Potential pathways for PH3 production
Potential PH3-production pathways in the Venusian environment are discussed in detail in the Supplementary Information (W. Bains et al., manuscript in preparation; summarized in Extended Data Fig. 10). Two possible classes of routes for the production of PH3 were considered: photochemical production or non-photochemical chemistry.
For photochemical modelling, we created a network of reactions of known kinetic parameters59 that could lead from H3PO4 (phosphoric acid) to PH3 (phosphine), by reaction with photochemically generated radicals in the Venusian atmosphere. Where reactions were possible but no kinetic data for the phosphorus species were known, homologous nitrogen species reaction kinetics were used instead, validated by comparing reactions of analogous nitrogen and phosphorus species. The maximum possible rate for reductive chemistry in this network was compared with the destruction rate as a function of altitude.
Non-photochemical reactions were modelled thermodynamically. For surface and atmospheric chemistry, we created a list of chemicals, their concentrations and reactions, for all potential PH3 production reactions. Phosphorus species abundances were calculated themodynamically and assumed to be in equilibrium with liquid/solid species at the cloud base. The free energy of reaction, indicating whether the net production of PH3 was thermodynamically favoured, was calculated using standard methods (see Supplementary Information for details). None of the reactions favour the formation of PH3, on average having a free energy of reaction of +100 kJ mol−1 (Extended Data Fig. 7).
Modelling the subsurface chemistry was approached via oxygen fugacity (fO2) (ref. 60), the notional concentration of free oxygen in the crustal rocks. We model the equilibrium between phosphate and PH3, for temperatures between 700 K and 1,800 K, at 100 or 1,000 bar, and with 0.01%, 0.2% and 5% water. Oxygen fugacity of plausible crust and mantle rocks based on Venus lander geological data is 8–15 orders of magnitude too high to support reduction of phosphate, so degassing of mantle rocks would produce only trivial amounts of PH3. Volcanic, lightning and meteoritic delivery were calculated based on parallels with terrestrial rates of events within Venusian atmosphere, and were calculated to be negligible.
Supplementary Info has even more detail and concepts, and another whole paper on it is pending