Neptune and Uranus: Commonality & Differences

In News

• 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.

Source: IE