Ice giant Uranus sees what pips and astronomers think they finally know why

Uranus and Neptune have an incredible amount in common. Only their color is, for inexplicable reasons until recently, slightly different. And scientists have now discovered – somewhat to their own surprise – why.

At the edge of our solar system are the two ice giants: Uranus and Neptune. And it could be twin brothers; they have about the same mass, about the same size and also the composition of their atmosphere is almost identical. But strikingly enough, they are very different outwardly; Uranus is a bit “paler” than bright blue Neptune.

haze
For a long time, no one knew how to explain this color difference. But in the Journal of Geophysical Research: Planets researchers are now offering a suitable explanation. According to scientists, the atmospheres of Uranus and Neptune contain a layer consisting of a thick nebula. In the case of Uranus, however, this nebula is slightly thicker, making the planet paler at visible wavelengths.

Model
The researchers draw this conclusion based on a model developed to describe the different layers of the atmospheres of Neptune and Uranus. And what makes this model unique – compared to comparable previous models – is that it takes into account how these layers are expressed in a wide range of wavelengths. “This is the first model to match observations of reflected sunlight in the ultraviolet to near-infrared range,” said study researcher Patrick Irwin. “It is also the first model that can explain the visible color difference between Uranus and Neptune.”

How it works?
In fact, using models, researchers test ideas or predictions about certain phenomena that cannot be easily tested in any other way. In this case, the model was supposed to give researchers a better picture of the atmospheres of Uranus and Neptune. And especially the clouds and mists that occur there. With the model, the researchers attempted to simulate an atmosphere that looked like telescopes observing at corresponding wavelengths would observe them at different wavelengths. In this case, the model was based on observations from the Gemini North Telescope, NASA’s Infrared Telescope Facility, and the Hubble Space Telescope.

color difference
And so, with the model, the researchers succeeded in describing the atmospheres of Neptune and Uranus in such a way that it was identical at different wavelengths to what the telescopes mentioned above saw. And the model describes atmospheres as being made up of three aerosol-filled layers located at different heights in that atmosphere. The color difference would then occur in the middle layer, referred to in the research paper as the “Aerosol-2 layer”. On both planets, aerosols could be found in this layer, on which methane ice condenses, after which the particles become heavier and are carried deeper into the atmosphere as methane snow. Because Neptune has a much more active and turbulent atmosphere than Uranus, Neptune would be much better (and faster) able to transport methane particles to this middle layer of the atmosphere and deposit them as methane snow. As a result, the nebula of this atmospheric layer on Neptune is thinner than that of Uranus. And that leads to the color difference we see, according to the researchers.

As mentioned, both Neptune and Uranus are blue at visible wavelengths. This is because their atmosphere is rich in methane, which primarily absorbs red sunlight and reflects blue sunlight. However, Uranus’ atmosphere has a much thicker layer of nebula that “muffles” the bright blue color. “The layer of Aerosol-2 in Uranus’ atmosphere is twice as thick as that of Neptune,” the researchers write. “And since these particles (which this nebula is made of, editor’s note) absorb ultraviolet light, this may explain why Uranus has a slightly bluer color to the human eye than Neptune.” Because the light that these particles reflect is – still in our eyes – a little whiter. And so the nebula makes Uranus a little lighter blue. If the Uranus nebula – like Neptune’s – had been half as thin, the planet would have been as bright a blue as Neptune. Image: NASA / JPL-Caltech / B. Jonsson.

Prime
It’s a somewhat surprising finding, said researcher Mike Wong. “We hoped this model would help us better understand clouds and nebulae in the atmospheres of ice giants. Explaining the color difference between Uranus and Neptune was an unexpected bonus!”

And with that, the pipse Uranus now seems to have given up one of its secrets. But there are many more; there is so much we don’t know about this planet. And that is why scientists urged the American space agency NASA a few months ago to set up a mission to Uranus. This should provide a wealth of information about the ice giant, as well as improve our understanding of similar planets orbiting other stars.