Phosphine absorption by liquid droplets on Venus: Implications for the maintenance of atmospheric PH₃

Phosphine gas has been reported to exist within the clouds of Venus at the 1 ppb level (Greaves et al 2021). A number of PH₃ loss mechanisms have been postulated including UV light photolysis and chemical reactions with radicals. An additional loss mechanism heretofore largely ignored is the absorption or uptake of PH₃ by the liquid sulfuric acid cloud droplets, similar to that which occurs for halide gases such as HCl and HBr (Delitsky and Baines 2023). Uptake of PH₃ into sulfuric acid and subsequent reaction will form phosphoric acid and other species. This loss mechanism is significant when evaluated over the global cloud at 47 to 70 km altitude. The liquid cloud particles on Venus absorb ~5.9 kg PH₃/km² every hour, which amounts to ~ 24 gigatons per year for the entire cloud deck. To maintain the 1 ppb observed in Venus clouds, this loss must be counterbalanced by an equal rate of PH₃ creation. Thus far, no plausible mechanism has been found to explain the existence of such large amounts of PH₃ (Bains W. et al, 2024). On Earth, PH₃ is released into the atmosphere at a rate of ~4 x 10^-5 gigatons per year from biological and anthropogenic sources (Fu and Zhang, 2020), which is 1 part in 6 x 10⁵ of the rate required to maintain the atmospheric 1 ppb PH₃ level on Venus against cloud absorption of PH₃. Thus, it is extremely unlikely that an Earth-like biological source flux can maintain the 1 ppb of PH₃ observed on Venus. However, Earth-like production rates could theoretically maintain a PH₃ abundance of 1.7 x 10^-15 within the Venusian clouds. Engineering studies have indicated that uptake of PH₃ and formation of phosphoric acids causes release of elemental sulfur particles. The reaction is PH₃ + H₂SO₄ -> H₃PO₄ + H₂S ; then H₂S + acid -> S_n (precipitate). At least 23 gigatons of S_n is formed and precipitated each year within the cloud droplets. Sulfur precipitation will drive the phosphine uptake and may change much of the sulfuric acid in the droplets into phosphoric acid. Re-examination of mass spectral data taken by the Pioneer Venus spacecraft during its mission in the 1970s indicated the presence of phosphorus species such as phosphine and phosphorus oxides in the clouds (Mogul et al 2021). Thus, phosphoric acid and other phosphorus species should be investigated as possible constituents of the clouds of Venus.