Results 1 to 15 of 15

Thread: What would cause a hypothetical gas giant to look green?

  1. #1
    Join Date
    Jun 2008
    Posts
    729

    Question What would cause a hypothetical gas giant to look green?

    Uranus and Neptune, for instance, appear blue due to atmospheric methane's absorption of infrared light. My question is what would make a gas giant have a green color. I'm assuming it's possible because I don't see why not.

    I wonder if the type of the host star would have anything to do with it.
    “Of all the sciences cultivated by mankind, Astronomy is acknowledged to be, and undoubtedly is, the most sublime, the most interesting, and the most useful. For, by knowledge derived from this science, not only the bulk of the Earth is discovered, but our very faculties are enlarged with the grandeur of the ideas it conveys, our minds exalted above their low contracted prejudices.” - James Ferguson

  2. #2
    Join Date
    Jul 2005
    Posts
    18,483
    Sudarsky Class V (very hot, with silicate and iron clouds) have predicted spectra that suggest they might look green. Methane, in high enough concentrations, is predicted to produce a green sky. And every now and then a science fiction writer invokes chlorophyll (from micro-organisms or buoyant plant life in the upper atmosphere) to turn a gas giant green.
    The host star is probably not particularly relevant. Our eyes are very good at adapting to the white point of whatever illuminant is available.

    ETA: It's not methane's infrared absorption that produces its colour in the visible spectrum.

    Grant Hutchison

  3. #3
    Join Date
    Jun 2008
    Posts
    729
    Quote Originally Posted by grant hutchison View Post
    Sudarsky Class V (very hot, with silicate and iron clouds) have predicted spectra that suggest they might look green. Methane, in high enough concentrations, is predicted to produce a green sky. And every now and then a science fiction writer invokes chlorophyll (from micro-organisms or buoyant plant life in the upper atmosphere) to turn a gas giant green.
    The host star is probably not particularly relevant. Our eyes are very good at adapting to the white point of whatever illuminant is available.

    ETA: It's not methane's infrared absorption that produces its colour in the visible spectrum.

    Grant Hutchison
    Thanks, Grant! I hadn't heard of that classification. Very interesting.

    What did you mean by that last line? Isn't methane the reason Uranus and Neptune appear blue?
    “Of all the sciences cultivated by mankind, Astronomy is acknowledged to be, and undoubtedly is, the most sublime, the most interesting, and the most useful. For, by knowledge derived from this science, not only the bulk of the Earth is discovered, but our very faculties are enlarged with the grandeur of the ideas it conveys, our minds exalted above their low contracted prejudices.” - James Ferguson

  4. #4
    Join Date
    Aug 2006
    Posts
    3,579
    Quote Originally Posted by Fiery Phoenix View Post
    Quote Originally Posted by grant hutchison View Post
    ETA: It's not methane's infrared absorption that produces its colour in the visible spectrum.
    What did you mean by that last line? Isn't methane the reason Uranus and Neptune appear blue?
    It's not the IR, it's the visible red. At least, I assume that's what Grant means.

  5. #5
    Join Date
    Jul 2005
    Posts
    18,483
    Yes, infrared absorption is irrelevant to visible colour.

    But although it seems to be an article of faith on the internet that "Uranus and Neptune are blue because methane is blue", methane really isn't that blue. It depends a bit on conditions, but methane's visible absorption spectrum takes out a fairly large amount of orange and red wavelengths, and a little bit of green - the metameric mix of blue and yellow should leave it green overall. Hence Peter Ward's book about the early Earth, Under A Green Sky.
    If you dig around, you'll find a few sources that say there must be some unidentified yellow-absorbing chromophores to account for the blue tint, particularly of Neptune.

    Grant Hutchison

  6. #6
    Join Date
    Aug 2006
    Posts
    3,579
    Quote Originally Posted by grant hutchison View Post
    Yes, infrared absorption is irrelevant to visible colour.

    But although it seems to be an article of faith on the internet that "Uranus and Neptune are blue because methane is blue", methane really isn't that blue. It depends a bit on conditions, but methane's visible absorption spectrum takes out a fairly large amount of orange and red wavelengths, and a little bit of green - the metameric mix of blue and yellow should leave it green overall. Hence Peter Ward's book about the early Earth, Under A Green Sky.
    If you dig around, you'll find a few sources that say there must be some unidentified yellow-absorbing chromophores to account for the blue tint, particularly of Neptune.

