On the abundance of non-cometary HCN on Jupiter

Abstract

Using one-dimensional thermochemical/photochemical kinetics and transport models, we examine the chemistry of nitrogen-bearing species in the Jovian troposphere in an attempt to explain the low observational upper limit for HCN. We track the dominant mechanisms for interconversion of N₂–NH₃ and HCN–NH₃ in the deep, high-temperature troposphere and predict the rate-limiting step for the quenching of HCN at cooler tropospheric altitudes. Consistent with some other investigations that were based solely on time-scale arguments, our models suggest that transport-induced quenching of thermochemically derived HCN leads to very small predicted mole fractions of hydrogen cyanide in Jupiter's upper troposphere. By the same token, photochemical production of HCN is ineffective in Jupiter's troposphere: CH₄–NH₃ coupling is inhibited by the physical separation of the CH₄ photolysis region in the upper stratosphere from the NH₃ photolysis and condensation region in the troposphere, and C₂H₂–NH₃ coupling is inhibited by the low tropospheric abundance of C₂H₂. The upper limits from infrared and submillimetre observations can be used to place constraints on the production of HCN and other species from lightning and thundershock sources.