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What do Uranuss cloud tops have in common with rotten eggs

Hydrogen sulphide, the gas that gives rotten eggs their charm, appears to permeate the upper atmosphere of the planet Uranus – as has been long debated, but never definitively proven.

Based on sensitive spectroscopic observations with the Gemini North telescope, planetary scientists, including Dr Leigh Fletcher from our Department of Physics and Astronomy, have uncovered the noxious gas swirling high above the giant planet’s cloud tops. This result resolves a stubborn, long-standing mystery of one of our neighbours in space.

Even after decades of observations, and a visit by the Voyager 2 spacecraft, Uranus has held on to one critical secret - the composition of its clouds. Now, one of the key components of the planet’s clouds has been found.

Patrick Irwin from the University of Oxford, UK and collaborators spectroscopically dissected the infrared light from Uranus captured by the 8-meter Gemini North telescope on Hawaii’s Maunakea.

They found hydrogen sulphide, the odiferous gas that most people avoid, in Uranus’s cloud tops. The long-sought evidence is published in the April 23rd issue of the journal Nature Astronomy.

Leigh explains that the differences between the cloud decks on 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 sulfide) was determined by the temperature and location of planet’s formation,” he says.

According to Leigh, 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.  

“Only a tiny amount remains above the clouds as a saturated vapour,” said Leigh. “And this is why it is so challenging to capture the signatures of ammonia and hydrogen sulphide above cloud decks of Uranus. The superior capabilities of Gemini finally gave us that lucky break.”

The new findings indicate that besides being one of the most hostile places for life in the Solar System, Uranus is also downright unpleasant. The study also confirms that 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.

The research has been covered by a range of national media outlets, including the BBC, the Mail Online and the Metro.

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