    Grant Hutchison
    Does Rayleigh scattering come into effect? Other planets might have coloured gasses strong enough to overshadow Rayleigh scattering, but perhaps on Neptune and Uranus, they would otherwise be pale and virtually colorless, leaving Reyleigh scattering to dominate. I'm speculating here.

  7. #7
    Join Date
    Jul 2005
    Posts
    18,483
    I'm sure Rayleigh contributes, but by viewing the atmosphere from different angles, planetary scientists are able to tease out the relative contributions of various mechanisms. In the rather magisterial textbook Neptune and Triton, the conclusion at the end of the long chapter on clouds and hazes was that the blue colour actually comes from some unidentified chromophore in the 3.8-bar base cloud (the lowest visible layer in the atmosphere), rather than the overlying methane-containing atmosphere.
    It's a pretty old text, but based on the only close-up observations we've ever had.

    Grant Hutchison

  8. #8
    Join Date
    Jun 2008
    Posts
    729
    Quote Originally Posted by grant hutchison View Post
    Yes, infrared absorption is irrelevant to visible colour.

    But although it seems to be an article of faith on the internet that "Uranus and Neptune are blue because methane is blue", methane really isn't that blue. It depends a bit on conditions, but methane's visible absorption spectrum takes out a fairly large amount of orange and red wavelengths, and a little bit of green - the metameric mix of blue and yellow should leave it green overall. Hence Peter Ward's book about the early Earth, Under A Green Sky.
    If you dig around, you'll find a few sources that say there must be some unidentified yellow-absorbing chromophores to account for the blue tint, particularly of Neptune.

    Grant Hutchison
    Understood.

    I've thought about chlorine, but I've only ever seen it associated with terrestrial planets. It has a distinct yellow-green tint.
    “Of all the sciences cultivated by mankind, Astronomy is acknowledged to be, and undoubtedly is, the most sublime, the most interesting, and the most useful. For, by knowledge derived from this science, not only the bulk of the Earth is discovered, but our very faculties are enlarged with the grandeur of the ideas it conveys, our minds exalted above their low contracted prejudices.” - James Ferguson

  9. #9
    Join Date
    Sep 2003
    Posts
    13,023
    Quote Originally Posted by DaveC426913 View Post
    Does Rayleigh scattering come into effect? Other planets might have coloured gasses strong enough to overshadow Rayleigh scattering, but perhaps on Neptune and Uranus, they would otherwise be pale and virtually colorless, leaving Reyleigh scattering to dominate. I'm speculating here.
    Though I'm not well-versed on the likelihood of other atmosphere color results, Rayleigh scattering explains the notable effect in at least two cases, ignoring Earth's sky: Mars and Saturn.

    Mars' blue sunsets in the region around the Sun apparently is due to selective scattering where the CO2, I think, is just large enough to scatter the slightly longer wavelengths of the spectrum allowing blue light to be the visible winner. Selective scattering may not ever work well such that a narrow band (i.e. green) within the visible spectrum would be favored over the other colors. Blue and red are at the end of the spectrum, so broader scattering effects will give them the advantage. I'm certainly no expert.

    When the upper atmosphere of Saturn clears from time to time, it's northern hemisphere can become quite blue... APOD image here. Though they are very likely natural colors, I don't recall if the colors in those Cassini images are enhanced.
    We know time flies, we just can't see its wings.

  10. #10
    Join Date
    Jul 2005
    Posts
    18,483
    Quote Originally Posted by Fiery Phoenix View Post
    Understood.

    I've thought about chlorine, but I've only ever seen it associated with terrestrial planets. It has a distinct yellow-green tint.
    Chlorine is so reactive, however, it's unlikely to stay in an atmosphere for any great length of time. You'd need a continuous source of production - science fiction writers have invoked chlorine biospheres, for instance, with chlorine occupying the role that oxygen (also very reactive) does on Earth.

    Grant Hutchison

  11. #11
    Join Date
    Jun 2008
    Posts
    729
    Quote Originally Posted by grant hutchison View Post
    Chlorine is so reactive, however, it's unlikely to stay in an atmosphere for any great length of time. You'd need a continuous source of production - science fiction writers have invoked chlorine biospheres, for instance, with chlorine occupying the role that oxygen (also very reactive) does on Earth.

