Exoplanet Science Strategy (2018)
Chapter: Appendix D: Biosignature Table
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 |
| 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.
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.
