Detection of hydrogen sulfide above the clouds in Uranus's atmosphere
Résumé
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 (H<SUB>2</SUB>S) ice. Here, we present evidence of a clear detection of gaseous H<SUB>2</SUB>S above this cloud deck in the wavelength region 1.57-1.59 mum 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 H<SUB>2</SUB>S below the observed cloud of (1.0 -2.5 ) ×1 0<SUP>-5</SUP>. The detection of gaseous H<SUB>2</SUB>S at these pressure levels adds to the weight of evidence that the principal constituent of 1.2-3-bar cloud is likely to be H<SUB>2</SUB>S ice.