The composition of the clouds above Uranus has long remained a mystery.

Image credit: Shutterstock / NASA

What do Uranus's cloud tops have in common with rotten eggs?

This blog post is adapted from an article published by the Gemini Observatory.

Even after decades of observations, and a visit by the Voyager 2 spacecraft, Uranus held on to one critical secret: the composition of its clouds. Now, one of the key components has finally been verified.

Professor Patrick Irwin from the University of Oxford's Department of Physics and global collaborators spectroscopically dissected the infrared light from Uranus captured by the eight-meter Gemini North telescope on Hawaii's Maunakea. They found hydrogen sulfide, the odiferous gas that most people avoid, in Uranus’s cloud tops. The long-sought evidence is published in the journal Nature Astronomy.

The Gemini data, obtained with the Near-Infrared Integral Field Spectrometer (NIFS), sampled reflected sunlight from a region immediately above the main visible cloud layer in Uranus's atmosphere. Professor Irwin said: 'While the lines we were trying to detect were just barely there, we were able to detect them unambiguously thanks to the sensitivity of NIFS on Gemini, combined with the exquisite conditions on Maunakea. Although we knew these lines would be at the edge of detection, I decided to have a crack at looking for them in the Gemini data we had acquired.'

Dr Chris Davis of the United States' National Science Foundation, a funder of the Gemini telescope, said: 'This work is a strikingly innovative use of an instrument originally designed to study the explosive environments around huge black holes at the centres of distant galaxies. To use NIFS to solve a longstanding mystery in our own solar system is a powerful extension of its use.'

Astronomers have long debated the composition of Uranus’s clouds and whether hydrogen sulfide or ammonia dominates the cloud deck, but lacked definitive evidence either way. Professor Irwin said: 'Now, thanks to improved hydrogen sulfide absorption-line data and the wonderful Gemini spectra, we have the fingerprint which caught the culprit.' The spectroscopic absorption lines (where the gas absorbs some of the infrared light from reflected sunlight) are especially weak and challenging to detect, according to Professor Irwin.

The detection of hydrogen sulfide high in Uranus's cloud deck (and presumably Neptune’s) contrasts sharply with the inner gas giant planets, Jupiter and Saturn, where no hydrogen sulfide is seen above the clouds, but instead ammonia is observed. The bulk of Jupiter and Saturn's upper clouds are comprised of ammonia ice, but it seems this is not the case for Uranus. These differences in atmospheric composition shed light on questions about the planets' formation and history.

Dr Leigh Fletcher, a member of the research team from the University of Leicester, adds that the differences between the cloud decks of the gas giants (Jupiter and Saturn), and the ice giants (Uranus and Neptune), were likely imprinted way back during the birth of these worlds. He said: 'During our solar system’s formation, the balance between nitrogen and sulphur – and hence ammonia and Uranus's newly detected hydrogen sulphide – was determined by the temperature and location of planet's formation.'

Another factor in the early formation of Uranus is the strong evidence that our solar system's giant planets likely migrated from where they initially formed. Therefore, confirming this composition information is invaluable in understanding Uranus's birthplace, evolution and refining models of planetary migrations.

According to Dr Fletcher, when a cloud deck forms by condensation, it locks away the cloud-forming gas in a deep internal reservoir, hidden away beneath the levels that we can usually see with our telescopes. He said: 'Only a tiny amount remains above the clouds as a saturated vapour. And this is why it is so challenging to capture the signatures of ammonia and hydrogen sulfide above cloud decks of Uranus. The superior capabilities of Gemini finally gave us that lucky break.'

Dr Glenn Orton of NASA's Jet Propulsion Laboratory, another member of the research team, said: 'We've strongly suspected that hydrogen sulfide gas was influencing the millimetre and radio spectrum of Uranus for some time, but we were unable to attribute the absorption needed to identify it positively. Now, that part of the puzzle is falling into place as well.'

While the results set a lower limit to the amount of hydrogen sulfide around Uranus, it is interesting to speculate what the effects would be on humans even at these concentrations. Professor Irwin said: 'If an unfortunate human were ever to descend through Uranus's clouds, they would be met with very unpleasant and odiferous conditions. However, suffocation and exposure in the -200C atmosphere made of mostly hydrogen, helium and methane would take its toll long before the smell.'

The new findings indicate that although the atmosphere might be unpleasant for humans, this far-flung world is fertile ground for probing the early history of our solar system and perhaps understanding the physical conditions on other large, icy worlds orbiting the stars beyond our Sun.