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Exoplanet Science Strategy (2018)

Chapter: Appendix D: Biosignature Table

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Suggested Citation:"Appendix D: Biosignature Table." National Academies of Sciences, Engineering, and Medicine. 2018. Exoplanet Science Strategy. Washington, DC: The National Academies Press. doi: 10.17226/25187.
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D

Biosignature Table

TABLE D.1 Biosignature Table: Spectral Features as a Function of Wavelength Range That Could Be Sought for Identification of an Oxygenic Photosynthetic Biosphere

Molecules/Feature 0.1-1.8 μm 1.8-2.5 μm 2.5-5.0 μm 5.0-20 μm Notes
O2 0.14, 0.2, 0.69, 0.76, 1.27 Biosignature sought—also disequilibrium pair with CH4, N2O.
O3 0.2-0.3 (strong), 0.38-0.65 4.75 9.6 Biosignature sought—also disequilibrium pair with CH4, N2O.
O4 (O2-O2 CIA)a 0.45, 0.48, 0.53, 0.57, 0.63, 1.06,1.27 (strong) False positive indicator—dense O2 from ocean runaway.b
CH4 0.1-0.14, 0.79, 0.89, 1.0, 1.1, 1.4, 1.7 2.31 (strong) 3.3 (strong) 7.7 Biosignature sought—disequilibrium pair with O2. Indicates presence of O2 sink.c May be disequilibrium pair with CO2.d
CO2 0.14, 1.05, 1.21, 1.32, 1.44, 1.6 2.01, 2.75 4.3 (strong) 9.4, 10.4, 15 False positive indicator, especially in combination with CO—ongoing CO2 photolysis.e
CO 1.6 2.35 4.65 False positive indicator, especially in combination with CO2—ongoing CO2 photolysis.e
Suggested Citation:"Appendix D: Biosignature Table." National Academies of Sciences, Engineering, and Medicine. 2018. Exoplanet Science Strategy. Washington, DC: The National Academies Press. doi: 10.17226/25187.
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Molecules/Feature 0.1-1.8 μm 1.8-2.5 μm 2.5-5.0 μm 5.0-20 μm Notes
N4 (N2-N2 CIA)f 4.1 False positive discriminant—helps quantify noncondensible gas fraction, disequilibrium biosignature when paired with N2/O2.g
N2O 0.13, 0.145, 0.185 2.11, 2.25 2.6, 2.67, 2.97, 3.6, 3.9, 4.3, 4.5 7.9, 17.0 Biosignature sought—disequilibrium pair with O2.
H2 0.64-0.66, 0.8-0.85 Possible bulk atmospheric constituent.
H2O 0.13, 0.17, 0.65, 0.72, 0.82, 0.94, 1.12, 1.4 1.85 2.7 6.3 Habitability indicator. False positive discriminant—could show ocean loss or presence of catalyst for CO2 recombination.h
SO2 0.2, 0.29, 0.37 4.0 7.3, 8.8, 19.0 Desiccation marker. High amounts are likely incompatible with a surface ocean.i
Ocean glintj 0.8-0.9 (optimal) Habitability indicator. False positive discriminant—disequilibrium biosignature when paired with O2/N2.k
Vegetation red edge 0.6 (halophile),l 0.7 (photosynthesis—G dwarf)m Biosignature sought. Other pigments may also generate spectral edges.n
Seasonal variability CO2 (1.6), CH4 (1.1 and 1.4), O2/O3 CO2 (15) Biosignature sought—seasonal variability in biomass building and metabolic output.o

NOTE: All values in the table are given in microns (μm) and molecular band wavelengths are derived from HITRAN; L.S. Rothman, I.E. Gordon, Y. Babikov, A. Barbe, D.C. Benner, P.F. Bernath, M. Birk, et al., The HITRAN2012 molecular spectroscopic database, Journal of Quantitative Spectroscopy and Radiative Transfer 130:4, 2013.

a G.D. Greenblatt, J.J. Orlando, J.B. Burkholder, and A.R. Ravishankara, Absorption measurements of oxygen between 330 and 1140 nm. Journal of Geophysical Research 95(D11):18577, 1990; C. Hermans, A.C. Vandaele, M. Carleer, S. Fally, R. Colin, A. Jenouvrier, B. Coquart, and M.-F. Mérienne, Absorption cross-sections of atmospheric constituents: NO2, O2, and H2O, Environmental Science and Pollution Research 6(3):151, 1999; B. Maté, C. Lugez, G.T. Fraser, and W.J. Lafferty, Absolute intensities for the O2 1.27 μm continuum absorption, Journal of Geophysical Research 104(D23):30585, 1999; R. Thalman and R. Volkamer, Temperature dependent absorption cross-sections of O2-O2 collision pairs between 340 and 630 nm and at atmospherically relevant pressure, Physical Chemistry Chemical Physics 15:15371, 2013.

b E.W. Schwieterman, V.S. Meadows, S.D. Domagal-Goldman, D. Deming, G.N. Arney, R. Luger, C.E. Harman, A. Misra, and R. Barnes, Identifying planetary biosignature impostors: Spectral features of CO and O4 resulting from abiotic O2/O3 production, Astrophysical Journal Letters 819(1):L13, 2016.

c S.D. Domagal-Goldman, A. Segura, M.W. Claire, T.D. Robinson, and V.S. Meadows, Abiotic ozone and oxygen in atmospheres similar to prebiotic Earth, Astrophysical Journal 792(2):90, 2014.

