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

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) × 10^-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.