Why so blue?: Differences between Neptune and Uranus' colours decoded

Ever wonder why Uranus and Neptune – the two farthest planets from the sun with similar masses, sizes, and atmospheric compositions – are different shades of blue?

Scientists now have the answer as to why Neptune has a distinctly bluer colour compared to Uranus, which is a pale shade of cyan.

What have we learned?

Research suggests that a layer of concentrated haze that exists on both planets is thicker on Uranus than a similar layer on Neptune. The layer ‘whitens’ Uranus' appearance more than Neptune’s. If there were no haze in the atmospheres of Neptune and Uranus, both would appear almost equally blue.

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Okay, but how did scientists find that out?

As per a press release from NASA, astronomers used  Gemini North telescope, the NASA Infrared Telescope Facility, and the Hubble Space Telescope to develop a single atmospheric model that matches observations of both planets.

This is important because previous investigations of these planets’ upper atmospheres had focused on the appearance of the atmosphere at only specific wavelengths.

However, this new model, consisting of multiple atmospheric layers, matches observations from both planets across a wide range of wavelengths. The new model also includes haze particles within deeper layers that had previously been thought to contain only clouds of methane and hydrogen sulfide ices.

“This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” explained Patrick Irwin, Professor of Planetary Physics at Oxford University and lead author of a paper presenting this result in the Journal of Geophysical Research: Planets.

“It’s also the first to explain the difference in visible colour between Uranus and Neptune,” he added.

How does the model work?

The team’s model consists of three layers of aerosols at different heights.

The key layer that affects the colours is the middle layer, which is a layer of haze particles  that is thicker on Uranus than on Neptune. The team suspects that, on both planets, methane ice condenses onto the particles in this layer, pulling the particles deeper into the atmosphere in a shower of methane snow.

Because Neptune has a more active, turbulent atmosphere than Uranus does, the team believes Neptune’s atmosphere is more efficient at churning up methane particles into the haze layer and producing this snow. This removes more of the haze and keeps Neptune’s haze layer thinner than it is on Uranus, meaning the blue colour of Neptune looks stronger.

An unexpected bonus!

“We hoped that developing this model would help us understand clouds and hazes in the ice giant atmospheres," said Mike Wong, an astronomer at the University of California, Berkeley, and a member of the research.

“Explaining the difference in colour between Uranus and Neptune was an unexpected bonus!”

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