Starship Asterisk*

Voyager 2 views of Uranus (left) and Neptune (right) during its 1980s flybys of the planets. Credit: NASA/JPL-Caltech/B. Jónsson

---

Astronomers may now understand why the similar planets Uranus and Neptune are different colors. Using observations from the Gemini North telescope, the NASA Infrared Telescope Facility (IRTF), and the Hubble Space Telescope, researchers have developed a single atmospheric model that matches observations of both planets. The model reveals that excess haze on Uranus builds up in the planet’s stagnant, sluggish atmosphere and makes it appear a lighter tone than Neptune.

Neptune and Uranus have much in common — they have similar masses, sizes, and atmospheric compositions — yet their appearances are notably different. At visible wavelengths Neptune has a distinctly bluer color whereas Uranus is a pale shade of cyan. Astronomers now have an explanation for why the two planets are different colors.

New research suggests that a layer of concentrated haze that exists on both planets is thicker on Uranus than a similar layer on Neptune and ‘whitens’ Uranus's appearance more than Neptune’s. If there were no haze in the atmospheres of Neptune and Uranus, both would appear almost equally blue. ...

• Journal of Geophysical Research: Planets (online 23 May 2022) DOI: 10.1029/2022JE007189
• arXiv > astro-ph > arXiv:2201.04516 > 12 Jan 2022 (v1), 28 Apr 2022 (v2)

Wikipedia wrote:

At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium.[24] A trace amount of methane is also present. Prominent absorption bands of methane exist at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune's vivid azure differs from Uranus's milder cyan. Because Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.

NOIRLab wrote:

New research suggests that a layer of concentrated haze that exists on both planets is thicker on Uranus than a similar layer on Neptune and ‘whitens’ Uranus's appearance more than Neptune’s. If there were no haze in the atmospheres of Neptune and Uranus, both would appear almost equally blue.