All the way back in In the late 1980s, the Voyager 2 spacecraft visited Uranus and Neptune. During the flybys we got to see the first close-ups of those ice giants. Even then, planetary scientists noticed a distinct color difference between the two. Yes, they both have shades of blue. But if you look closely at Uranus, you will see a colorless light blue planet. Neptune, on the other hand, has interesting clouds, dark streaks and dark spots that come and go. They are all set against a dark blue background.
So, why the difference? Planetary scientists have long-suspect aerosols (gas droplets in which liquids or dust are suspended) in any atmosphere. But according to a team of scientists studying the layers of the planets, the nebulae that create those aerosols may be only part of the story.
Uranus and Neptune: the big picture
To understand what is happening, let’s look at what we know about Uranus and Neptune. They are called “ice giants” because their cores contain large amounts of oxygen, carbon, nitrogen and sulfur. Those elements are called “ice” 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 main components. These blankets of gases are where each planet’s “weather” takes place. It turns out that you need a lot of observations — both visually and in other wavelengths of light — over long periods of time to see the weather play out in these two worlds.
Voyager gave astronomers a taste of what’s “out there.” That led to more long-term observations using other observatories on the ground and in space. Those studies reveal details about the weather in those worlds and what it does specifically to make Uranus so pale.
Why so blue on Neptune and not so blue on Uranus? hares. But scientists had to explain the existence and activity of nebulae in the upper atmosphere of giant ice planets. So they made a model. The work was done by a team led by Patrick IrwinProfessor of Planetary Physics at the University of Oxford in England.
Their model actually uses observations of multiple atmospheric layers on Uranus and Neptune.
“This is the first model simultaneously suitable for observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” explained Irwin in a press release. He is the lead author of a paper presenting the team’s model in an upcoming issue of the 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 ultraviolet, visible and near infrared wavelengths (from 0.3 to 2.5 micrometers). The data came from the Near-Infrared Integral Field Spectrometer (NIFS) on the Gemini North telescope (part of the NOIRLab) and archival 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 a surprising structure and activity in both spheres.
The resulting model reveals striking differences between two worlds that otherwise look quite similar. When we look at each planet in visible light, we naturally see the different shades of blue. The infrared and other data go deeper, revealing details about nebula 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, it is thick with haze particles and is called the Aerosol-2 layer. Both planets have that layer, but it’s the one that looks thicker on Uranus than on Neptune.
The Cause and Effect of Nebulae on Uranus and Neptune:
Let’s take a look at how nebulae are created on both planets. It turns out that the process is pretty much the same for everyone. Both have outer atmospheres rich in methane, which freezes at about 91 K. That methane ice condenses on particles in the above-mentioned Aerosol-2 layer, making the atmospheric particles slightly more massive. The result is methane “snow” that settles on the underlying layers. It actually seems like a case of “constant winter” at certain levels of each atmosphere.
However, there is a final twist that explains the color differences between the two planets. Neptune has an active, turbulent atmosphere. That churns up the methane “snow” particles and sends more of the snow and mist deeper into the atmosphere. Neptune “looks after itself”, thus has a thinner haze layer and retains its beautiful blue color. That same churning could also explain the dark spots on the planet.
Uranus, on the other hand, has a slower atmosphere. There’s not much churning of the methane “snow” there, and the mist particles aren’t pulled down. That means that the haze layer persists and is thicker, giving Uranus a lighter pale blue hue.
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