Uranus, the first planet discovered in modern times, has the poetic name of the Greek god of the heavens. In the English language, it is, unfortunately, the literal butt of every astronomy joke and I’m afraid that this latest discovery will make things worse. Astronomers have discovered that the upper atmosphere of Uranus is dominated by hydrogen sulfide, a molecule that smells like rotten eggs and is often found in human flatulence. I’ll give you a moment to crack a few jokes.
Got it out of your system? Good. Let’s get down to the science. These observations, presented in Nature Astronomy, have an impact beyond Uranus. They not only expand our knowledge of the ice giant planet's composition but also provide important clues about the formation of the Solar System itself.
The composition of the highest clouds of Uranus has never been unambiguously identified before. Planetary scientists could detect hints of ammonia and hydrogen sulfide but there was not enough evidence to determine their presence. Many suspected that the latter was responsible for the extra haze seen in radio and microwave observations. The height of the top clouds also seemed wrong if ammonia was the dominant component. But scientists lacked positive proof. The new observations change that.
“We have definitely detected the presence of hydrogen sulfide gas above the main cloud deck, and this is the smoking gun that the main cloud deck is mostly made by hydrogen sulfide ice,” lead author Professor Patrick Irwin, from the University of Oxford, told IFLScience.
“We finally have a more definite detection of what the clouds are made of. This means that on Uranus, and we think probably on Neptune, the bulk abundance of sulfur is greater than that of nitrogen. On Jupiter and Saturn, you end up with an ammonia cloud layer at the top.”
This discovery places a strong constraint on the formation of planets. Where Uranus and Neptune are located (over 18 times the Earth-Sun distance), both ammonia and hydrogen sulfide are solid, so they could have been easily absorbed by fledgling planets. But closer in, hydrogen sulfide is less easily absorbed by the giant planets. And this is a key factor.
It suggests that the four giant planets of the Solar System roughly formed where they are now. If they have migrated, they haven’t migrated that much. And they definitely haven't swapped positions. There are important discussions of planetary formation in our own and other Solar Systems. Planetary migration is often key to explain objects like hot Jupiters, giant planets found closer to their star than Mercury is to the Sun.
The observations were performed almost a decade ago but researchers were lacking an extensive spectral analysis of hydrogen sulfide. When that became available a few years ago, the team went back to the data to search for these signatures. It’s a testament to the good quality data collected that this was possible.
The team is now looking a bit further out at Neptune to see if they can spot the same signature there as well. “For Neptune, there’s more overlying haze and we are not sure. It’s likely to be there but if it’s there, it’s even harder to see,” added Professor Irwin.
If you truly want to know what Uranus smells like, be aware that hydrogen sulfide only smells of rotten eggs at concentrations of about 3-5 parts per million. Above 30, it actually smells sweet although it has been described as sickeningly sweet and at that dosage causes fatigue, loss of appetite, and dizziness.