New Near-UV H₂O Cross-Sections & Photochemistry of Early Earth-like Atmospheres

We use new laboratory measurements of the absorption properties of water vapor to dramatically revise theoretical predictions of the concentrations of spectrally active trace gases in early-Earth like exoplanet atmospheres, and eliminate a key O₂ false positive scenario. We study the photochemistry of abiotic, habitable planets with anoxic CO₂-N₂ atmospheres. Such worlds are representative of early Earth, Mars and Venus, and analogous exoplanets. H₂O photodissociation controls the atmospheric photochemistry of these worlds through production of reactive OH, which dominates the removal of atmospheric trace gases. The near-UV (NUV; >200 nm) absorption cross-sections of H₂O were previously unmeasured at habitable temperatures (< 373 K). We present the first-ever measurements of NUV H₂O absorption at 292 K, and show it to absorb orders of magnitude more than previously assumed. The enhanced OH production due to these higher cross-sections leads to efficient recombination of CO and O₂, suppressing both by orders of magnitude relative to past predictions and eliminating the low-outgassing “false positive” scenario for O₂ as a biosignature. Enhanced [OH] increases rainout of reductants to the surface, relevant to prebiotic chemistry, and may also suppress CH₄ and H₂. Overall, our work advances the state-of-the-art of photochemical models by providing crucial new H₂O cross-sections, and suggests that detection of spectrally active trace gases like CO in rocky exoplanet atmospheres may be more challenging than previously considered. While we focus on CO₂-rich worlds, our results are relevant to anoxic planets in general.