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

Uranus and Neptune share an incredible amount. Only her color, for reasons inexplicable until recently, is slightly different. Scientists have now discovered – somewhat to their surprise – why.

At the edge of our solar system are two ice giants: Uranus and Neptune. It could be two brothers. They have approximately the same mass, approximately the same volume, and the composition of their atmosphere is almost identical. But what is remarkable is that outwardly they are very different; Uranus is somewhat “pale” than Neptune’s bright blue.

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For a long time it was not clear how this difference in color could be explained. But in Journal of Geophysical Research: Planets The researchers are now coming up with a suitable explanation. According to scientists, the atmospheres of both Uranus and Neptune contain a layer consisting of thick nebulae. But in the case of Uranus, this nebula is a bit thicker, which makes the planet appear fainter at visible wavelengths.

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

How it works?
In fact, using models, researchers test ideas or predictions about specific 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. Especially the clouds and fog that occur in them. With this model, the researchers attempted to simulate an atmosphere that would be very similar to telescopes observing at corresponding wavelengths that would be observing 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.

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color difference
Thus, using the model, the researchers were able to describe the atmospheres of Neptune and Uranus in a way that was identical at different wavelengths to what the aforementioned telescopes saw. The model describes the atmosphere as consisting of three layers of aerosols located at different altitudes in that atmosphere. The color difference will then appear in the middle layer, referred to in the research article as the “aerosol layer-2”. On both planets, aerosols can be found in this layer, where methane ice condenses, after which the particles become heavier and are drawn deeper into the atmosphere as methane ice. Since Neptune’s atmosphere is more energetic and turbulent than that of Uranus, Neptune would be much better (and faster) able to transport methane particles into that middle layer of the atmosphere and drop them as methane snow. As a result, the nebula in this atmospheric layer on Neptune is thinner than the nebula on Uranus. This results in the color difference we see, according to the researchers.

As mentioned, Neptune and Uranus are both blue in visible wavelengths. This is because its atmosphere is rich in methane, which mainly absorbs red light from the sun and reflects blue light from the sun. However, Uranus’ atmosphere has a noticeably thicker nebula layer that masks the bright blue color. “The aerosol-2 layer in the atmosphere of Uranus is a factor of more than twice as thick as that of Neptune,” the researchers wrote. “And since these particles (which make up this nebula, ed.) absorb ultraviolet light, this may explain why Uranus is a bit bluer to the human eye than Neptune.” Because the light these particles reflect back – back into our eyes – is a little whiter. Thus the nebula makes Uranus a little lighter than blue. If the nebula on Uranus – like the Neptune nebula – were half thinner, the planet would be a bright blue like Neptune. Photo: NASA/JPL-Caltech/B. Jónsson.

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

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With that, it now appears that pulsing Uranus has given up one of its secrets. But there is much more than that; There is a lot we do not know about this planet. That is why scientists urged NASA a few months ago to create a mission to Uranus. This should provide a wealth of information about the ice giant, as well as increase our understanding of similar planets orbiting other stars.

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