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

way back In the late 1980s, the Voyager 2 spacecraft visited Uranus and Neptune. During the flybys we got the first close-ups of these ice giants. Even then, planetary scientists noticed a distinct color difference between the two. Yes, they both wear shades of blue. But if you look closely at Uranus, you will see a faceless light blue planet. Neptune, on the other hand, features interesting clouds, dark bands, and dark spots that come and go. They are all set against a darker blue background.

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

Uranus and Neptune: The Big Picture

Voyager 2’s observations of Uranus and a false color image of the planet.NASA

To understand what’s happening, let’s look at what we know about Uranus and Neptune. They are called “ice giants” because their cores are high in oxygen, carbon, nitrogen, and sulfur. These elements are called “ice” because they are volatile chemical compounds that freeze at around 100K. However, the key to the different colors lies in the atmosphere of the planets. Each has hydrogen, helium and methane as their main components. It is in these gas blankets that the “weather” of each planet takes place. It turns out that to observe how the weather plays out in these two worlds, you need a lot of observations – both visually and in other wavelengths of light – over long periods of time.

Voyager gave astronomers a taste of what’s “out there.” This led to more long-term observations with other ground-based and space-based observatories. These studies are revealing details about the weather on these worlds and what exactly is causing Uranus to turn so pale.

Neptune as imaged by Voyager.NASA

Why so blue on Neptune and not so blue on Uranus? Fog. However, scientists have had to explain the existence and activity of nebulae in the upper atmospheres of giant ice planets. So they made 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 from multiple atmospheric layers on Uranus and Neptune.

“This is the first model to allow simultaneous observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” Irwin said in a press release. He is the lead author of an article that will showcase the team’s model in an upcoming issue of the magazine Journal of Geophysical Research: Planets.

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

Irwin’s team analyzed a series of observations of both planets in the ultraviolet, visible and near-infrared (from 0.3 to 2.5 microns). The data comes from the Near-Infrared Integral Field Spectrometer (NIFS) at the Gemini North Telescope (part of the NOIRLab) and archive data from the NASA 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 shows striking differences between two worlds that otherwise look quite similar. Of course, when we look at each planet in visible light, we see the different shades of blue. The infrared and other data goes deeper, revealing details about haze layers. The team’s model shows three layers of aerosols at different altitudes 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 appears thicker on Uranus than on Neptune.

The cause and effect of haze on Uranus and Neptune:

This diagram shows three aerosol layers in the atmospheres of Uranus and Neptune as modeled by a team of scientists led by Patrick Irwin. The altitude scale in the diagram represents pressure above 10 bar. The deepest layer (the Aerosol 1 layer) is thick and consists of a mixture of hydrogen sulfide ice and particles produced by the interaction of the planets’ atmospheres with sunlight. The key layer affecting the colors is the middle layer, a layer of haze particles (dubbed the Aerosol 2 layer in the publication), which is thicker on Uranus than on Neptune. Above these two layers is an extensive layer of haze (the Aerosol 3 layer) that is similar to the layer below but thinner. On Neptune, large particles of methane ice are also forming over this layer. Courtesy International Gemini Observatory/NOIRLab/NSF/AURA, J. da Silva/NASA/JPL-Caltech/B. Jonsson

Let’s look at how nebulae form on both planets. It turns out the process is roughly the same for everyone. Both have outer atmospheres rich in methane, which freezes out at about 91 K. This methane ice condenses on particles in the aforementioned Aerosol 2 layer, making the atmospheric particles slightly more massive. The result is methane “snow” that rains down on the layers below. Indeed, it appears to be a case of “permanent 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, turbulent atmosphere. This kicks up the methane “snow” particles, sending more snow and haze deeper into the atmosphere. Therefore, Neptune “cleans itself”, has a thinner haze layer and retains its pretty blue color. The same movement could also explain the dark spots on the planet.

Uranus, on the other hand, has a more sluggish atmosphere. There, not so much methane “snow” is whirled up and the haze particles are not pulled down. This means the haze layer persists and is thicker, giving a lighter pale blue hue on Uranus.

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

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