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Uranus is called a green planet, although it appears blue in many of the photographs including those taken by spacecraft, like Voyager 2, Hubble, and JWST. NASA's Uranus page mentions it to be bluish-green.

One of the reasons mentioned is due to the presence of methane in the atmosphere of Uranus. But if that's the case, why can't we see this green color in the photos? I am unable to exactly grasp the mechanism behind the green color.

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    $\begingroup$ Those photos you linked look green to me. I imagine you are like other people I have met who label all of the in-betweeny colours beweteen blue and green as blue. If you want to explain this to your 6th graders get a high quality gradient print out between 500nm and 450nm so the range is greatly expanded and ask the different students to point to where they think blue stops and green begins. $\endgroup$
    – DKNguyen
    Sep 17, 2023 at 20:35
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    $\begingroup$ Who calls Uranus a Green Planet? $\endgroup$
    – Nacht
    Sep 18, 2023 at 2:42
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    $\begingroup$ blue and green are not very distinctive due to the closeness of the wavelengths, so the definition varies and many languages don't even differentiate them $\endgroup$
    – phuclv
    Sep 18, 2023 at 11:48
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    $\begingroup$ Some photos like this one from Voyager 2 are definitely green. But I don't know if it is meant to be a true colour representation or not. $\endgroup$ Sep 18, 2023 at 12:17
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    $\begingroup$ Of the 3 first links I could only find a picture of Uranus in the first link: en.m.wikipedia.org/wiki/Voyager_2#/media/… $\endgroup$ Sep 18, 2023 at 23:04

5 Answers 5

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I think your issue is linguistic not astronomical. The way a language cuts up the space of colours into discrete names is varies strongly from one language to another, and even from one dialect to another. Translating colour names is fraught with difficulty.

Uranus has a colour that is approximated by an RGB value with approximately equal amounts of green and blue (I've found the value #B2D6DB, but monitors vary, so this isn't absolute). You might describe this as "light green-blue" or "light blue-green" or "pale teal" or "pale aqua" or "duck-egg", or some form of "turquoise".

However, the usual contrast in terms of colour is with Neptune, which much closer to an azure or cerulean blue. Uranus may not be "green", but it is "greener than Neptune".

The images below show the RGB value and a colour photo of Uranus by an Earth based observer which shows a turquoise colour

enter image description hereenter image description here

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    $\begingroup$ The image is cropped from four - each of them shows the same basic color, even the infrared ones. This one is called "straight RGB" but no information is given about color correction, white balance, etc. There's no reason to believe this one has any bearing on the "true color" of the light, which is a murky concept at best to begin with (cf. Billmeyer and Saltzman's Principles of Color Technology 3rd/4th edition) The image's color has been process SEVERAL times, for "internet color" possibly for your device's screen's pallet. $\endgroup$
    – uhoh
    Sep 17, 2023 at 9:55
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    $\begingroup$ to even begin to get "the color of Neptune" it will take either a proper RGB recording through a good quality Bayer filter followed by fastidious color correction for the camera's own bias, or better yet, staring from a recorded, calibrated photometric spectrum, then apply the proper projection on to RGB space. Good luck! See for example this answer to Need help simulating solar limb darkening $\endgroup$
    – uhoh
    Sep 17, 2023 at 10:00
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    $\begingroup$ "Translating colour names is fraught with difficulty." - So is specifying colours objectively. Monitor RGB values simply won't do it, for a large number of reasons. Better to speak in terms of the spectrum that is actually reflected from the planet's surface (intensity at each wavelength), either with or without the filtering of Earth's atmosphere. $\endgroup$ Sep 17, 2023 at 18:43
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    $\begingroup$ @KarlKnechtel I agree completely. ..."colours are hard"... But working with spectra is well beyond what is needed to explain this to a bunch of sixth grade kids. All you need is "It looks greeny-blue. More green than Neptune (because of the methane)" In this sense #B2D6DB is a "lie to children". $\endgroup$
    – James K
    Sep 17, 2023 at 19:34
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    $\begingroup$ Specifying or reproducing colors is immensely complicated, but you don't really need an objective definition of what exact shades qualify as "green" here. If you show someone pictures of Neptune and Uranus taken with similar processing, as long as it's reasonably accurate, and say "the green one", it'll be immediately obvious which one is meant. It's not really an attempt to describe the precise color. $\endgroup$ Sep 17, 2023 at 22:23
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This answer looks into why Uranus is "green" or at least greener compared with Neptune's blue.

Both planets have similar atmospheric compositions and would ordinarily be expected to show similar colors. But according to a study by the National Science Foundation's NOIRLab program, the greater atmospheric activity of Neptune's atmosphere tends to break up the smog of more complex hydrocarbons which, on Uranus, tends to make the externally observed color of Uranus whiter and greener than the methane would otherwise be expected to give. The technical reference is from [Irwin et al.1](https://doi.org/10.1029/2022JE007189), and is open access. The figure below, taken from this reference, shows the brighter transmission in Neptune at blue and violet wavelengths compared with Uranus, whereas Uranus becomes slightly brighter at longer wavelengths and less blue/violet selective overall.

enter image description here

We actually see a similar effect on Earth. In this question the whitening of the sky near the horizon is discussed in terms of increased Rayleigh scattering, which would be caused by the smoggy conditions on Uranus too. One answer demonstrates through photography that the sky hue on Earth also tends to become greener as it becomes whiter, matching well with the difference between Uranus and Neptune.

