Neptune and Uranus: Commonality & Differences

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Neptune and Uranus have much in common, yet their appearance is in different shades of blue.

Key Points

Observations came from Gemini Observatory, a Program of NSF’s NOIRLab, and other telescopes reveal that excess haze on Uranus makes it paler than Neptune.
Commonality between Uranus and Neptune: They have similar masses, sizes, and atmospheric compositions.
At visible wavelengths, Neptune is a rich, deep azure hue, while Uranus is a distinctly pale shade of cyan.

Image Courtesy: NASA

Reason for difference in colours

One reason why this difference had not been explained so far was the absence of similar data for comparison. Previous studies of the spectrum of each planet concentrated on individual wavelength regions.
Uranus’s stagnant, sluggish atmosphere makes it appear to have a lighter tone than Neptune.
- If there was no haze in the atmospheres of Neptune and Uranus, both would appear almost equally blue as a result of blue light being scattered in their atmospheres.
The model leading to this conclusion describes three aerosol layers in the atmospheres of Neptune and Uranus.
- The key layer that affects the colours is the middle layer, which is a layer of haze particles that is thicker on Uranus than on Neptune.
- On both planets, methane ice condenses onto the particles in this layer, pulling the particles deeper into the atmosphere.
Neptune has a more active, turbulent atmosphere than Uranus does; Neptune’s atmosphere is more efficient at churning up methane particles into the haze layer.
- This removes more of the haze and keeps Neptune’s haze layer thinner than it is on Uranus, with the result that the blue colour of Neptune looks stronger.

Regarding Dark Spots

The model also helps explain the dark spots that are occasionally visible on Neptune and less commonly detected on Uranus.
While astronomers were already aware of the presence of dark spots in the atmospheres of both planets, they didn’t know which aerosol layer was causing these dark spots or why the aerosols at those layers were less reflective.
The team’s research sheds light on these questions by showing that a darkening of the deepest layer of their model would produce dark spots similar to those seen on Neptune and perhaps Uranus.

Single atmospheric model

Using observations from the Gemini North telescope, the NASA Infrared Telescope Facility, and the Hubble Space Telescope, researchers have developed a single atmospheric model that matches observations of both planets.
The model shows that the haze around Uranus is thicker than that around Neptune.

Additional Information

The red colours of the sunlight scattered from the haze and air molecules are more absorbed by methane molecules in the atmosphere of the planets.
- This process — referred to as Rayleigh scattering — is what makes skies blue here on Earth (though in Earth’s atmosphere sunlight is mostly scattered by nitrogen molecules rather than hydrogen molecules).
- Rayleigh scattering occurs predominantly at shorter, bluer wavelengths.
An aerosol is a suspension of fine droplets or particles in a gas.
- Common examples on Earth include mist, soot, smoke, and fog.
- On Neptune and Uranus, particles produced by sunlight interacting with elements in the atmosphere (photochemical reactions) are responsible for aerosol hazes in these planets’ atmospheres.
A scientific model is a computational tool used by scientists to test predictions about a phenomena that would be impossible to do in the real world.
Layers:
- The deepest layer (referred to in the paper as the Aerosol-1 layer) is thick and is composed of a mixture of hydrogen sulphide ice and particles produced by the interaction of the planets’ atmospheres with sunlight.
- The top layer is an extended layer of haze (the Aerosol-3 layer) similar to the middle layer but more tenuous.
- On Neptune, large methane ice particles also form above this layer.

Source: IE

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