Detection of hydrogen sulfide above the clouds in Uranus’s atmosphere

Abstract

Visible-to-near-infrared observations indicate that the cloud top of the main cloud deck on Uranus lies at a pressure level of between 1.2 bar and 3 bar. However, its composition has never been unambiguously identified, although it is widely assumed to be composed primarily of either ammonia or hydrogen sulfide (H2S) ice. Here, we present evidence of a clear detection of gaseous H2S above this cloud deck in the wavelength region 1.57–1.59 μm with a mole fraction of 0.4–0.8 ppm at the cloud top. Its detection constrains the deep bulk sulfur/nitrogen abundance to exceed unity (>4.4–5.0 times the solar value) in Uranus’s bulk atmosphere, and places a lower limit on the mole fraction of H2S below the observed cloud of \((1.0-2.5)\times 1{0}^{-5}\). The detection of gaseous H2S at these pressure levels adds to the weight of evidence that the principal constituent of 1.2–3-bar cloud is likely to be H2S ice.

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Fig. 1: The appearance and spectrum of Uranus at the near-infrared wavelengths observed by Gemini/NIFS and associated absorption spectra of CH4, NH3 and H2S.
Fig. 2: Pressure variation of temperature and condensable abundances assumed in this study for Uranus.
Fig. 3: Fits to average Gemini/NIFS observation of Uranus, made on 2 November 2010 at 15.3° N, using three different assumptions for the a priori imaginary refractive index spectrum, and excluding H2S and NH3 absorption.
Fig. 4: Fits to the coadded Gemini/NIFS observation of Uranus in the wavelength range 1.56–1.6 μm.

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Acknowledgements

We are grateful to the United Kingdom Science and Technology Facilities Council for funding this research and to our support astronomers: R. McDermid and C. Trujillo. The Gemini Observatory is operated by the Association of Universities for Research in Astronomy under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência e Tecnologia (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). We thank L. Sromovsky for providing the code used to generate our Rayleigh-scattering opacities. G.A.O. was supported by NASA funding to the Jet Propulsion Laboratory, California Institute of Technology. L.N.F. was supported by a Royal Society Research Fellowship at the University of Leicester.

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P.G.J.I. wrote the proposal to make the original observations, and reduced and reanalysed the data using the NEMESIS code; B.B. and R.G. assisted in identifying and validating the line data used. G.A.O. provided the Spitzer temperature–pressure profile used. All authors contributed to the analysis and interpretation of the results, and all authors wrote the final paper.

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Correspondence to Patrick G. J. Irwin.

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Irwin, P.G.J., Toledo, D., Garland, R. et al. Detection of hydrogen sulfide above the clouds in Uranus’s atmosphere. Nat Astron 2, 420–427 (2018). https://doi.org/10.1038/s41550-018-0432-1

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