Reference (open access)

  1. Irwin, P. G. J., Teanby, N. A., Fletcher, L. N., Toledo, D., Orton, G. S., Wong, M. H., et al. (2022). "Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including dark spots". Journal of Geophysical Research: Planets, 127, e2022JE007189. https://doi.org/10.1029/2022JE007189
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  • $\begingroup$ It's amazing that the spectra of Neptune and Uranus are almost identical! $\endgroup$
    – dtn
    Sep 25, 2023 at 7:09
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This is just a mere illusion. In reality, the peak wavelength is actually between green and blue, that is slightly cyanish/turquoise. The reason this happens is because the atmosphere of Uranus contains methane, which has an absorption wavelength between yellow to red leading to blue/green reflected wavelength. In the albedo of 30% of the sunlight reflected by Uranus, blue green light is absorbed the most.

This wavelength is however scattered in our atmosphere, via atmosphere refraction, specifically Rayleigh scattering which leads to telluric absorption which leads to a few wavelengths being scattered more like blue wavelengths, which is precisely observed in the ground-based telescope images. In space-based telescopes where there is nill telluric scattering, the actual color is perceived.

Even though the green color is more sensitive to our light cones, green light is already absorbed by our atmosphere, before reaching our eye.

This is quite similar to how the Sun's peak wavelength is green, but we perceive it as yellow, except the atmospheric refraction is different in this case.

Space telescope:

Enter image description here

vs.

Ground-based telescope:

Enter image description here

It's not leaned towards pure green or pure blue. The Rayleigh scattering absorbs only few nms, like around 30-40 nm.

In conclusion:

This is a mere mirage by the atmospheric scattering of the Earth, aerosols like to absorb more green than blue due to their electrons and quantum mechanics even though Uranus is lying between green and blue.

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  • $\begingroup$ Actually, I have to explain this to 6th graders and I wasn't exactly able to understand how to explain this to them, since, I myself was a bit perplexed by reading the info. available on the internet. $\endgroup$ Sep 17, 2023 at 7:25
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    $\begingroup$ Hope it helped you $\endgroup$
    – Arjun
    Sep 17, 2023 at 7:49
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    $\begingroup$ "Rayleigh scattering absorbs only few nms, like around 30-40 nm" What does that mean? $\endgroup$
    – PM 2Ring
    Sep 17, 2023 at 13:38
  • $\begingroup$ It is a continuous "stretched out" function, not narrow. Related to the slope of it? $\endgroup$ Sep 17, 2023 at 16:18
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    $\begingroup$ Atmosphere does not have anything to do with sun looking yellow and not green. Objects of the same temperature viewed up close will also look white and not green. Even if the spectrum peaks at that wavelength, that is just a small fraction of the spectrum so all together it adds up to white. $\endgroup$ Sep 18, 2023 at 10:59
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A convenient pattern

Uranus’ color is somewhere between green and blue. Since a lot of space trivia is in the purview of elementary education, and since patterns are useful for memorizing, it’s convenient to fudge color names to fit the pattern:

  • Mars is red.
  • Jupiter is orange*.
  • Saturn is yellow.
  • Uranus is green.
  • Neptune is blue.

*If you squint, red + yellow = orange, and brown is a dark shade of orange. Again, fudging. “thick red, brown, yellow and white clouds“. “shades of white, orange, brown and red”. “shades of white, orange, brown, and red”.

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    $\begingroup$ Jupiter is not orange! $\endgroup$
    – Mike G
    Sep 18, 2023 at 4:49
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    $\begingroup$ Added footnote for orange Jupiter. $\endgroup$ Sep 18, 2023 at 5:41
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    $\begingroup$ @MikeG how can you say that! I ate Jupiter only yesterday, and the citrussey zest I experienced could not possibly have arisen if it were anything else but orange. $\endgroup$ Sep 18, 2023 at 10:58
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    $\begingroup$ @leftaroundabout So that's why I couldn't see it this morning. Before that, it was white with brown stripes. $\endgroup$
    – Mike G
    Sep 18, 2023 at 17:40
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    $\begingroup$ Ah, so that's why Pluto was delisted as a planet: It breaks the pattern by not being indigo or purple. $\endgroup$
    – dan04
    Sep 18, 2023 at 23:57
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Although both Uranus and Neptune exhibit a color palette ranging from deep blue to pale green, there is a prime difference in the atmospheric composition which determines the color difference.

In the outer layers of the atmosphere, Uranus has other hydrocarbons which are heavier than methane, mainly ethane and acetylene. These substances form tiny crystals that hang in the atmosphere which causes formation of fog and it lets through waves of different wavelengths with different efficiency and it is the deep blue color that is absorbed more efficiently and leaving behind a pale-green color which we perceive. Moreover, the atmosphere of Uranus is calmer than Neptune. So, this fog is not being torn into separate bands of clouds and hence it covers the entire planet more or less evenly.

On the other hand, Neptune also has such a layer of fog but it is much thinner and cannot obscure the blue color properly as there is not enough fog to absorb it.

enter image description here

Source: https://universemagazine.com/en/green-planet-for-the-green-holiday/

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