Uranus smells like rotten eggs

There's a lot of really smelly stuff wafting around Uranus.

The clouds in Uranus' upper atmosphere are composed largely of hydrogen sulfide, the molecule that makes rotten eggs so stinky, a new study suggests.

"If an unfortunate human were ever to descend through Uranus' clouds, they would be met with very unpleasant and odiferous conditions," study lead author Patrick Irwin, of Oxford University in England, said in a statement.

But that wayward pioneer would have bigger problems, he added: "Suffocation and exposure in the negative 200 degrees Celsius [minus 328 degrees Fahrenheit] atmosphere, made of mostly hydrogen, helium, and methane, would take its toll long before the smell."

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 instrument on the 26-foot (8 meters) Gemini North telescope in Hawaii. NIFS scrutinized sunlight reflected from the atmosphere just above Uranus' cloud tops — and spotted the signature of hydrogen sulfide.

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