Phosphine photochemistry in Saturn’s atmosphere

The phosphine photochemistry on Saturn is studied with a 1D photochemical model. The PH 3 concentration is rapidly depleted with height (scale height 3.5 km) in the upper troposphere. Formation of P, a probable precursor of P 4 , (a potential red chromophore in the atmosphere), is highly improbable unless the rate constant for the recombination reaction PH + H 2 + M → PH 3 + M is less than 10 −41 cm 6 molec −2 sec −1 . Coupling of PH 3 and hydrocarbon photochemistry, specifically the C 2 H 2 catalyzed photodissociation of CH 4 , is important. Column production rates of the organophosphorus compounds CH 3 PH 2 and HCP of ∼3 × 10 8 cm −2 sec −1 are predicted, with potentially observable column densities of >1 × 10 17 cm −2 .