Suggested Citation:"Appendix D: Biosignature Table." National Academies of Sciences, Engineering, and Medicine. 2018. Exoplanet Science Strategy. Washington, DC: The National Academies Press. doi: 10.17226/25187.
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d J. Krissansen-Totton, S. Olson, and D.C. Catling, Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life, Science Advances 4(1):eaao5747, 2018.

e Schwieterman (2016), op cit.; Y. Wang, F. Tian, T. Li, and Y. Hu, On the detection of carbon monoxide as an anti-biosignature in exoplanetary atmospheres, Icarus 266:15, 2016.

f W.J. Lafferty, A.M. Solodov, A. Weber, W.B. Olson, and J.M. Hartmann, Infrared collision-induced absorption by N(2) near 4.3 μm for atmospheric applications: Measurements and empirical modeling, Applied Optics 35(30):5911, 1996.

g J. Krissansen-Totton, D.S. Bergsman, and D.C. Catling, On detecting biospheres from chemical thermodynamic disequilibrium in planetary atmospheres, Astrobiology 16(1):39, 2016; Schwieterman (2016), op cit.

h P. Gao, R. Hu, R.D. Robinson, C. Li, and Y.L. Yung, Stability of CO2 atmospheres on desiccated M dwarf exoplanets, Astrophysical Journal 806(2):249, 2015; Schwieterman (2016), op cit.; G. Tian, K. France, J.L. Linsky, P.J.D. Mauas, and M.C. Vieytes, High stellar FUV/NUV ratio and oxygen contents in the atmospheres of potentially habitable planets, Earth and Planetary Science Letters 385:22, 2014.

i V.S. Meadows, G.N. Arney, E.W. Schweiterman, J. Lustig-Yaeger, A.P. Lincowski, T. Robinson, S.D. Domagal-Goldman, et al., The habitability of Proxima Centauri b: Environmental states and observational discriminants, Astrobiology 18(2):133, 2018.

j T.D. Robinson, V.S. Meadows, and D. Crisp, Detecting oceans on extrasolar planets using the glint effect, Astrophysical Journal Letters 721(1):L67, 2010.

k Krissansen-Totton (2016), op cit.; Robinson (2010), op cit; M.E. Zugger, J.F. Kasting, D.M. Williams, T.J. Kane, and C.R. Philbrick, Searching for water earths in the near-infrared, Astrophysical Journal 739(1):12, 2011.

l E.W. Schwieterman, T.D. Robinson, V.S. Meadows, A. Misra, and S. Domagal-Goldman, Detecting and constraining N2 abundances in planetary atmospheres using collisional pairs, Astrophysical Journal 810(1):57, 2015.

m D.M. Gates, Energy, plants, and ecology, Ecology 46(1-2):1, 1965.

n L. Arnold, Earthshine observation of vegetation and implication for life detection on other planets, Space Science Review 135:323, 2008; N.Y. Kiang, J. Siefert, Govindjee, and R.E. Blankenship, Spectral signatures of photosynthesis. I. Review of Earth organisms, Astrobiology 7:222, 2007; and Schwieterman (2015), op cit.

o V.S. Meadows, Modelling the diversity of extrasolar terrestrial planets, Proceedings of the International Astronomical Union 1:25, 2006; S.L. Olson, E.W. Schwieterman, C.T. Reinhard, A. Ridgwell, S.R. Kane, V.S. Meadows, and T.W. Lyons, Atmospheric seasonality as an exoplanet biosignature, Astrophysical Journal Letters 858(2):L14, 2018.

SOURCE: Table modified from V.S. Meadows, Reflections on O2 as a biosignature in exoplanetary atmospheres, Astrobiology 17(10):1022, 2017.

Suggested Citation:"Appendix D: Biosignature Table." National Academies of Sciences, Engineering, and Medicine. 2018. Exoplanet Science Strategy. Washington, DC: The National Academies Press. doi: 10.17226/25187.
×
Page 155
Suggested Citation:"Appendix D: Biosignature Table." National Academies of Sciences, Engineering, and Medicine. 2018. Exoplanet Science Strategy. Washington, DC: The National Academies Press. doi: 10.17226/25187.
×
Page 156
Suggested Citation:"Appendix D: Biosignature Table." National Academies of Sciences, Engineering, and Medicine. 2018. Exoplanet Science Strategy. Washington, DC: The National Academies Press. doi: 10.17226/25187.
×
Page 157
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The past decade has delivered remarkable discoveries in the study of exoplanets. Hand-in-hand with these advances, a theoretical understanding of the myriad of processes that dictate the formation and evolution of planets has matured, spurred on by the avalanche of unexpected discoveries. Appreciation of the factors that make a planet hospitable to life has grown in sophistication, as has understanding of the context for biosignatures, the remotely detectable aspects of a planet’s atmosphere or surface that reveal the presence of life.

Exoplanet Science Strategy highlights strategic priorities for large, coordinated efforts that will support the scientific goals of the broad exoplanet science community. This report outlines a strategic plan that will answer lingering questions through a combination of large, ambitious community-supported efforts and support for diverse, creative, community-driven investigator research.

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