Secret behind blue hues of Neptune, Uranus discovered by scientists

The secret behind what gives ice planets Neptune and Uranus their distinctive blue hues has been discovered by Oxford University scientists. Neptune is a deeper blue than Uranus because it has a thinner layer of haze, scientists claim.

Like looking at distant hills on a summer evening when dust, smoke and other dry particles cast blue shadows over the landscape, the haze of these distant ice giants forms an atmospheric curtain which gives the two planets their distinctive blue color. They share many similarities in mass, size and atmospheric compositions. Neptune, however, has a noticeably deeper shade of blue than its planetary neighbor.

New research led by Oxford professor Patrick Irwin suggests that differences in the layers of haze are responsible. Irwin believes that if there were no haze in the atmospheres of either planet, they would both appear equally blue.

This is an image of the planet Uranus taken by the spacecraft Voyager 2 in 1986. (Credit: Oxford University / SWNS)

Using observations from the Hubble Space Telescope, the NASA Infrared Telescope Facility and the Gemini North Telescope, an international team of researchers has developed a model to understand the aerosol layers in the atmospheres of Neptune and Uranus.

“This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” Irwin explains in a statement. “It’s also the first to explain the difference in visible color between Uranus and Neptune.”

The team modeled three haze layers at different heights in the atmospheres of each planet. The middle layer of haze particles was found to be thicker on Uranus than on Neptune. It is this layer which is key to the color difference.

On both planets, methane ice condenses on the particles in this middle layer and it forms a shower of methane snow that pulls haze particles deeper into the atmosphere. Here, these haze particles cause condensation of hydrogen sulphide ice which forms a separate, deeper layer of cloudy haze.

Neptune. (Credit: Oxford University)

Because Neptune has a more active, turbulent atmosphere than Uranus, it is better at churning up the methane gas. This causes it to condense, produce snow and pull those middle-layer haze particles deeper into the atmosphere. Thus, the middle layer is thinner on Neptune than on Uranus and it therefore appears bluer.

In contrast, the excess haze on Uranus builds up in the planet’s stagnant, sluggish atmosphere, giving it a lighter, paler tone.

The team’s research also shows the presence of a second, deeper layer in the model that, when darkened, could account for dark spots occasionally visible on Neptune and more sporadically on Uranus, such as the famous Great Dark Spot (GDS-89) on Neptune observed by Voyager 2 in 1989.

While astronomers were already aware of the presence of dark spots in the atmospheres of both planets, they didn’t know which haze layer was causing them or whether they were caused by a thinning or darkening of a layer.

“We hoped that developing this model would help us understand clouds and hazes in the ice giant atmospheres,” says Dr Mike Wong, an astronomer and team member from the University of California, Berkeley. “Explaining the difference in color between Uranus and Neptune was an unexpected bonus.”

The paper is published in the Journal of Geophysical Research: Planets.

This diagram shows three layers of aerosols in the atmospheres of Uranus and Neptune, as modeled by a team of scientists led by Patrick Irwin. The height scale on the diagram represents the pressure above 10 bar. The deepest layer (the Aerosol-1 layer) is thick and composed of a mixture of hydrogen sulfide ice and particles produced by the interaction of the planets’ atmospheres with sunlight. The key layer that affects the colors is the middle layer, which is a layer of haze particles (referred to in the paper as the Aerosol-2 layer) 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 color of Neptune looks stronger.  Above both of these layers is an extended layer of haze (the Aerosol-3 layer) similar to the layer below it but more tenuous. On Neptune, large methane ice particles also form above this layer. (Credit: Oxford University)

Report by South West News Service writer Danny Halpin

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