Why Uranus and Neptune are different shades of blue?

Uranus is pale blue in colour while Neptune is a deeper shade of blue, and an atmospheric model can explain the difference

Why Uranus and Neptune are different shades of blue?
Uranus (left) and Neptune, imaged by Voyager 2
(Credit: NASA/JPL-Caltech)

Uranus and Neptune are different shades of blue, and we may finally know why.

In visible-light images, Uranus appears a pale blue, while Neptune is a deeper, cobalt hue. Using data from the Hubble Space Telescope and ground-based observatories, as well as information gleaned from the Voyager 2 spacecraft, Patrick Irwin at the University of Oxford and his colleagues have developed detailed models of the atmospheres of both worlds that might explain the colour difference.

The thick, enshrouding skies of these planets are made of hydrogen and helium as well as methane. But hazes formed of other chemicals are thought to be floating at different altitudes too. These are probably created when methane is broken down by ultraviolet radiation from the sun, before being remade into larger hydrocarbons.

In their new models, the scientists identified a haze layer, thought to be present on both worlds, which is roughly double the thickness on Uranus as on Neptune. This feature, which the team calls the Aerosol-2 layer, would look whitish at visible wavelengths. So the greater thickness of the Aerosol-2 hazes on Uranus would lighten the planet’s appearance – similar to how tracing paper placed over a picture makes its vibrant hues more milky.

“This explains why Uranus is a paler blue colour than Neptune,” says Irwin.

As for why the two gas giants are blue in the first place, that is down to the methane in their atmospheres, says team member Leigh Fletcher at the University of Leicester, UK. “Methane absorbs red light, leaving blue to be reflected back,” he says.

“Seeing both planets successfully compared directly in analysis is rare,” says Naomi Rowe-Gurney, who researches the atmospheres of Uranus and Neptune at NASA’s Goddard Space Flight Center. “The authors state that future observations will help to answer remaining questions and I am sure that the James Webb Space Telescope will help with this during the planned observations of both planets within the first year of operations,” she says.


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