Shades of Uranus: Scientists know why the planet and Neptune are different hues of blue

June 1, 2022

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Why are Uranus and Neptune two different hues of blue?

The two worlds are quite similar in mass to each other – Uranus is roughly 15 times the mass of Earth, while Neptune is at 17 – with nearly identical atmospheric compositions of hydrogen (more than 80% each), helium and methane. But now, new research suggests that a “stagnant, sluggish” atmosphere on Uranus allows haze to build up and concentrate on the gas giant, making it a “whiter” cyan blue than Neptune’s cerulean.

A new model, using wavelengths from ultraviolet to near-infrared, investigates multiple atmospheric layers in each of the planets. The study shows that embedded in the inner atmospheric planetary layers, there is even more haze than thought, rather than just icy clouds of methane and hydrogen sulfide. It is the first time a study took into account wavelengths from ultraviolet to near-infrared, rather than concentrating on a handful of light waves, the authors said.

“It’s also the first [study] to explain the difference in visible color between Uranus and Neptune, “lead author Patrick Irwin, a planetary physics professor at Oxford University, said in a statement (opens in new tab) from the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory, or NOIRLab.

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Previously, scientists had suggested it was the methane of Neptune that makes that planet so blue, as the gas absorbs a lot of red light and reflects bluer colors. But scientists were at a loss to explain what was going on at Uranus, given it has even more methane (2.3% of the atmospheric mass, compared with 1.9% at Neptune.)

That discrepancy suggests something else must be responsible for the color difference. But other key differences between the planets revealed few clues to the mystery. And both planets are understudied, each only having been visited by one spacecraft, NASA’s Voyager 2 in the 1980s. Since then, scientists have relied on telescopic check-ins to monitor the two blue orbs.

Uranus is an odd world, rotating on its side compared to the plane of the solar system in a way that leads to extreme seasons persisting for about 20 years. (A 2018 study suggested a world twice the size of Earth smacked into Uranus, causing its odd orientation.) Uranus also has little or no internal heat to supplement sunlight, presenting the coldest atmosphere of any solar system planet.

Infrared images of Uranus (1.6 and 2.2 microns) obtained on Aug. 6, 2014, showing a very large storm on the planet. (Image credit: Imke de Pater (UC Berkeley) & WM Keck Observatory images)

One of Neptune’s distinctive atmospheric features is its storms; Neptune’s winds can reach up to 1,500 mph (2,400 kph), the fastest yet detected in the solar system. When Voyager 2 flew by Neptune, it found a large storm nicknamed the “Great Dark Spot,” which was large enough to contain the entire Earth. While that spot disappeared, others have popped up in observations from the Hubble Space Telescope.

Uranus has its own stormy temperament. The year 2014, for example, had an impressive show when storms were quite active on the usually quiescent world. Astronomers were astonished given that the sunlight was strongest on the planet in 2007, when the sun’s light fell directly upon the equator. The reasons for that time gap are poorly understood.

A storm was spotted in Neptune's atmosphere in 2016, with the Hubble Space Telescope.