Why are Uranus and Neptune different colors?  The answer is surprisingly banal

Why are Uranus and Neptune different colors? The answer is surprisingly banal

The way back In the late 1980s, the Voyager 2 spacecraft visited Uranus and Neptune. During the overflights, we were able to see the first close-up views of these ice giants. Even then, planetary scientists noticed a stark color difference between the two. Yes, they both sport shades of blue. But, if you look closely at Uranus, you see a featureless pale blue planet. Neptune, on the other hand, has some interesting clouds, dark bands and dark spots that come and go. They are all placed on a darker blue background.

So why the difference? Planetary scientists have long suspected the presence of aerosols (gas droplets containing suspended liquids or dust) in every atmosphere. But, according to a team of scientists studying the layers of planets, the haze created by these aerosols may only be part of the story.

Uranus and Neptune: overview

Voyager 2 observations of Uranus and a false color image of the planet.Nasa

To understand what’s going on, let’s look at what we know about Uranus and Neptune. They are called “ice giants” because their cores contain high proportions of oxygen, carbon, nitrogen and sulphur. These elements are called “ices” because they are volatile chemical compounds that freeze at around 100 K. However, the clue to the different colors lies in the atmospheres of the planets. Each has hydrogen, helium and methane as its main components. These gas blankets are where each planet’s “time” occurs. It turns out that you need a lot of observations – both visually and in other wavelengths of light – over long periods of time to watch the weather unfold in these two worlds.

Voyager has given astronomers a taste of what’s “out there.” This led to longer-term observations using other ground-based and space-based observatories. These studies reveal details about the weather in these worlds and what it does specifically to make Uranus look so pale.

Neptune, photographed by Voyager.Nasa

Why so blue on Neptune and not so blue on Uranus? Mists. But scientists had to explain the existence and activity of haze in the upper atmospheres of giant ice planets. So they created a model. The work was carried out by a team led by Patrick Irwin, professor of planetary physics at the University of Oxford in England.

Their model actually uses observations of several atmospheric layers on Uranus and Neptune.

“This is the first model to simultaneously scale observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” Irwin explains in a press release. He is the main author of an article presenting the team model in an upcoming issue of the Journal of Geophysical Research: Planets.

“It’s also the first to explain the visible color difference between Uranus and Neptune.”

Irwin’s team analyzed a set of observations of the two planets in ultraviolet, visible and near-infrared wavelengths (from 0.3 to 2.5 micrometers). The data comes from the Near Infrared Integral Field Spectrometer (NIFS) of the Gemini North Telescope (part of the NOIRLab) as well as archival data from NASA’s Infrared Telescope Facility. Images and data from the NASA/ESA Hubble Space Telescope also contributed to the study. Together, the data revealed surprising structure and activity in both atmospheres.

blue planets

The resulting model reveals striking differences between two worlds that otherwise seem quite similar. If we look at each planet in visible light, of course, we see the different shades of blue. Infrared and other data go deeper and reveal details about haze layers. The team’s model shows three layers of aerosols at different heights in the atmosphere. The layer that affects the colors is the middle layer, which is thick with haze particles and is called the Aerosol-2 layer. Both planets have this layer, but it is the one that appears thicker on Uranus than on Neptune.

The cause and effect of the mists on Uranus and Neptune:

This diagram shows three aerosol layers in the atmospheres of Uranus and Neptune, modeled by a team of scientists led by Patrick Irwin. The height scale on the diagram represents the pressure above 10 bar. The innermost layer (the Aerosol-1 layer) is thick and composed of a mixture of hydrogen sulfide ice and particles produced by the interaction of planetary atmospheres with sunlight. The key layer that affects colors is the middle layer, which is a layer of haze particles (referred to in the article as the Aerosol-2 layer) that is thicker on Uranus than on Neptune. Above these two layers is an extended layer of haze (the Aerosol-3 layer) similar to the layer below but more tenuous. On Neptune, large methane ice particles also form above this layer. Courtesy of Gemini International Observatory/NOIRLab/NSF/AURA, J. da Silva/NASA/JPL-Caltech/B. Jonsson

Let’s look at how the mists are created on both planets. It turns out the process is pretty much the same for everyone. Both have methane-rich outer atmospheres, which freeze at around 91 K. This methane ice condenses on the particles in the Aerosol-2 layer mentioned above, making the atmospheric particles slightly more massive. The result is a “snow” of methane that falls on the lower layers. It actually looks like a case of “constant winter” at certain levels of each atmosphere.

However, there is one final twist that explains the color differences between the two planets. Neptune has an active and turbulent atmosphere. This causes methane “snow” particles to rise and sends more snow and haze deeper into the atmosphere. Thus, Neptune “grooms”, has a finer layer of haze and retains its attractive blue color. This same churning may also explain the dark spots on the planet.

Uranus, on the other hand, has a slower atmosphere. There isn’t as much churning of the methane “snow,” and the haze particles aren’t pulled downward. This means the haze layer persists and is thicker, providing a lighter shade of pale blue over Uranus.

This article was originally published on Universe today by Carolyn Collins-Petersen. Read the original article here.

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