Researchers have long wondered about the composition of the clouds high up in Uranus' sky — specifically, whether they're dominated by ammonia ice, as at Jupiter and Saturn, or by hydrogen sulfide ice. The answer has proved elusive, because it's tough to make observations with the required detail on distant Uranus. (Not only are Jupiter and Saturn closer to Earth, they have also hosted dedicated orbiter missions. Uranus has been visited just once — a brief flyby by NASA's Voyager 2 probe in January 1986.) Irwin and his colleagues studied Uranus' air using the Near-Infrared Integral Field Spectrometer (NIFS), an...