μm. The 10 bar O2 atmospheres are easily distinguished from Earth-like atmospheres by their deep, wide O2–O2 (O4) collision-induced absorption bands. Moist versus desiccated abiotic O2 cases are distinguished primarily by the presence or absence of water features. Around M dwarf stars, Earth-like planets with biological and geophysical sources of methane result in longer atmospheric lifetimes and correspondingly deep, observable methane features (Segura et al., ; Rugheimer et al., 2015).
<b>FIG. 9.</b>
FIG. 9.
Synthetic transmission spectrum with photochemical O2 and CO features. Model prebiotic atmosphere in photochemical equilibrium around GJ876 from Harman et al. (2015) containing ∼6% abiotic O2 and ∼1% CO. Data and error bars (1σ) for simulated JWST-NIRISS (left) and JWST-NIRSpec (right) were calculated with the noise model of Deming et al. (2009), assuming 65 h integrations (10 transits of an Earth-size planet around GJ876) and photon-limited noise. Figure adapted from Schwieterman et al. (2016).
<b>FIG. 10.</b>
FIG. 10.
The impact of N4 absorption in Earth's disk-averaged NIR spectrum. This plot shows the spectral differences between the cases with N4 absorption (blue and green) and the case without it (red), when compared with the EPOXI observations of the Earth in this wavelength range (black). N4 is clearly required to match the observed spectrum of the Earth near 4.1 μm. The gray band shows the calibration uncertainty for the EPOXI data (Klassen et al., 2008). Figure adapted from Schwieterman et al. (2015b).
<b>FIG. 11.</b>
FIG. 11.
A flowchart cartoon of possible steps to be taken in searching for a photosynthetic biosphere on an extrasolar planet, and in interpreting detection or nondetection of O2. The process starts with a survey of the background, and includes searching not only for the biosignature O2 but also for other atmospheric molecules such as CH4, which may support the biological interpretation of any O2 observed, and CO2 and O4, which may indicate abiotic processes that could be producing O2 without life. This flowchart also shows how observations of molecules other than O2 can be used to classify environments like the early Earth's, thereby identifying potentially habitable, but anoxic environments. (Credit: Shawn Domagal-Goldman).
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