Green-appearing sun atmosphere composition

Question

What would the composition of a planetary atmosphere would cause a sun to appear green (i.e. green sunlight) for most of the day? I don't mean a green sky, as similar questions have asked. If there were to be such an atmosphere, what colors would appear at sunrise or sunset?

Answer 1

This is remarkably difficult to achieve, and may be impractical.

The sun appears yellowish to humans. because the shorter wavelength bluer light gets scattered preferentially by Rayleigh scattering, and the remaining light is a mix of greens and yellows and reds that kinda looks yellowish to human eyes. If you scatter more light, there's less green in the remaining sunlight and it goes orangey. If you scatter less light, there's more blue in the remaining sunlight and it looks more white.

This means that unless you're literally tinting the atmosphere with colored particles, you can't easily have a green sky and a green sun at the same time. If you are tinting the atmosphere, you're going to a) have problems with the residence time of the colored particles (they're gonna fall or be washed out of the sky eventually) and b) replacing them as older particles are removed. Thick greenish clouds will also make it harder to actually see the sun in the first place, and you're more likely to end up with an effect like an overcast day.

If you were able to conjure up a stable gas that absorbed longer wavelengths of light but was largely transparent to shorter wavelengths like greens and blues, the combination of scattered blue light and absorbed red light might do the job you want. You'd still probably have a bluish sky as a result of Rayleigh scattering, but the gas would cause the colour of the remaining transmitted sunlight to be greener. The sky would also likely be much darker at dawn and dusk though I'm not sure what sort of colour would result... some orangey-reds might get through, but you might have dark blue fading into a sort of dark yellowy-green fading into black at sunset, for example.

Methane might be one candidate here. It is a nice simple gas that can be produced naturally and abiotically and can be found in great quantities in the outer solar system, and is what gives Neptune and Uranus their blue colors. It is a heavy enough molecule that you don't need a super-Earth's worth of gravity to hold on to it. It doesn't play well with atmospheric oxygen, of course, so you must have a reducing atmosphere. Actually simulating a methane-rich atmosphere of sufficient thickness to produce greenish sunlight is left as an exercise for the reader, as that's far too complex a task for me right now, but it doesn't seem completely implausible.

Another problem to consider is the formation of hazes in the atmosphere... prebiotic Earth had a methane rich atmosphere which was believed to be quite hazy, and Titan's nitrogen/methane atmosphere is similarly hazy. This is at least in part due to UV light from the sun causing photodissociation of methane (which also means you need a way to replenish atmospheric methane... possibly a surface rich in methanogens?). The freed hydrogen escapes into space, and the remaining hydrocarbon radicals hook up to form stuff like tholins. Tholin hazes are what makes Titan's sky as brown as it is. Keeping a methane world's atmosphere crystal clear is probably impractical, but at least you now have a completely different set of technical problems to worry about.

I'm not aware of any other conveniently available and stable gasses which will do the job.

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ETA: star colors.

These curves are derived from Planck's law of black body radiation. The first bar undeneath shows the portion of the emitted spectrum that's visible, and a plausible representation of its color. Underneath that is a color temperature indicator which shows the rough color of a star whose spectrum peaks at that point.

This should help show you some of the tradeoffs here... a bright G-class star like the sun has lots of shorter wavelength light, but the downside is that it has more blue light to scatter away and more UV light to form hazes. A cooler K-class star produces proportionally half as much UV (there's a hand calculator for thus on hyperphysics) but there's still a lot of blue and proportionally more red that needs to be absorbed. A cooler still M-class star produces much less blue and UV but then there's much less green and a lot more red to deal with.

Basically, outside of chlorophyll, nature hates green. Consider having your color sensitivity adjusted so cyan and yellow things look greener instead... this might be the simplest solution!