Scientists explain why Uranus and Neptune have different colours

Uranus and Neptune. NASA’s Voyager 2 spacecraft captured these views of Uranus (on the left) and Neptune (on the right) during its flybys of the planets in the 1980s. Credit: NASA/JPL-Caltech/B. Jónsson

Hubble Space Telescope, NASA Infrared Telescope, and Gemini Observatory observations show that Uranus is paler than Neptune because it has more haze, and that dark spots are caused by a darkerening of a second, deeper cloud/haze layer.

Now, astronomers may be able to figure out why Uranus and Neptune are different colours even though they are both planets. Researchers used data from the Hubble Space Telescope, the NASA Infrared Telescope Facility, and the Gemini North telescope to make a single model of both planets’ atmospheres. The model shows that too much haze builds up in Uranus’s slow-moving atmosphere, giving the planet a lighter colour than Neptune. The model also shows that there is a second, deeper layer that can cause dark spots in these atmospheres, like the famous Great Dark Spot (GDS) that Voyager 2 saw in 1989.

Even though Neptune and Uranus have a lot in common, like having similar masses, sizes, and atmospheres, they look very different. At wavelengths that humans can see, Neptune is much bluer than Uranus. Astronomers have now figured out why this might be the case.

According to new research, a layer of concentrated haze that covers both Uranus and Neptune is thicker on Uranus than on Neptune and “whitens” Uranus’s appearance more than it does Neptune’s. If there wasn’t any haze in Neptune and Uranus’s atmospheres, they would both look almost the same shade of blue.

A model made by an international team led by Patrick Irwin, Professor of Planetary Physics at Oxford University, to describe the aerosol layers in the atmospheres of Neptune and Uranus led to this conclusion. Previous studies of the upper atmospheres of these planets had only looked at how the atmospheres looked at certain wavelengths. But this new model, which has many layers of atmosphere, matches observations from both planets at the same time over a wide range of wavelengths. In the deeper layers, where clouds of methane and hydrogen sulphide ices were thought to be all that was there before, the new model adds haze particles.

Professor Irwin, who is the lead author of a paper in the Journal of Geophysical Research: Planets about this result, says, “This is the first model that fits observations of reflected sunlight from ultraviolet to near-infrared wavelengths at the same time.” “It’s also the first to explain why Uranus and Neptune look different from each other.”

The team’s model is made up of three layers of different-sized aerosols. The middle layer, which is made up of haze particles and called “Aerosol-2” in the paper, is the most important layer for the colours. It is thicker on Uranus than on Neptune. The team thinks that on both planets, methane ice forms on the particles in this layer. The methane ice then pulls the particles deeper into the atmosphere like snow. The team thinks that because Neptune’s atmosphere is more active and turbulent than Uranus’s, it is better at churning up methane particles into the haze layer and making this snow. This gets rid of more of the haze and keeps the haze layer on Neptune thinner than on Uranus. This makes Neptune bluer than Uranus.

“We hoped that making this model would help us understand how clouds and hazes form in the atmospheres of ice giants,” says Mike Wong, an astronomer at the University of California, Berkeley, and a member of the team that came up with this result. “Telling me why Uranus and Neptune are different colours was a pleasant surprise!”

To make this model, Professor Irwin’s team looked at observations of the planets made in ultraviolet, visible, and near-infrared wavelengths (from 0.3 to 2.5 micrometres) with the NASA/ESA Hubble Space Telescope, the NASA Infrared Telescope Facility near the summit of Maunakea in Hawaii, and the Gemini North Telescope, also in Hawaii.

The model also helps explain why sometimes you can see dark spots on Neptune and less often on Uranus. Astronomers already knew that both planets’ atmospheres had dark spots, but they didn’t know which aerosol layer was to blame or why the aerosols at those layers were less reflective. The team’s research helps answer these questions by showing that darkening the particles in the deepest layer of their model would make dark spots very similar to those seen on Neptune and sometimes on Uranus.

Further information: P.G.J. Irwin et al, Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots, Journal of Geophysical Research: Planets (2022). DOI: 10.1029/2022JE007189.

Journal information: Journal of Geophysical Research: Planets

Source: University of Oxford