Chapter 12 of 15
Molecules and life
Each gas absorbs at its own wavelengths. A molecule's shape sets how it vibrates, and its vibrations set where it absorbs in the infrared. No single gas proves life; a biosignature needs context.
Illustration
A molecule with N atoms has 3N − 6 ways to vibrate (3N − 5 if it is straight): water has three, carbon dioxide four (its bend comes twice, at right angles), methane nine. A mode absorbs infrared light only if it changes the molecule's dipole. That is why CO₂'s symmetric stretch (7.50 µm), O₂'s stretch and two of methane's four fundamentals are silent in the infrared. The bands the modes make are the ones the Reading atmospheres chapter marks: water's two stretches at 2.66 and 2.73 µm make its 2.7 µm band, methane's ν₃ the 3.3 µm band, CO₂'s asymmetric stretch the 4.26 µm band, and CO's stretch the 4.67 µm band.
Band positions: the atlas's NIST CCCBDB fundamentals, λ = 10⁴/ν̃. Displacements are symmetry-correct sketches with the centre of mass fixed and no net spin, not a force-field calculation. In every mode the atom that moves most swings 0.2 Å, and the stage states how much that exaggerates its zero-point motion, the root-mean-square displacement from ħ/2ω at the mode's frequency: for water's hydrogens in ν₁ that is 0.047 Å, so they are drawn ×4. Time is slowed 10¹³ to 10¹⁴ times, and the stage gives the factor for each mode.
H₂O Water
- Geometry
- bent, bond 0.958 Å, angle 104.48° (CCCBDB)
- Bands (µm)
- 1.4, 1.9, 2.7 (fundamentals 2.66 and 2.73), 6.27
- Vibrations
- ν₁ symmetric stretch · 2.73 µm · infrared-active · drawn ×4 · the 2.7 µm band in Reading atmospheres ›
- ν₂ bend · 6.27 µm · infrared-active · drawn ×3 · beyond the 0.5–5.1 µm window of Reading atmospheres
- ν₃ asymmetric stretch · 2.66 µm · infrared-active · drawn ×4 · the 2.7 µm band in Reading atmospheres ›
- Seen by
- HST/WFC3 near 1.4 µm; JWST NIRISS, NIRSpec, MIRI
- Exoplanet detections
- HD 209458 b and XO-1 b with WFC3 (Deming et al. 2013)
- WASP-39 b with JWST (JWST ERS Team 2023)
- Why it matters
- The commonest molecule in warm planet atmospheres; sets the shape of most spectra.
- Without life?
- Made everywhere by ordinary chemistry and outgassing. Not a biosignature.
CO₂ Carbon dioxide
- Geometry
- linear, bond 1.162 Å (CCCBDB)
- Bands (µm)
- 4.26 (very strong), 15.0; the 7.50 µm symmetric stretch is IR-silent
- Vibrations
- ν₁ symmetric stretch · 7.50 µm · infrared-silent (the dipole does not change) · drawn ×10 · beyond the 0.5–5.1 µm window of Reading atmospheres
- ν₂ bend (two, at right angles) · 15.00 µm · infrared-active · drawn ×5 · beyond the 0.5–5.1 µm window of Reading atmospheres
- ν₃ asymmetric stretch · 4.26 µm · infrared-active · drawn ×10 · the 4.26 µm band in Reading atmospheres ›
- Seen by
- JWST NIRSpec G395H/PRISM (4.3 µm), MIRI (15 µm)
- Exoplanet detections
- WASP-39 b, 26σ at 4.3 µm (JWST ERS Team 2023)
- K2-18 b, 3σ (Madhusudhan et al. 2023)
- Why it matters
- Traces the planet's metal content and chemistry; a context gas for any biosignature claim.
- Without life?
- Volcanism and thermochemistry make it readily.
CO Carbon monoxide
- Geometry
- diatomic, bond 1.128 Å (CCCBDB)
- Bands (µm)
- 4.67 fundamental
- Vibrations
- ν stretch · 4.67 µm · infrared-active · drawn ×10 · the 4.67 µm band in Reading atmospheres ›
- Seen by
- JWST NIRSpec; ground high-resolution spectrographs near 2.3 µm
- Exoplanet detections
- HD 209458 b, with its orbital motion (Snellen et al. 2010)
- Why it matters
- With CO₂ and H₂O it fixes the carbon-to-oxygen balance of hot atmospheres.
- Without life?
- CO₂ photolysis and hot thermochemistry; often read as a sign against life, not for it.
