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

26 April 2018

Uranus

Uranus
(Credit: NASA)

One of the planet Uranus’s secrets, the composition of its clouds, has finally been uncovered by a team of UK researchers.

One of the key components of Uranus’s clouds, the foul-smelling hydrogen sulfide gas has been confirmed by a team from the UK led by Professor Patrick Irwin from the University of Oxford.

STFC is one of the organisations funding the research undertaken by the team, that also included UK researchers from the universities of Bristol and Leicester, who spectroscopically dissected the infrared light from Uranus captured by the eight-meter Gemini North telescope on Hawaii’s Maunakea.

Even after decades of observations, and a visit by the Voyager 2 spacecraft the long-sought evidence has eluded scientists until now. 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.’   

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 and Gemini data, 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.”

The detection of hydrogen sulfide high in Uranus’s cloud deck contrasts sharply with the Solar Systems 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, said: “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. “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.”

Professor Irwin said: ‘If an unfortunate human were ever to descend through Uranus’s clouds, they would be met with very unpleasant 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.

Notes to Editors

Article on the Nature website

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Last updated: 26 April 2018

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