On the abundance of non-cometary HCN on Jupiter

TL;DR

This study uses thermochemical and photochemical models to explain the low observed levels of HCN in Jupiter's troposphere, concluding that both transport and photochemistry produce negligible HCN.

Contribution

It provides a detailed analysis of nitrogen chemistry in Jupiter's atmosphere, highlighting the limited role of photochemical and transport processes in HCN production.

Findings

Transport-induced quenching of HCN is minimal in Jupiter's troposphere.

Photochemical production of HCN is ineffective due to spatial separation of reactants.

Observed HCN upper limits constrain lightning and thunderstorm contributions.

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, hightemperature troposphere and predict the rate-limiting step for the quenching of HCN at cooler tropospheric altitudes. Consistent with 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 submillimeter observations can be used to place constraints on the production of HCN and other species from lightning and thundershock sources.