Table 4.
Potential Biosignature Gases and Associated Information
| Biosignature | UV-Visible-NIR band center, μm and (cm−1) | Visible-NIR band interval, cm−1 | Thermal IR spectral band center, μm | Biogenic source | Abiogenic false positive |
|---|---|---|---|---|---|
| O2 | 1.58 (6329) 1.27 (7874) 1.06 (9433) 0.76 (13158) 0.69 (14493) 0.63 (15873) 0.175–0.19 [Schumann–Runge] |
6300–6350 7700–8050 9350–9400 12850–13200 14300–14600 14750–15900 |
— | Photosynthesis: splitting of water | Cases of water and CO2 photodissociation and preferential escape of hydrogen, with lack of O2 sinks |
| O3 | 4.74 (2110) 3.3 (3030) 0.45–0.85 [Chappuis] 0.30–0.36 [Huggins] 0.2–0.3 [Hartley] |
2000–2300 3000–3100 10600–22600 |
>15 (rotation), 14.3, 9.6, 8.9, 7.1, 5.8 | Photosynthesis: photochemically derived from O2 | As above |
| CH4 | 3.3 (3030) 2.20 (4420) 1.66 (6005) <0.145 continuum |
2500–3200 4000–4600 5850–6100 |
6.5, 7.7 | Methanogenesis: reduction of CO2 with H2, often mediated by degradation of organic matter | Geothermal or primordial methane |
| N2O | 4.5 (2224) 4.06 (2463) 2.87 (3484) 0.15–0.20 0.1809, 0.1455, 0.1291 |
2100–2300 2100–2800 3300–3500 |
7.78, 8.5, 16.98 | Denitrification: reduction of nitrate with organic matter | Chemodenitrification but not truly abiotic on Eartha; also strong coronal mass injection affecting an N2–CO2 atmosphereb |
| NH3 | 4.3 3.0 (3337) 2.9 (3444) 2.25, 2, 1.5, 0.93, 0.65, 0.55, 0.195, 0.155 |
2800–3150 | 6.1, 10.5 | Ammonification: Volatilization of dead or waste organic matter | Nonbiogenic, primordial ammonia |
| (CH3)2S | 3.3 (2997) 3.4 (2925) 0.205, 0.195, 0.145, 0.118 |
2900–3100 | 6.9, 7.5, 9.7 | Plankton | No significant abiotic sources |
| CH3Cl | 3.3 (3291) 3.4 (2937) 0.175, 0.160, 0.140, 0.122 |
2900–3100 | 6.9, 9.8, 13.7 | Algae, tropical vegetation | No significant abiotic sources (Keppler et al., 2005) |
| CH3SH | 3.3 (3015) 3.4 (2948) 0.204 |
2840–3100 | 6.9, 7.5, 9.3, 14.1 | Mercaptogenesis: Methanogenic organisms can create CH3SH instead of CH4 if given H2S in place of H2 (Moran et al., 2008). | No significant abiotic sources |
| C2H6 | 3.37 (2969) 3.39 (2954) 3.45 (2896) <0.16 |
2900–3050 | 6.8, 12.15 | Photochemically derived from CH4, CH3SH, and other biologically produced organic compounds | Could be derived from geothermal or primordial methane |
Shown are absorption band centers or band ranges in the UV-visible to NIR, as well as thermal IR. Particularly strong bands are marked in bold because of their strength and/or lack of contamination from other gases. Square brackets contain the names of particular bands.
N2O has been generated from “chemodenitrification,” whereby nitrite (NO2−) or nitrate (NO3−) reacts with Fe2+-containing minerals in brines (Jones et al., 2015; Samarkin et al., 2010). However, on Earth, the source of natural oxidized nitrogen ultimately comes from nitrifying bacteria or atmospheric chemistry that relies upon oxygen, which comes from photosynthesis. Also N2O can be released from UV photoreduction of ammonium nitrate (Rubasinghege et al., 2011), where the latter comes from humans as industrial fertilizer. Another N2O source comes from very weak in situ atmospheric gas phase reactions.
Airapetian et al. (2016).