Uranus’ Stratospheric HCl Upper Limit from Herschel/SPIRE

Herschel/SPIRE observations of Uranus are used to search for stratospheric hydrogen chloride (HCl) emission at 41.74 cm−1. HCl was not detected and instead 3σ upper limits were determined; <6.2 ppb (<2.0 × 1014 molecules cm−2) for a 0.1 mbar step profile and <0.40 ppb (<1.2 × 1014 molecules cm−2) for a 1 mbar step profile. HCl is expected to have an external source and these upper limits are consistent with abundances of other external species (CO, H2O, CO2) and a solar composition source.