Why do Neptune and Uranus appear in different colors

• Neptune and Uranus have similar masses, sizes, and atmospheric compositions yet their appearances are in different shades of blue.

Reasons for different colours

• At visible wavelengths, Neptune is a rich, deep azure hue, while Uranus is a distinctly pale shade of cyan.
• So far difference was not explained because of the absence of similar data for comparison.
• Previous studies of the spectrum of each planet concentrated on individual wavelength regions.
• New comparison: researchers have developed a single atmospheric model that matches observations of both planets from NASA/ESA Hubble Space Telescope, as well as Gemini North telescope and NASA Infrared Telescope Facility.
  • This model shows that the haze around Uranus is thicker than that around Neptune.

Difference in atmosphere

• Uranus’s stagnant, the sluggish atmosphere makes it appear lighter tone than Neptune.
• If there were no haze in the atmospheres of both the planets, they would appear almost equally blue because of blue light being scattered in their atmospheres.
• Model leading to this conclusion describes three aerosol layers in the atmospheres of Neptune and Uranus.
• Critical layer that affects 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 particles in this layer, pulling particles deeper into the atmosphere.
  • Neptune has a more active, turbulent atmosphere than Uranus does, it is believed Neptune’s atmosphere is more efficient at churning up methane particles into the haze layer.
  • This removes haze and keeps Neptune’s haze layer thinner than it is on Uranus, with the result that the blue colour of Neptune looks stronger.

Exam track

Prelims take away

• Hubble Space Telescope
• Gemini North telescope
• Solar system