Table 3.
Specific Atmospheric Substances, Their Spectral Bands in the UV-Visible to Thermal-Infrared, and Their Significance for Providing Environment Context for Establishing Whether an Earth-Like Exoplanet Is Truly Habitable
| Substance | Spectral band center or feature, μm | Significance for the planetary environment and habitability |
|---|---|---|
| CO2 | 15, 4.3, 4.8, 2.7, 2.0, 1.6, 1.4, 0.1474, 0.1332, 0.119 | Noncondensable greenhouse gas at T > 140 K (i.e., except at the outer edge of a conventional HZ) |
| Well mixed gas, enabling retrievals of atmospheric structure | ||
| Could be an antibiosignature if it coexists with a large amount of H2 | ||
| Could be a substrate for biological C fixation | ||
| N2 | 0.1–0.15 For N2–N2: 4.3, 2.15 |
Pressure broadening that enhances the greenhouse effect |
| Possible disequilibrium biogenic gas if detected with O2 and a surface of liquid water | ||
| O2 | 6.4, 1.57, 1.27, 0.765, 0.690, 0.630, 0.175–0.19 For O2–O2: 1.27, 1.06, 0.57, 0.53, 0.477, 0.446 |
Possible bulk constituent that enhances greenhouse effect through pressure broadening and weak thermal IR absorption (also a possible biosignature and hence also in Table 4). |
| O3 | >15 (rotation), 14.5, 9.6, 8.9, 7.1, 5.8, 4.7, 3.3, 0.45–0.85, 0.30–0.36 0.2–0.3 |
Possible indicator of O2 from which it derives |
| Greenhouse gas | ||
| H2O | Continuum, >20 (pure rotation), 6.2, 2.7, 1.87, 1.38, 1.1, 0.94, 0.82, 0.72, 0.65, 0.57, 0.51, 0.17, 0.12 | Condensable greenhouse gas |
| Abundances near saturation inferred from spectral features may suggest a wet planetary surface or clouds | ||
| CO | 4.67, 2.34, 1.58, 0.128–0.16 | Antibiosignature gas |
| May indicate lack of liquid water | ||
| H2 | 2.12, NIR continuum, <0.08 continuum | Antibiosignature gas if a relatively high abundance coexists with abundant CO2 |
| If abundant, pressure broadening that enhances the greenhouse effect | ||
| Greenhouse effect from pressure-induced absorption with self and other key species (e.g., CO2, CH4) | ||
| CH4 | 7.7, 6.5, 3.3, 2.20, 1.66, <0.145 continuum | Greenhouse gas |
| In the absence of oxidized species, could indicate a reducing atmosphere. Also a potential biosignature (Table 4) | ||
| C2H6 | 12.1, 3.4, 3.37, 3.39, 3.45, <0.16 continuum | Together with CH4, in the absence of oxidized species, could indicate a reducing atmosphere |
| HCN | 14.0, 3.0, <0.18 continuum | In the absence of oxidized species, could indicate a reducing atmosphere |
| H2S | 7, 3.8, 2.5, 0.2 | Potentially volcanic gas |
| SO2 | 20, 8.8, 7.4, 4, 0.22–0.34 | Potentially volcanic gas |
| H2SO4 (aerosol) | 11.1, 9.4, 8.4, 3.2a | Transient behavior potentially indicates active volcanism |
| May indicate an oxidizing atmosphere | ||
| Climate effects (cloud condensation nuclei; albedo) | ||
| Organic haze | Continuum opacity in visible-NIR | Indicates a reducing atmosphere with CO2/CH4 < 0.1 |
| May derive from biogenic or abiotic methane | ||
| Climate effects (antigreenhouse effect; shortwave absorption) | ||
| Rayleigh scattering | 0.2–1 | May indicate cloud-free atmosphere and help constrain the main scattering molecule (bulk atmospheric composition) |
| Clouds | UV, visible, NIR, TIR | Climate effects |
| Radiative transfer calculations with scattering (Rayleigh and Mie multiple scattering) may constrain cloud particle sizes and possibly composition |
Also noted is the interpretation of potential biosignature gases.
a
Exact wavelengths depend on the concentration of H2SO4 and size distribution of the aerosols.
HZ = habitable zone; IR = infrared; NIR = near-infrared; TIR = thermal infrared.