Roses are red. Neptune it’s deep blue.
Why Uranus isn’t it too? the scientists wondered.
It’s an intriguing question. Uranus and Neptune, the two outermost planets in our solar system, are ice giants — cold worlds made of gas and ice, with similar chemical compositions.
They are also not very different in mass, with Uranus being 15 times greater than Earth’s and Neptune’s being 17 times greater. And both are roughly four times the size of Earth, with Uranus being slightly larger.
However, the two worlds look decidedly different. Uranus, as it was first revealed by the spacecraft Voyager 2 and Nasa in 1986, it is a pale blue blob with no special features.
When the same spacecraft encountered Neptune in 1989, it revealed a world with the most powerful winds in the solar system, ripping through a real blue atmosphere, with giant storms and even a mysterious dark spot. Why the difference?
Patrick Irwin, a planetary physicist at the University of Oxford, at United Kingdom, and his colleagues developed an answer. They gathered a detailed knowledge of each world’s atmosphere using the Gemini North telescope in Hawaii, the Hubble Space Telescope and other observations.
The two planets are blue because they have methane in their atmospheres, a gas that absorbs the red color of sunlight. But an average layer of methane haze on Uranus appeared to be twice as thick as the layer on Neptune. It is the presence of this additional fog that produces the different appearances.
“This fog looks kind of whitish,” Irwin said. “That’s why Uranus looks paler than Neptune.”
The research was published Tuesday in the Journal of Geophysical Research: Planets.
Imke de Pater, a planetary scientist at the University of California at Berkeley, said the finding makes sense. “The abundance of methane on the two planets is very similar,” she said. “Something has to explain the color difference.”
Why Uranus has a thicker haze layer than Neptune’s may be the result of a giant impact early in its existence that tipped the planet, said Leigh Fletcher, a planetary scientist at the University of Leicester in the UK and co-author of the study. .
“All its energy and internal heat sources could have been released in that huge collision,” he said. “So what we see today is a more stagnant world.”
Both worlds would lose fog as the methane ice pulled it into the lower atmosphere, falling as methane snow. But on more active Neptune, methane snow falls more often, leading to a thinner fog layer.
Erich Karkoschka, a planetary scientist at the University of Arizona, said he “wouldn’t make that assumption,” that Uranus’ collision with another object would explain why it is less active than Neptune. He suggested that the planets might be physically different enough to account for differences in their atmospheres.
The work may also explain the origin of Neptune’s vast and mysterious dark spots, Irwin said, which appear to be caused by a dimming of haze particles, possibly due to evaporation of hydrogen sulfide ice.
A future Uranus orbiter and atmospheric probe is a priority for NASA, to be launched in the 2030s. They could reveal more to scientists about the haze layers, as could observations with the James Space Telescope Webb.
“There’s still a lot of uncertainty,” Irwin said. “We don’t really know what particles are made of. The only way to really know what’s going on is to launch a probe into these deep atmospheres.”
Translated by Luiz Roberto M. Gonçalves
–
