Predicted chemistry of the deep atmosphere of Uranus before the Voyager 2 encounter

Abstract

The Voyager 2 spacecraft will encounter Uranus in January 1986,and will provide the first spacecraft observations of that planet.It carries an infrared interferometer spectrometer (IRIS) whichhas already detected a variety of infrared active gases in theatmospheres of Jupiter, Saturn and Titan^1–4. Some of the detected gases are not in chemical equilibrium but have sources in the deep atmosphere. IRIS observations of these non-equilibrium gases, in particular PH₃ (mixing ratio $X_{{\text{PH}}_{\text{3}}}$ ≃6×10⁻⁷) and GeH₄ ($X_{{\text{GeH}}_{\text{4}}}$ ≃ 7×10⁻¹⁰) on Jupiter¹ and PH₃ ($X_{{\text{PH}}_{\text{3}}}$ ≃ 1×10⁻⁶) on Saturn³, can be used to deduce the strength of convective mixing in the deepatmospheres of Jupiter and Saturn^5,6. Similar deductions could be made about convective mixing rates in the deep atmosphere of Uranus if models of the equilibrium chemistry and thermochemicalkinetics were available to help interpret IRIS observations of the visible Uranus atmosphere. We describe here the results of comprehensive thermochemical equilibrium and chemical kinetic calculations for Uranus. We predict that the most abundant non-equilibrium trace gas derived from the deep atmosphere of Uranus is N₂; other important non-equilibrium species include HCl, HF, GeH₄, C₂H₆, PH₃, H₂Se, CH₃SH, CO, CH₃NH₂, CH₃OH, and CO₂. Some of these species are detectable potentially by the Voyager instruments.