Why Uranus and Neptune do not have the same blue color

The two ice giants of the solar system, Neptune and Uranus, are blue, but not the same color. Can we explain exactly why?

Earth is not the only one holding the title of blue planet. On the edge of the solar system, we find the two ice giants, Neptune and Uranus. They are distinguished by a soft bluish color, but slightly different from each other: Neptune is navy blue, Uranus is pale blue. There is also a slight distinction of texture, as Uranus is a very uniform blue, while Neptune has nuanced streaks that show spots.

Uranus (left, light blue and pale, uniform) and Neptune (right, darker blue, with visible spots) photographed by Voyager 2. // Source: Nasa

However, the two planets are quite similar. They are almost the same size (Neptune: 49,244 km in diameter; Uranus: 50,724 km) and, above all, their composition has many points in common. The difference in color is therefore of interest to scientists, because in astronomy, this type of observation element constitutes information in itself.

Do we have, so far, the answer to this color difference? Yes and no. In early January 2022, a post was published announcing that he had identified the key. But the study has not been published in a scientific journal, it is currently only in pre-publication, so it has only a weak weight of evidence. What are the authors’ explanations?

Uranus and Neptune do not have the same atmospheric opacity

If the planets have a very similar structure, the key to the color difference is probably in the atmosphere. Therefore, the authors produced models by collecting observations in the visible and near infrared. The aim is to be able to observe the different atmospheric layers more accurately.

And in doing so, they were able to detect an important distinction in the photochemical fog layer. On most planets (including Earth), these fog particles appear when ultraviolet radiation from the Sun breaks down aerosol particles into the atmosphere.

In Uranus, this layer called Aerosol-2 is twice as opaque as in Neptune, which explains this paler, more uniform bluish appearance: “ Because these particles absorb UV, this explains the lower UV reflectivity observed in Uranus, and also explains why Uranus appears to have a paler blue color to the human eye than Neptune, as these particles are reflected in a visible spectrum close to white. “, write the authors.

The fact that the Aerosol-2 layer is less opaque on Neptune also explains why we can distinguish more shades — rats, spots — on its surface: being less opaque, less reflective, it allows us to see more “through” than for Uranus. .

This explanation does not provide a complete answer: it is only the How? “Or” Whatno Because. Why don’t the two planets have the same Aerosol-2 layer thickness? It seems, according to the authors, that the answer lies … in an even lower layer, Aerosol-1. At this level, in Neptune, methane would dissipate more efficiently than in Uranus, causing greater fog dissipation in the upper Aerosol-2 layer.

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To better understand Uranus and Neptune, we need to send a probe

While these hypotheses are firmly entrenched in the data obtained so far, sending a probe to these planets would go a long way in advancing research. Unfortunately, Neptune and Uranus are the big forgotten ones in space exploration at the moment: the last time a spacecraft looked at them was Voyager 2, in 1986, and it was just passing through because it was destined to leave the system. solar.

But that could change: in the spring of 2022, a committee report US National Academies points to Uranus as a “high priority” mission. The idea would be to deploy a probe in orbit around the planet (Uranus Orbiter and Probe). It is clear that this instrument would allow a better understanding of the atmosphere, its layers, its composition.

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