CH₄ Methane
- Geometry
- tetrahedral, bond 1.087 Å, angle 109.47° (CCCBDB)
- Bands (µm)
- 1.65, 2.4, 3.3 (ν₃ 3.31), 7.66
- Vibrations
- ν₁ symmetric stretch · infrared-silent (the dipole does not change) · no wavelength in the atlas
- ν₂ bend (two) · infrared-silent (the dipole does not change) · no wavelength in the atlas
- ν₃ asymmetric stretch (three) · 3.31 µm · infrared-active · drawn ×6 · the 3.3 µm band in Reading atmospheres ›
- ν₄ asymmetric bend (three) · 7.66 µm · infrared-active · drawn ×4 · beyond the 0.5–5.1 µm window of Reading atmospheres
- Seen by
- JWST NIRSpec, NIRISS, MIRI
- Exoplanet detections
- WASP-80 b (Bell et al. 2023)
- K2-18 b, 5σ (Madhusudhan et al. 2023)
- Why it matters
- With O₂ it is the classic disequilibrium pair that a biosphere could sustain.
- Without life?
- Serpentinisation of ultramafic rock (the main abiotic source on Earth today) and thermochemistry in cool H₂ atmospheres.
O₂ Oxygen
- Geometry
- diatomic, bond 1.208 Å (CCCBDB)
- Bands (µm)
- 0.76 (A band), 0.69, 0.63; O₂–O₂ pairs at 1.06 and 1.27; the 6.43 µm vibrational fundamental is IR-silent
- Vibrations
- ν stretch · 6.43 µm · infrared-silent (the dipole does not change) · drawn ×11 · beyond the 0.5–5.1 µm window of Reading atmospheres
- Seen by
- Reflected light (future HWO); ground high-resolution at 0.76 µm
- Exoplanet detections
- None in exoplanets so far.
- Why it matters
- Earth's most prominent biosignature gas, but only meaningful in context.
- Without life?
- CO₂ photolysis around some stars, or massive hydrogen escape after water loss.
O₃ Ozone
- Geometry
- bent, bond 1.278 Å, angle 116.8° (CCCBDB)
- Bands (µm)
- 0.25 (UV), 0.5–0.7 (Chappuis), 9.65 (strongest mid-IR); fundamentals 9.01, 9.60 and 14.2
- Vibrations
- ν₁ symmetric stretch · 9.01 µm · infrared-active · drawn ×10 · beyond the 0.5–5.1 µm window of Reading atmospheres
- ν₂ bend · 14.20 µm · infrared-active · drawn ×9 · beyond the 0.5–5.1 µm window of Reading atmospheres
- ν₃ asymmetric stretch · 9.60 µm · infrared-active · drawn ×8 · beyond the 0.5–5.1 µm window of Reading atmospheres
- Seen by
- HWO in UV/visible; mid-IR missions such as LIFE at 9.6 µm
- Exoplanet detections
- None in exoplanets so far.
- Why it matters
- A sensitive tracer of O₂, visible even when O₂ itself is hard to see.
- Without life?
- Photochemistry from abiotic O₂, favoured around some M and F dwarfs.
N₂O Nitrous oxide
- Geometry
- linear, bond 1.128 Å (CCCBDB)
- Bands (µm)
- 3.7, 4.5, 7.8, 8.6, 17
- Vibrations
- N–N stretch, mostly · 4.50 µm · infrared-active · drawn ×12 · not drawn in Reading atmospheres
- N–O stretch, mostly · 7.78 µm · infrared-active · drawn ×9 · beyond the 0.5–5.1 µm window of Reading atmospheres
- bend (two, at right angles) · 17.00 µm · infrared-active · drawn ×5 · beyond the 0.5–5.1 µm window of Reading atmospheres
- Seen by
- JWST MIRI; LIFE
- Exoplanet detections
- None in exoplanets so far.
- Why it matters
- Made by life on Earth, with few abiotic sources at that level.
- Without life?
- Flares, energetic particles and lightning; these make far more NOₓ than N₂O, which is diagnostic.
NH₃ Ammonia
- Geometry
- pyramidal, bond 1.012 Å, angle 106.67° (CCCBDB)
- Bands (µm)
- 2.90, 3.00, 6.15, 10.5
- Vibrations
- ν₁ symmetric stretch · 3.00 µm · infrared-active · drawn ×5 · not drawn in Reading atmospheres
- ν₂ umbrella · 10.50 µm · infrared-active · drawn ×3 · beyond the 0.5–5.1 µm window of Reading atmospheres
- ν₃ asymmetric stretch (two) · 2.90 µm · infrared-active · drawn ×4 · not drawn in Reading atmospheres
- ν₄ asymmetric bend (two) · 6.15 µm · infrared-active · drawn ×3 · beyond the 0.5–5.1 µm window of Reading atmospheres
- Seen by
- JWST MIRI (10.5 µm), NIRSpec
- Exoplanet detections
- None in exoplanets so far.
- Why it matters
- Abundant in cool giant planets; its depletion has been used to infer internal heat.
- Without life?
- Thermochemistry deep in hydrogen atmospheres; destroyed quickly by UV.
