Pale blue, deep blue: how Uranus and Neptune get their colors

Roses are red. Neptune is deep blue.

Why, scientists ask themselves, is not Uranus either?

It’s an interesting question. Uranus and Neptune, the two outer planets of our solar system, are both ice giants – cold worlds that are part of gas, part ice, with similar chemical compositions.

They are also not far away in mass, Uranus is 15 times that of Earth, and Neptune 17 times. And they are both about four times the size of Earth, Uranus is slightly larger.

Yet the two worlds certainly look different. Uranus, as first discovered by NASA’s past Voyager 2 space probe in 1986, is a light blue spot without character. When the same spaceship Neptune encountered in 1989, it revealed a world with the most powerful winds in the solar system, ripe by a royal blue atmosphere, with gigantic storms and even a mysterious dark place. Why the difference?

Patrick Irwin, a planetary physicist at Oxford University, and colleagues have now developed an answer. They made a detailed understanding of the atmosphere of each world with the Gemini North Telescope in Hawaii, the Hubble Space Telescope and other observations.

Both worlds are blue because they have methane in their atmosphere, which absorbs the color red from the light of the sun. But an important middle layer of methane haze on Uranus turned out to be twice as thick as the layer on Neptune. It is the presence of this extra haze that leads to the various visas.

“That haze is kind of whitish in appearance,” said Drs. said Irwin. “Therefore Uranus looks paler than Neptune does.”

The study was published Tuesday in the Journal of Geophysical Research: Planets.

Imke de Pater, a planetary scientist at the University of California, Berkeley, said the discovery made sense. “The abundance of methane on the two planets is very similar,” she said. “Something has to explain the difference in colors.”

Why Uranus has a thicker haze than Neptune may be the result of a gigantic influence early in life that knocked the planet to its side, said Leigh Fletcher, a planetary scientist at the University of Leicester in England and a co-author on paper.

“All of his internal energy and sources of heat could be given up in that enormous collision,” he said. “So what you see today is a more stagnant world.”

Both worlds would lose mist if methane ice pulled it into the lower atmosphere, falling like methane snow. But on the activated Neptune more often falls methane snow, which leads to a thin haze layer.

Erich Karkoschka, a planetary scientist at the University of Arizona, said he “would not make this assumption” that Uranus’ collision with another object explained why it was less active than Neptune. He suggested that the worlds might just be physically different enough to account for the differences in their atmospheres.

The work can also explain the origin of the great and mysterious dark places of Neptune, Dr. Irwin said, which appear to be caused by a darkening of the haze particles, possibly caused by evaporating hydrogen sulfide ice.

A future Uranus orbiter and atmospheric probe is now a top priority for NASA to launch in the 2030s. That could tell scientists more about the haze, such as observations with the James Webb Space Telescope.

“There is still a lot of uncertainty,” said Drs. said Irwin. “We do not really know what the particles are made of. The only way to really know what’s going on is to fall into a sin in these deep atmospheres.

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