H2S Cloud Properties in Uranus and Neptune: Sensitivity to Deep Composition and Vertical Mixing

Radiative transfer analyses of Uranus and Neptune spectra have revealed a cloud layer at pressures greater than ~2 bar (1,2), with H₂S gas detected above it on both planets (3,4), suggesting H₂S ice as its main constituent. However, the properties of these clouds and their dependence on the deep atmospheric composition remain poorly constrained.

We present an extended version of a one-dimensional cloud microphysics model [5,6] previously applied to simulate CH₄ and H₂S clouds in the Ice Giants (7,8). The model now incorporates NH₄SH chemistry, extending the simulation domain to ~50 bar. Since NH₃ reacts with H₂S to form NH₄SH at depth, the deep N/S ratio controls how much H₂S is available to condense at higher altitudes. We explore how the deep NH₃ and H₂S abundances, together with the vertical mixing profile, determine the properties of the H₂S cloud layer, including its base pressure, total opacity, and particle size distribution.

Preliminary results and the implications of this work for the interpretation of current and future observations of Uranus and Neptune will be discussed.

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