Pale Blue, Deep Blue: How Uranus and Neptune Get Their Colors

Roses are red. The deep blue of Neptune.

Why, the scientists wondered, isn’t Uranus also?

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

Their mass isn’t far off either, with Uranus being 15 times that of Earth and Neptune 17 times. And they’re both about four times the size of Earth, with Uranus being slightly larger.

Yet the two worlds seem decidedly different. Uranus, as first revealed by NASA’s Voyager 2 spacecraft in 1986, is a featureless light blue spot. When the same spacecraft encountered Neptune in 1989, it revealed a world with the strongest winds in the solar system, ripping through a royal blue atmosphere, with giant storms and even a mysterious dark spot. Why the difference?

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

Both worlds are blue because they contain methane in their atmosphere, which absorbs the red color of sunlight. But a key intermediate layer of methane haze on Uranus appeared to be twice as thick as the layer on Neptune. It is the presence of this additional haze that leads to the different faces.

“This mist looks quite white,” Dr. Irwin said. “That’s why Uranus is 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, Berkeley, said the finding makes sense. “The abundance of methane on the two planets is very similar,” she said. “Something must explain the color difference.”

Why Uranus has a thicker haze layer than Neptune may be the result of a giant impact early in its life that flipped the planet, said Leigh Fletcher, planetary scientist at the University of Leicester in England and co-author On paper.

“All of its internal energy and heat sources could have been given up in this huge collision,” he said. “So what you see today is a more stagnant world.”

Both worlds would lose haze as methane ice dragged it into the lower atmosphere, falling as methane snow. But on the more active Neptune, methane snow falls more often, leading to a thinner haze layer.

Erich Karkoschka, a planetary scientist at the University of Arizona, said he “wouldn’t make the assumption” that Uranus’ collision with another object explained why it was less active than Neptune. He suggested that the worlds 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, Dr Irwin said, which appear to be caused by a darkening of haze particles, likely caused by the evaporation of hydrogen sulfide ice.

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

“There is still a tremendous amount of uncertainty,” Dr Irwin said. “We don’t really know what the particles are made of. The only way to really know what’s going on is to launch a probe into those deep atmospheres.

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