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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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.

Author information


  1. Department of Physics (Atmospheric, Oceanic and Planetary Physics), University of Oxford, Oxford, UK

    • Patrick G. J. Irwin
    • , Daniel Toledo
    •  & Ryan Garland
  2. School of Earth Sciences, University of Bristol, Bristol, UK

    • Nicholas A. Teanby
  3. Department of Physics & Astronomy, University of Leicester, Leicester, UK

    • Leigh N. Fletcher
  4. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

    • Glenn A. Orton
  5. LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 6, Université Paris-Diderot, Sorbonne Paris Cité, Meudon, France

    • Bruno Bézard


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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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The authors declare no competing interests.

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Correspondence to Patrick G. J. Irwin.

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