PH₃ Phosphine
- Geometry
- pyramidal, bond 1.421 Å, angle 93.3° (CCCBDB)
- Bands (µm)
- 4.30, 8.94, 10.1; the Venus claim used a millimetre line at 267 GHz
- Vibrations
- ν₁ symmetric stretch · 4.30 µm · infrared-active · drawn ×4 · not drawn in Reading atmospheres
- ν₂ umbrella · 10.10 µm · infrared-active · drawn ×3 · beyond the 0.5–5.1 µm window of Reading atmospheres
- ν₃ asymmetric stretch (two) · 4.30 µm · infrared-active · drawn ×3 · not drawn in Reading atmospheres
- ν₄ asymmetric bend (two) · 8.94 µm · infrared-active · drawn ×2 · beyond the 0.5–5.1 µm window of Reading atmospheres
- Seen by
- JWST MIRI; ALMA and JCMT for Venus
- Exoplanet detections
- None in exoplanets so far.
- Why it matters
- Proposed as a biosignature for rocky planets, where it is hard to make without life.
- Without life?
- Deep, hot chemistry in giant planets; otherwise thought hard to make abiotically on rocky worlds (Bains et al. 2021).
DMS Dimethyl sulfide
- Geometry
- dms, bond 1.802 Å, angle 98.9° (CCCBDB)
- Bands (µm)
- 3.3–3.4 (C–H stretches), 6–7, ≈10, ≈15
- Seen by
- JWST MIRI LRS; NIRSpec (3.3 µm overlaps methane)
- Exoplanet detections
- K2-18 b: tentative and contested (see below) (Madhusudhan et al. 2025)
- Why it matters
- On Earth it is made almost only by marine life.
- Without life?
- Few known abiotic sources on modern Earth; broken apart quickly by starlight.
SO₂ Sulfur dioxide
- Geometry
- bent, bond 1.432 Å, angle 119.5° (CCCBDB)
- Bands (µm)
- ≈4.05 (combination band), 7.34, 8.69, 19.3
- Vibrations
- ν₁ symmetric stretch · 8.69 µm · infrared-active · drawn ×10 · beyond the 0.5–5.1 µm window of Reading atmospheres
- ν₂ bend · 19.30 µm · infrared-active · drawn ×7 · beyond the 0.5–5.1 µm window of Reading atmospheres
- ν₃ asymmetric stretch · 7.34 µm · infrared-active · drawn ×13 · beyond the 0.5–5.1 µm window of Reading atmospheres
- Seen by
- JWST NIRSpec G395H/PRISM, MIRI LRS
- Exoplanet detections
- WASP-39 b: photochemical SO₂ (Tsai et al. 2023)
- WASP-39 b in the mid-IR (Powell et al. 2024)
- Why it matters
- The first photochemical product seen in an exoplanet atmosphere: starlight driving chemistry.
- Without life?
- Photochemistry and volcanism.
Na Sodium
- Geometry
- an atom (CCCBDB)
- Bands (µm)
- 0.589 (D lines, 589.0 and 589.6 nm)
- Seen by
- Optical transmission (HST, ground high-resolution)
- Exoplanet detections
- HD 209458 b: the first exoplanet atmosphere (Charbonneau et al. 2002)
- Why it matters
- Predicted by Seager & Sasselov 2000 and the first atmosphere ever detected.
- Without life?
- Atomic sodium in hot atmospheres; not a biosignature.
K Potassium
- Geometry
- an atom (CCCBDB)
- Bands (µm)
- 0.767 (766.5 and 769.9 nm)
- Seen by
- Optical transmission
- Exoplanet detections
- XO-2 b (Sing et al. 2011)
- Why it matters
- With sodium, a probe of clouds and temperature in hot giants.
- Without life?
- As sodium; not a biosignature.
Biosignature checklist
- Detection significance. How many sigma, from how many independent analyses?
- Context. The star, the planet's temperature and the other gases present (Meadows et al. 2018).
- Disequilibrium. Gases that would react away without a source, such as O₂ with CH₄ (Krissansen-Totton et al. 2018).
- Abiotic sources ruled out. Every non-living way to make the gas, checked (Schwieterman et al. 2018; Catling et al. 2018).
No gas has passed this checklist on any planet. Life has not been detected anywhere beyond Earth.
Phosphine on Venus: contested
2020: phosphine reported in Venus's clouds from JCMT and ALMA (Greaves et al. 2020). Re-analyses found no significant detection (Snellen et al. 2020; Villanueva et al. 2021), with a reply by the original team (Greaves et al. 2021). Still disputed.
DMS on K2-18 b: tentative, debated
2023: methane and CO₂ in an H₂-rich atmosphere, with "potential signs" of DMS (Madhusudhan et al. 2023). 2025: a ~3σ MIRI feature consistent with DMS and/or DMDS (Madhusudhan et al. 2025). Independent analyses found insufficient evidence (Schmidt et al. 2025; Luque et al. 2025). Not established.