A Novel Abiotic Pathway for Phosphine Synthesis over Acidic Dust in Venus' Atmosphere

Recent ground-based observations of Venus have detected a single spectral feature consistent with phosphine (PH₃) in the middle atmosphere, a gas which has been suggested as a biosignature on rocky planets. The presence of PH₃ in the oxidized atmosphere of Venus has not yet been explained by any abiotic process. However, state-of-the-art experimental and theoretical research published in previous works demonstrated a photochemical origin of another potential biosignature-the hydride methane-from carbon dioxide over acidic mineral surfaces on Mars. The production of methane includes formation of the HC · O radical. Our density functional theory (DFT) calculations predict an energetically plausible reaction network leading to PH₃, involving either HC · O or H· radicals. We suggest that, similarly to the photochemical formation of methane over acidic minerals already discussed for Mars, the origin of PH₃ in Venus' atmosphere could be explained by radical chemistry starting with the reaction of ·PO with HC·O, the latter being produced by reduction of CO₂ over acidic dust in upper atmospheric layers of Venus by ultraviolet radiation. HPO, H₂P·O, and H₃P·OH have been identified as key intermediate species in our model pathway for phosphine synthesis.