Phosphine photochemistry in the atmosphere of Saturn

A model is presented for the photochemistry of PH3 in the upper troposphere and lower stratosphere of Saturn that includes the effects of coupling with NH3 and hydrocarbon photochemistry, specifically the C2H2 catalyzed photodissociation of CH4. PH3 is rapidly depleted with altitude (scale height ∼35 km) in the upper troposphere when K∼104cm2sec−1; an upper limit for K at the tropopause is estimated at ∼105cm2sec−1. If there is no gas phase P2H4 because of sublimation, P2 and P4 formation is unlikely unless the rate of the spin-forbidden recombination reaction PH + H2 + M → PH3 + M is exceedingly slow. An upper limit P4 column density of ∼2×1015cm−2 is estimated in the limit of no recombination. If sublimation does not remove all gas phase P2H4, P2 and P4 may be produced in potentially larger quantities, although they would be restricted almost entirely to the lowest levels of our model, where T≳100°K. Potentially observable amounts of the organophosphorus compounds CH3P2H2 and HCP are predicted, with column densities of >1017 cm−2 and production rates of ∼2×108cm−2sec−1. The possible importance of electronically excited states of PHx and additional PH3/hydrocarbon photochemical coupling paths are also considered.