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.

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Meet the planets Our solar system Depending on who you ask, there are eight or nine planets in our solar system - some experts still count Pluto, while the International Astronomical Union (IAU) took away its planetary status in 2006. People were still excited when NASA presented the first high-res images of Pluto this week. Its neighbors all had their portrait taken as early as the 1960s.

Meet the planets Mercury The spacecraft Mariner 10 left for the planet closest to the Sun in 1973. It took this picture of Mercury's moon-like surface in March 1974. The planet's distance to the Sun varies between 28.5 million miles (46 kilometers) and 43.5 million miles (70 kilometers), because its orbit isn't a perfect circle. Scientists were surprised to discover that Mercury had a small magnetic field.

Meet the planets Venus Mariner 10 took this first close-up of Mercury's direct neighbor on February 5, 1974. The picture was color-enhanced by NASA to bring out Venus' cloudy atmosphere - the planet is perpetually blanketed by a thick veil of clouds rich in carbon dioxide. Mariner 10's journey to Venus was a rocky one: the spacecraft's high-gain antenna developed problems and a mechanical issue caused a large fuel-loss.

Meet the planets Earth The first full-on photo of our planet as seen from outer space was taken by Lunar Orbiter 1 in August 1966. That was three years before a human being had ever set foot on the Moon, which can be seen in the foreground of this picture as a shadow. The now-iconic photo was one of a series of pictures taken in preparation for the Apollo missions that would eventually put a human on the Moon.

Meet the planets Mars This close-up of Earth's neighbor is the first picture ever taken of another planet by a spacecraft. Mariner 4 snapped it on July 15, 1965. Scientists who had expected to see lakes, valleys and mountains were disappointed - instead of an Earth-like planet, they were treated to craters similar to those on the Moon. The New York Times wrote: "Mars is probably a dead planet."

Meet the planets Jupiter Spacecraft Pioneer 10 took the planet's first close-up from roughly 80,780 miles (130,000 kilometers) away on November 19, 1973. Jupiter is our solar system's largest planet. At its equator, Jupiter's diameter is a whopping 88,846 miles (142,984 kilometers). Its mass is two-and-a-half times larger than the masses of all other planets combined.

Meet the planets Saturn This first shot of the ringed planet was taken on Pioneer 10's follow-up mission, Pioneer 11, on August 31, 1979. It was a perilous adventure: as the spacecraft flew through Saturn's outer rings, it almost crashed into one of two new moons it discovered. Visible at the upper left-hand corner in this photo is Saturn's moon Titan.

Meet the planets Uranus One of the first glimpses scientists got of Uranus was of its rings. Voyager 2 took this shot of them in 1986. Scientists had to remote-fix the spacecraft's camera for it to be able to photograph the planet with the coldest atmosphere in our solar system (as low as -366 degrees Fahrenheit or -221 degrees Celsius). The device had malfunctioned while Voyager 2 was passing Saturn.

Meet the planets Neptune Voyager 2 also took the first picture of Neptune in August 1989. The planet has four cloud features that scientists know about. For those who don't count Pluto, Neptune is the planet in our solar system that's furthest away from the sun: at an average of 2.8 billion miles (4.5 billion kilometers), that distance is 30 times greater than the one between the Sun and Earth.

Meet the planets Pluto The fact that Pluto is not officially a planet anymore didn't detract from the excitement scientists and lay-people all over the world experienced when NASA released this first close-up of the copper-colored (dwarf-) planet taken by New Horizons on July 13, 2015. The spacecraft traveled 3 billion miles (4.88 billion kilometers) to the solar system's farthest reaches for this shot. Author: Carla Bleiker



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.

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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."