Microphysical Modeling of Hydrogen Sulfide Clouds in the Atmospheres of the Ice Giants

Radiative transfer analyses of spectra obtained from Uranus and Neptune have revealed the presence of
a cloud layer at pressures greater than ~2 bar (1,2). The detection of hydrogen sulfide (H₂S) gas above
this cloud layer on both planets (3,4) suggests that H₂S ice is the most likely main constituent. This
interpretation is further supported by the expectation that methane (CH₄) clouds condense at higher
altitudes (5). However, due to their depth and observational limitations, our understanding of the
properties of H₂S clouds on these planets remains very limited.
To investigate the properties of H₂S clouds in the atmospheres of Uranus and Neptune, we employed a
one-dimensional cloud microphysics model originally developed for Titan and Mars (6,7). The model
includes nucleation, condensation, evaporation, coagulation, and precipitation processes, and has
previously been used to simulate haze and CH₄ cloud microphysics in the Ice Giants (5,8,9).
Figure 1 shows, as an example, simulated H₂S ice profiles for Uranus using this microphysical model.
The vertical transport of H₂S gas is simulated using an eddy diffusion coefficient (Keddʏ), which controls
the supply of vapor for cloud nucleation and particle growth. We employed the Keddʏ profiles derived
in [10] for H₂S abundances of 10× and 30× solar. Since several cloud microphysical parameters for H₂S
remain uncertain (e.g., the contact parameter), different values are tested in the simulations. In the
example shown, the model indicates cloud bases near 5.3 bar for 10× solar abundance and 6.4 bar for
30× solar. Near the cloud base, particle mean radii range from 40 to 55 μm, depending on the assumed
contact parameter and abundance. At higher altitudes, particle sizes decrease; for instance, at ~3 bar,
mean radii are around 20 μm. In general, H₂S cloud simulations produce higher opacities than CH₄
clouds.
In this work, we will present a series of cloud microphysical simulations of H₂S clouds in the Ice Giants.
Various cloud properties, such as particle size distributions and precipitation rates, will be constrained.
We will also discuss the implications of our results for the atmospheric circulation of these planets and
for the future exploration of Uranus.

Figure 1. Vertical distributions of H2S ice (g/m³) for Uranus, simulated for different values of the cloud
contact parameter and deep H2S abundances. These simulations employ the Keddʏ profiles calculated in
[10] for the corresponding H2S abundances.

References: [1] P. G. Irwin, et al., JGR: Planets, 127, e2022JE007189. [2] L. Sromovsky, et al., Icarus,
Volume 317, (2019) [3] P. G. Irwin, et al., Nature Astronomy 2, 420 (2018). [4] P. G. Irwin, et al.,
Icarus 321, 550 (2019). [5] D. Toledo, et al., A&A, 694, A81 (2025). [6] P. Rannou, et al., Science 311,
201 (2006). [7] F. Montmessin, et al., JGR: Planets 107, 4 (2002). [8] D. Toledo, et al., Icarus, 333, 1-
11, (2019). [9] D. Toledo, et al., Icarus, Volume 350, (2020). [10] H. Ge, et al., The Planetary Science
Journal,5, 101(2024).