Laurel Kornfeld
April 26th, 2018
Arriving at Uranus in 1986, Voyager 2 observed a bluish orb with extremely subtle features. A haze layer hid most of the planet’s cloud features from view. Photo Credit: NASA / JPL-Caltech
Uranus’s cloud tops are composed of hydrogen sulfide, a gas that smells like rotten eggs, according to a study led by Glenn Orton of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
Most of the planet’s clouds are hidden by a layer of haze surrounding the bluish world. NASA’s Voyager 2, the only spacecraft to visit Uranus, was not capable of detecting the makeup of its clouds, leaving researchers to debate whether they are composed of hydrogen sulfide or ammonia.
Using the Near-Infrared Integral Field Spectrometer (NIFS) on the 26.25-foot (eight-meter) Gemini North telescope on the Hawaiian island of Mauna Kea, Orton and his team identified the hydrogen sulfide by using spectroscopy to study infrared light emitted by the planet.
Cloud tops on Jupiter and Saturn are composed of ammonia with no hydrogen sulfide. Neptune’s cloud tops probably have a composition similar to that of Uranus. The different atmospheric compositions likely stem from differences in the planets’ formation and evolution processes.
“We’ve strongly suspected that hydrogen sulfide gas was influencing the millimeter and radio spectrum of Uranus for some time, but we were unable to attribute the absorption needed to identify it positively,” Orton said in a news release. “Now, that part of the puzzle is falling into place as well.”
NIFS analyzed reflected sunlight just above Uranus’s main layer of clouds.
“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 Mauna Kea,” said researcher Patrick Irwin of the University of Oxford and lead author of a paper on the findings published in the journal Nature Astronomy.
Differences in the atmospheric compositions of gas giants Jupiter and Saturn and ice giants Uranus and Neptune are likely due to the locations and temperatures where the planets originally formed, said Leigh Fletcher of the University of Leicester. When clouds form through condensation, they trap the gas that formed them in their interiors, where telescopes cannot detect it, she said.
“Only a tiny amount remains above the clouds as a saturated vapor,” Fletcher said. “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.”
Even though the level of hydrogen sulfide in Uranus’s atmosphere is low, the compound’s effect on humans exposed to it would be very unpleasant. The planet’s atmospheric temperature of minus 328 degrees Fahrenheit (minus 200 degrees Celsius) and its atmospheric composition of hydrogen, helium, and methane would freeze and suffocate any humans exposed to them.
This image of a crescent Uranus, taken by Voyager 2 on Jan. 24, 1986, shows the planet’s icy blue atmosphere. Photo Credit: NASA / JPL
Laurel Kornfeld
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.
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