Biosignatures & Paleobiology

An Overview Of Exoplanet Biosignatures

By Keith Cowing
Status Report
April 25, 2024
Filed under , , , , , , , , , , ,
An Overview Of Exoplanet Biosignatures
Graphical illustration of potential scenarios leading to potentially detectable abiotic O2 and/or O3. Circle molecules identify spectrally active species such as O4 (O2-O2 collisionally induced absorption) that could be used to fingerprint the origin(s) of O2 and/or O3, while the red strike-through indicates species that should be absent. Modern Earth is distinguished by the combination of spectrally active O2, O3, H2O, CO2, and CH4 together with low levels of CO. See section “False Positives for Oxygen and Ozone Biosignatures” in this text for additional details. This figure is reproduced from Meadows et al. (2018) under Creative Commons Attribution License CC-BY. Graphic artist: R. Hasler. — astro-ph.EP

This chapter reviews proposed exoplanet biosignatures, including their biological origins, observable features, atmospheric sinks, and potentially confounding abiotic sources.

Emphasis is placed on material published since past comprehensive reviews while providing a foundational understanding of each named biosignature.

Topics include possible gaseous biosignatures (e.g., O2, O3, CH4, N2O, DMS, CH3Cl, C5H8, NH3, PH3), surface biosignatures (e.g., vegetation red edge, other pigment features, polarization signatures), and temporal biosignatures (e.g., atmospheric seasonality). Potential frameworks for assessing remote biosignatures are described. Text and table summaries provide references to relevant original research articles.

N2O concentrations in parts-per-million (ppm) as a function of N2O flux (y-axes) and atmospheric O2 (x-axes) predicted by a photochemical model for an Earth-like planet orbiting within the habitable zone of each named star. The stars listed here correspond to those whose spectra are shown in Figure 8. The inset Earth image in the top left panel indicates Earth’s N2O molecular flux (0.4 Tmol/yr; 1.5×109 molecules/cm2 /s). The green horizontal line indicates Earth’s current N2O atmospheric mixing ratio (~330 parts-per-billion; 3.3×10-7 ). Data subpanels were adapted from Schwieterman et al. (2022) under Creative Commons Attribution License CC-BY. — astro-ph.EP

Summary of gaseous (left), surface (center), and temporal (right) biosignatures. Gaseous biosignatures include spectrally active, volatile metabolic products, such as O2 produced by oxygenic photosynthesis, and potential photochemical byproducts, such as ozone (O3) from O2 photochemistry. Surface biosignatures include the vegetation red edge (VRE), which results from the sharp contrast between chlorophyll absorption at visible wavelengths and scattering at infrared wavelengths in photosynthetic organisms. Temporal biosignatures include time-dependent modulation of gases linked to life—such as seasonal changes in CO2 consumed by photosynthesis and released by the decay of organic materials—or variations in albedo from the growth and decay of vegetation. This figure is reproduced from Schwieterman (2021) under Creative Commons Attribution License CC-BY. Sub image credits: NASA and the Encyclopedia of Life (EOL). — astro-ph.EP

Simulated spectrum of Earth from 0.2-22 μm at quadrature-phase (half illumination) showing various spectral features that include biosignature gases O2, O3, and CH4; the vegetation red edge (VRE) surface feature; and habitability marker gases H2O, CO2, and N2. Rayleigh scattering is indicative of atmospheric pressure. Top: Apparent spectral albedo in reflected light from 0.2-2 μm (ultraviolet/visible/near-infrared) at quadrature phase. Middle: Near-infrared (2-5 μm) spectral radiance (units: W m-2 μm-1 sr-1 ), including reflected and emitted light components. Bottom: Thermal infrared (5-22 μm) spectral radiance. The synthetic spectrum was calculated using the Virtual Planetary Laboratory 3D spectral Earth model (Robinson et al. 2011; Schwieterman et al. 2015b). This figure is reproduced with minor modifications from Schwieterman et al. (2018a) under Creative Commons Attribution License CC-BY. Modifications include additional gas feature labels, an inset of a simulated Earth at half illumination, and an inset color bar indicating the visible wavelength range. — astro-ph.EP

Earth’s infrared (2-15 μm) cloud-free transmission spectrum from Macdonald and Cowan (2019; black line) and fit using the SMARTER retrieval model (blue line; Lustig-Yaeger et al. 2023). This spectrum shows gaseous habitability markers CO2 (2.7, 4.3, 15 μm), H2O (6 μm), and N2 (4.2 μm); biosignatures CH4 (3.3, 7.7 μm), O3 (4.7, 9.7 μm) and N2O (4, 7.7, 8.5 μm), and industrial pollutants CFC-11 (CCl3F; 11.8 μm) and CFC-12 (CCl2F2; 10.8 μm), and NO2 (6.2 μm). This figure is taken from Lustig-Yaeger et al. (2023) under Creative Commons Attribution License CC-BY. — astro-ph.EP

Absorption features for potential biosignature molecules and other gases important for terrestrial planet characterization. Shown are line intensities for the most abundant isotopologue from the HITRAN 2020 database (Gordon et al. 2022) for O2, O3, CH4, N2O, CH3Cl, NH3, PH3, H2O, and CO2 in units of cm-1 /(molec·cm-2 ) and absorption cross-sections from the PNNL database (Sharpe et al. 2004) for C5H8, (CH3)2S (DMS), and C2H6 in units of cm2 /molecule. Note that opacities are often incomplete for λ < 2 μm either because these data are not in HITRAN but are available elsewhere (e.g., O3) or because they haven’t been measured (e.g., CH3Cl). — astro-ph.EP

Edward W. Schwieterman, Michaela Leung

Comments: Chapter 13 accepted for publication in the Reviews in Mineralogy and Geochemistry (RiMG) Volume 90 on “Exoplanets: Compositions, Mineralogy, and Evolution” edited by Natalie Hinkel, Keith Putirka, and Siyi Xu; 44 pages, 14 figures, and 4 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR); Geophysics (physics.geo-ph)
Cite as: arXiv:2404.15431 [astro-ph.EP] (or arXiv:2404.15431v1 [astro-ph.EP] for this version) Submission history From: Edward W. Schwieterman [via Natalie Hinkel as proxy] [v1] Tue, 23 Apr 2024 18:19:42 UTC (11,024 KB)


Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