A global team of researchers found evidence of hydrogen sulfide, a foul-smelling gas usually occurring in rotten eggs, in the upper layers of clouds on Uranus.
"Even after decades of observations and a visit by NASA's Voyager 2 spacecraft, Uranus held on to one critical secret — the composition of its clouds," NASA said in press release on Tuesday.
The mystery was finally solved by the research published this week in the Nature Astronomy journal.
The group of scientists, which included physicist Glenn Orton of NASA's Jet Propulsion Laboratory in California, managed to detect the gas by analyzing reflected infrared light from Uranus. Their research was based on the data gathered by the giant Gemini North telescope in Mauna Kea, Hawaii.
Team ammonia vs. team hydrogen sulfide
"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. "Now, that part of the puzzle is falling into place as well."
Scientists suspect that clouds on Uranus' neighbor Neptune also contain hydrogen sulfide. This provides a "striking difference" from the other two gas planets, Jupiter and Saturn. Researchers detected ammonia above their clouds but not hydrogen sulfide, NASA said. Jupiter and Saturn are closer to the Sun than Uranus and Neptune.
"These differences in atmospheric composition shed light on questions about the planets' formation and history," NASA said in a press release.
Uranus is an 'unpleasant' place
Uranus is the seventh planet from the Sun and its diameter is about four times that of Earth. It was named after the Greek god Uranus, who was the father of Kronos and grandfather of Zeus in Greek mythology. Similar to Saturn, the planet has a ring system, but its rings are fainter and harder to spot.
Conditions on the planet are hostile to humans, with the stench of hydrogen sulfide a comparatively minor problem.
"If an unfortunate human were ever to descend through Uranus's clouds, they would be met with very unpleasant and odiferous conditions," says lead author of the research, Patrick Irwin of the University of Oxford. However, "[s]uffocation and exposure in the negative 200 degrees Celsius [392 degrees Fahrenheit] atmosphere made of mostly hydrogen, helium and methane would take its toll long before the smell."