Uranus and Neptune have an incredible amount in common. Only their color is, for reasons inexplicable 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 that 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 somewhat ‘paler’ than bright blue Neptune.

mist

For a long time it was unclear how this color difference can be explained. But in the Journal of Geophysical Research: Planets researchers are now coming up with an appropriate explanation. According to the scientists, the atmospheres of both Uranus and Neptune contain a layer consisting of a thick nebula. In the case of Uranus, however, that nebula is slightly thicker, making the planet appear paler at visible wavelengths.

Fashion model

The researchers draw that 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 previous, comparable models – is that it takes into account how those layers are expressed at a wide range of wavelengths. “This is the first model to match observations of reflected sunlight at ultraviolet to 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 does it work?

Using models, researchers actually 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 the researchers a better picture of the atmospheres of Uranus and Neptune. And especially the clouds and mists that occur in it. With the model, the researchers tried to simulate an atmosphere that looked just like telescopes that observe at corresponding wavelengths observe them at different wavelengths. In this case, the model was based on observations from the Gemini North Telescope, the NASA Infrared Telescope Facility and the Hubble Space Telescope.

Color difference

And so, with the model, the researchers managed to describe 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 the atmospheres as consisting of three aerosol-filled layers located at different heights in that atmosphere. The color difference would then arise in the middle layer, referred to in the research article 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 drawn 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 that middle layer of the atmosphere and drop them down as methane snow. As a result, the nebula in this atmospheric layer on Neptune is thinner than that on Uranus. And that leads to the color difference that we see, according to the researchers.

As mentioned, Neptune and Uranus are both blue at visible wavelengths. This is because their atmosphere is rich in methane, which mainly absorbs red light from the sun and reflects blue light from the sun. However, Uranus’s atmosphere has a significantly thicker nebula layer that ‘muffles’ the bright blue color. “The Aerosol-2 layer in Uranus’s atmosphere is a factor of two thicker than Neptune’s,” the researchers write. “And since these particles (from which that nebula is made up, ed.) absorb ultraviolet light, that may explain why Uranus has a slightly bluer color to the human eye than Neptune.” Because the light that these particles reflect is – again in our eyes – a bit whiter. And so the nebula makes Uranus a bit lighter blue. Had the nebula on Uranus — like Neptune’s — been half thinner, the planet would have been as bright blue as Neptune. Image: NASA / JPL-Caltech / B. Jónsson.

Bonus

It’s a somewhat surprising discovery, said researcher Mike Wong. “We hoped this model would help us better understand the clouds and nebulae in the atmospheres of ice giants. Explaining the difference in color 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 an awful lot we don’t know about this planet. And that’s why scientists urged the American space agency NASA a few months ago to set up a mission to Uranus. That should provide a wealth of information about the ice giant, as well as increase our understanding of similar planets orbiting other stars.