In the solar system, there are two planets which are almost similar, Uranus and Neptune. Their size is about the same, and so is their composition and structure. But when looking at images of the two planets, one can immediately see that Neptune has a color approaching azure, with swirling storms visible. For its part, Uranus has a teal blue color and does not present any visible stain. We can therefore wonder why the colors of these two planets, which are so similar, are different.
A team of researchers has looked into the question of the colors of Neptune and Uranus, and their paper on the subject is awaiting peer review on the arXiv pre-print server. According to the scientists behind the study, led by planetary physicist Patrick Irwin of the University of Oxford, an extensive layer of haze would dilute the hue of Uranus.
It is thus the presence of this layer that would make Uranus paler than Neptune.
The structure of the atmospheres of the two planets
According to the measurements made by scientists, the structure of the two planets is almost the same. Both have a rocky core surrounded by a mantle of water, ammonia, and methane ice. Then there is a gaseous atmosphere made mainly of hydrogen, helium and methane. And finally, there is the upper atmosphere. According to scientists, this atmosphere would be arranged in layers.
During their study, Irwin and his colleagues analyzed visible and infrared observations of Uranus and Neptune. They then created new models of the atmospheric layers. In these models, the two planets each had a layer of photochemical haze. This is created when ultraviolet radiation from the Sun breaks down aerosol particles in the atmosphere.
This layer is called Aerosol-2 by scientists, and they have found that on Uranus it appears twice as opaque as on Neptune. This is what would differentiate the two planets.
The explanation behind the difference
According to what can be read in the article, the particles in the Aerosol-2 layer can absorb UV rays. This explains why Uranus has a lower UV reflectivity and appears to have a paler blue color to the naked eye. The lower opacity on Neptune also explains why dark spots can be seen there.
Below the Aerosol-2 layer is the Aerosol-1 layer which has a deeper tint. The spectral signature of this layer matches that of ice and dark haze. The team of scientists thus thinks that it is in this Aerosol-1 layer that the dark forms such as the spots and bands that can be observed on Neptune are born. If Neptune’s Aerosol-2 layer is thinner and more transparent, these dark shapes will be all the more visible.
For the moment, researchers do not yet know why the Aerosol-2 layer of Neptune is thinner than that of Uranus. They believe, however, that Neptune’s atmosphere may be better able to clear haze by snowing methane more efficiently than on Uranus.