    Grant Hutchison
    I figured as much. Like a photosynthesis that produces chlorine in favor of oxygen, though I imagine this would require an underlying supply of hydrogen chloride and/or water, which isn't something you'd expect on gas giants.

    Class V gas giants are probably what I'm looking for.
    “Of all the sciences cultivated by mankind, Astronomy is acknowledged to be, and undoubtedly is, the most sublime, the most interesting, and the most useful. For, by knowledge derived from this science, not only the bulk of the Earth is discovered, but our very faculties are enlarged with the grandeur of the ideas it conveys, our minds exalted above their low contracted prejudices.” - James Ferguson

  12. #12
    Join Date
    Aug 2006
    Posts
    3,579
    Quote Originally Posted by George View Post
    Though I'm not well-versed on the likelihood of other atmosphere color results, Rayleigh scattering explains the notable effect in at least two cases, ignoring Earth's sky: Mars and Saturn.

    Mars' blue sunsets in the region around the Sun apparently is due to selective scattering where the CO2, I think, is just large enough to scatter the slightly longer wavelengths of the spectrum allowing blue light to be the visible winner. Selective scattering may not ever work well such that a narrow band (i.e. green) within the visible spectrum would be favored over the other colors. Blue and red are at the end of the spectrum, so broader scattering effects will give them the advantage. I'm certainly no expert.

    When the upper atmosphere of Saturn clears from time to time, it's northern hemisphere can become quite blue... APOD image here. Though they are very likely natural colors, I don't recall if the colors in those Cassini images are enhanced.
    Yeah, although given the OP's wording we can assume s/he was asking about what a planet would look like from a distance, not from the surface.

  13. #13
    Join Date
    Sep 2003
    Posts
    13,023
    Quote Originally Posted by DaveC426913 View Post
    Yeah, although given the OP's wording we can assume s/he was asking about what a planet would look like from a distance, not from the surface.
    Yes, but the scattering affects both observing angles, which is obvious in the case for Saturn.

    But your point raises an interesting question for Mars, the selective scattering there causes more of the red end of the spectrum to leave the atmosphere, favoring a redder Mars. But color seen is based on proportionality of each color (SED) and I doubt the scattering from the vacuum-like density of air on Mars gives much of a bump to its color, but I could easily be wrong. If the horizon looks blue for an observer on the surface then obviously enough scattering took place to cause that so perhaps Mars does look a little (perhaps a tiny bit) more red than otherwise for observers above Mars.
    We know time flies, we just can't see its wings.

  14. #14
    Join Date
    Aug 2006
    Posts
    3,579
    Quote Originally Posted by George View Post
    Yes, but the scattering affects both observing angles,
    I know. It's just tangential to the OP's question.

    Quote Originally Posted by George View Post
    But your point raises an interesting question for Mars, the selective scattering there causes more of the red end of the spectrum to leave the atmosphere, favoring a redder Mars.
    Hrm. Blue scatters more than red. So the red will go straight through (and not be seen from space).


    Quote Originally Posted by George View Post
    But color seen is based on proportionality of each color (SED) and I doubt the scattering from the vacuum-like density of air on Mars gives much of a bump to its color, but I could easily be wrong.
    Agree. I doubt it has much effect.

    The OP is talking about gas giants, which are all atmo.

  15. #15
    Join Date
    Sep 2003
    Posts
    13,023
    Quote Originally Posted by DaveC426913 View Post
    Hrm. Blue scatters more than red. So the red will go straight through (and not be seen from space).
    That is the usual scheme but not on Mars...

    Mars blue horizon.jpg

    Ironic huh? The reds are scattered more than the blues because of "selective scattering". It's my understanding (meaning subject to change by others) that a significant number of particles about the same size as the wavelength of red light (say 700nm) is causing red to scatter far more than the blue light, contrary to our sky and most others, no doubt.

    The OP is talking about gas giants, which are all atmo.
    Yes, but Saturn is a gas giant. What if the upper atmosphere of Saturn had particle sizes that of red light, similar to Mars, would we see a red color for that atmosphere? Perhaps, but a mixture of sizes, which is likely the case for hypothetical atmospheres, will produce a more balanced scattering effect and yield the color of the light source (ie white).
    We know time flies, we just can't see its wings.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •