Uranus has stinky clouds

Uranus stinks. The planet’s upper clouds are made of hydrogen-sulfide ice. That molecule gives rotten eggs their terrible odor.

“At the risk of schoolboy sniggers, if you were there, flying through the clouds of Uranus, yes, you’d get this pungent, rather disastrous smell,” says Leigh Fletcher. He’s a planetary scientist at the University of Leicester in England.

Fletcher and his colleagues recently studied the cloud tops of Uranus. The team used the Gemini North telescope in Hawaii. The telescope has a spectrograph. This instrument splits light into different wavelengths. Those data reveal what an object is made of. They showed the clouds of Uranus have hydrogen sulfide. The researchers shared their findings April 23 in Nature Astronomy.

The result wasn’t a complete surprise. Scientists found hints of hydrogen sulfide in the planet’s atmosphere in the 1990s. But the gas hadn’t been conclusively detected back then.

Now, it has. And, the clouds aren’t just smelly. They offer clues about the early solar system. For example, its clouds of hydrogen sulfide set Uranus apart from the gas giants, Jupiter and Saturn. The cloud tops on those planets are mostly ammonia.

Ammonia freezes at warmer temperatures than does hydrogen sulfide. So it’s more likely that ice crystals of hydrogen sulfide would have been abundant far out in the solar system. There, the crystals could have glommed onto newly forming planets. That suggests Uranus and the other ice giant, Neptune, were born farther from the sun than were Jupiter and Saturn.

“This tells you the gas giants and the ice giants formed in a slightly different way,” Fletcher explains. He says, “They had access to different reservoirs of material” when our solar system was forming.

Stinky clouds don’t deter Fletcher. He and other planetary scientists want to send a spacecraft to Uranus and Neptune. It would be the first mission to the ice giant planets since the Voyager spacecraft visited in the 1980s.