Updated April 24, 2018 19:26:56

Photo: Scientists spotted the noxious gas swirling above the clouds in Uranus's atmosphere. (Supplied: NASA/JPL/Michael Benson, Kinetikon Pictures)

Scientists have solved a "longstanding mystery", confirming Uranus smells bad.

They discovered hydrogen sulphide gas — also known as rotten egg gas — swirling in the planet's cloud tops.

"If an unfortunate human were ever to descend through Uranus's clouds, they would be met with very unpleasant and odiferous conditions," lead author of a study, published in Nature Astronomy, Professor Patrick Irwin said in a statement.

The composition of Uranus's clouds, and whether their main component was hydrogen sulphide or ammonia, has long been debated.

The scientists found the answer by looking at Uranus through an eight-metre-diametre telescope at the Gemini Observatory in Hawaii, and spotted the noxious gas swirling above its clouds.

"We were able to detect the 'fingerprint' of [hydrogen sulphide] in the light reflected from Uranus's clouds," Professor Irwin said.

"From the strength of the fingerprint, we were able to figure out how much [hydrogen sulphide] is present at the cloud tops."

But Professor Irwin noted Uranus's bad smell would be far from the biggest concern for a human visitor.

"Suffocation and exposure in the minus-200-degrees-Celsius atmosphere — made of mostly hydrogen, helium and methane — would take its toll long before the smell," Professor Irwin said.

In contrast, no hydrogen sulphide was seen above the clouds of inner gas giant planets Jupiter and Saturn.

Instead, most of their upper-clouds were made of ammonia ice.

The Gemini Observatory said the planets' differences in atmospheric composition "shed light on questions about the planets' formation and history".

The differences were likely imprinted in their early history, determined by the conditions where they formed, study co-author Dr Leigh Fletcher said.

"This composition information is invaluable in understanding Uranus' birthplace, evolution and refining models of planetary migrations," the Gemini Observatory said.

Professor Irwin said there was now another mystery about Uranus to solve.

"We should also see a very thick cloud of methane about the [hydrogen sulphide] cloud, but actually only see clear methane clouds in small, occasional storm cloud systems," he said.

"Uranus has many more secrets to reveal."

Topics: planets-and-asteroids, science-and-technology, astronomy-space, physics, telescopes, hawaii

First posted April 24, 2018 17:16:29