What color is the sunset on other planets?

Jun 1, 2020
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It depends on whether the planet's atmosphere is predominantly filled with gas or dust particles.

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Nice article, and great paper. I'll need more time to absorb more and reduce how scattered I am now. ;)

But... I would bet them an ice cream sundae that, to a human-Martian through a neutral filter, the disk of the Sun would not appear blue, and neither will it appear pink as in this article's depiction from the NASA rover. It will likely look just plain white because the brain likes to color correct the whitest and brightest object by making, in this case, the Sun the white object. Cameras do it also. [see Color Constancy]

I enjoyed the ironic fact that the Martian sky absorbs blue making it less likely to see the regions near the Sun as blue, but they note that the blues are only slightly less absorbed vs. the reds. The selective scattering, therefore, greatly dominates the blue effect since the particle sizes, apparently of dust not CO2, is in that ideal micron or slightly less range -- too big for blue photons to scatter but fine for scattering the red photons.

What is all too often misstated, but not in the article I don't think, the Sun's dominant energy wavelength isn't yellow or green, but blue. So blue light gives this scattering a boost. [Most seem to use a Planck distribution that puts the peak in the green, but we have very accurate spectral irradiance data that shows blue since the Sun is not a perfect blackbody, after all.

Speaking of irony, most spectral distributions use energy (in a derivative form) vs. wavelength. But light comes in the form of photons, though photon packets seems to model better. So what is the peak color if you plot photon flux density vs. wavelength. Hint - I don't really like the result. ;)

Hmmm, this gives me an idea...
 
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Another cool question is what happens to an extended object (non-point source) if your telescope aperture is, say, 10x that of the eye (pupil). How much will it gain in surface brightness (mag. per unit area)?
 
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Color is dependent on context, so it would be difficult to say exactly what the eye were to perceive colorwise on other planets. The brain has a tendency to want to “normalize” the color we see as well which would play a factor. Try reading under a colored lightbulb. At first everything is saturated that color but slowly your brain reacts making that color seem more like normal full spectrum light. Turn off the lamp and go in a normal lit room and suddenly it seems to be the color of the colored lightbulb for a few minutes. Try it out it’s quite a neat effect!
 
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The brain has a tendency to want to “normalize” the color we see as well which would play a factor. Try reading under a colored lightbulb. At first everything is saturated that color but slowly your brain reacts making that color seem more like normal full spectrum light. Turn off the lamp and go in a normal lit room and suddenly it seems to be the color of the colored lightbulb for a few minutes. Try it out it’s quite a neat effect!
Yes. There are many dramatic examples. The one I like perhaps the best is looking at an older style (tungsten) head lamp in the daytime as it will appear distinctively yellow on a clear day. At night, it becomes much whiter. "White balance" is the term used for cameras, though color constancy is often used for how the eye color shifts.

The key point, however, is that the brain favors the whitest and brightest light to be the reference to allow the best color shift. So, in broad daylight, the car lamp with its much lower color temperature will appear yellow compared the ambient white sunlight. But at night, the best "white" source is the headlamp so the brain shifts this source light white or close to white in order to color correct all that it illuminates in order to simulate what we would have seen under normal sunlight.

If so, then looking directly at the Sun from Mars, the Sun that might appear, by calculation, slightly blue, per the article, or slightly pink, per the image shown will be color shifted by our brains to become the dominant reference light source, as the Sun always is. I don't know this for certain, but I'm fairly sure this is correct.
 

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