,,- University of Oxford, Department of Atmospheric, Oceanic, and Planetary Physics, Oxford, United Kingdom of Great Britain – England, Scotland, Wales (joseph.penn@physics.ox.ac.uk)
The spectral signature of hydrogen sulphide (H2S) above the cloud tops in Uranus’ atmosphere was detected in 2018 [1]. The H2S humidity can be used as a tracer of Uranus’ overturning circulation [2] - peaks and troughs in the latitudinal humidity distribution may correspond to regions of local upwelling and downwelling near the H2S condensation level.
We analysed observations from Gemini-NIFS and VLT-SINFONI, taken between 2009 and 2014, to study the H2S humidity distribution of Uranus. In our previous analysis of H2S on Neptune [3], we found a significant degeneracy between the methane (CH4) and H2S distributions, so we prescribe a latitudinally varying deep methane abundance previously derived from HST-STIS spectra [4]. We deconvolve the observations and extract spectra using the Minnaert limb-darkening approximation, which has been applied in several analyses of Ice Giant observations [3,4,5].
We fit a parameterised aerosol model and the H2S humidity to our extracted spectra with nested sampling using our open-source radiative transfer code, archNEMESIS [6]. Our atmospheric model has a large number of parameters, and to make nested sampling computationally feasible we utilise a trained neural network for early exploration of the parameter space during our retrievals.
Since our observations span several years, we search for temporal changes. We find changes in the aerosol structure and aerosol spectral properties corresponding to the development of Uranus' north polar hood, in agreement with previous work [4,8]. If we assume that the CH4 distribution is stable over time, then our results show no significant changes in the H2S distribution.
Our results show a general equator-to-pole decrease in the H2S humidity, similar to what has been found in microwave analyses that are sensitive to the deep H2S distribution [7]. Superimposed on this are local increases, which are fairly evenly spaced in latitude. We found a somewhat similar pattern in our analysis of H2S on Neptune [3], and an analysis of Neptune with VLT/MUSE also found peaks in reflectivity with a similar spacing [5]. These results are suggestive of a complex circulation pattern near the deep H2S aerosol layer.
[1] Irwin, P. G. J., et al. (2018). Detection of hydrogen sulfide above the clouds in Uranus's atmosphere. Nature Astronomy, 2(5), 420-427.
[2] Fletcher, L.N., et al. (2020). Ice Giant Circulation Patterns: Implications for Atmospheric Probes. Space Sci Rev 216, 21
[3] Penn, J., et al. (2026). Reconciling Near-Infrared and Microwave Analyses of Neptune’s Hydrogen Sulphide Distribution. Monthly Notices of the Royal Astronomical Society, 548, 2,
[4] James, A., et al. (2023). The Temporal Brightening of Uranus' Northern Polar Hood From HST/WFC3 and HST/STIS Observations. Journal of Geophysical Research: Planets, 128(10), e2023JE007904.
[5] Irwin, P. G. J., et al. (2023). Latitudinal Variations in Methane Abundance, Aerosol Opacity and Aerosol Scattering Efficiency in Neptune's atmosphere determined from VLT/MUSE. Journal of Geophysical Research: Planets, 128(11), e2023JE007980.
[6] Alday, J., et al. (2025). archNEMESIS: An Open-Source Python Package for Analysis of Planetary Atmospheric Spectra. Journal of Open Research Software, doi:10.5334/jors.554.
[7] Molter, E. M., et al. (2021). Tropospheric Composition and Circulation of Uranus with ALMA and the VLA. The Planetary Science Journal, 2(1), 3.
[8] Sromovsky, L. A., et al. (2024). The puzzling north polar region of Uranus: Continued zero-shear winds and increasing brightness from 2015 through 2022 according to 7 years of Keck AO imaging. Icarus, 420, 116186.
How to cite: Penn, J., Irwin, P., and Dobinson, J.: The Latitudinal Variation of H2S Humidity on Uranus , Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-52, https://doi.org/10.5194/epsc2026-52, 2026.
