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Based on the information given, what would be the color of the planet's sky?

Here is the composition of the planet's atmosphere:

  • Nitrogen: 64%
  • Ammonia: 19.7% (edit: as answers have pointed out, it wouldn't be possible to have this much ammonia in the atmosphere anyways, so I guess all of this percentage should become nitrogen)
  • Methane: 7%
  • Neon: 5%
  • Argon: 1.1%
  • Ethylene: 1%
  • Ethane: 1%
  • Hydrogen: 0.9%
  • Propylene: 0.2%
  • Others: 0.1%

The planet has a mass of 4 Earths and has a radius 1.6 times that of Earth. The atmospheric pressure is 4 atmospheres, and the average surface temperature is 225 K (-54.67 F or -48.15 C). In addition, the surface gravity is about 15.343 m/s² or 1.56 g.

The star type is G5V, which has a mass of 0.95 Solar masses, and a 0.95 Solar radius.

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    $\begingroup$ your Atmosphere is mostly made out of colourless gases with a star highly similar to our sun. Given that it would probably just look like earth around the poles. The only thing that may make a difference is methane but with 6% i don't know if this is has any big impact. $\endgroup$ Commented Apr 26 at 23:43
  • $\begingroup$ Methane clouds? Maybe, but sparse. In fact, not likely, from the phase-diagram. $\endgroup$ Commented Apr 26 at 23:48
  • $\begingroup$ @Escaped dental patient. Yeah maybe i just read that metan can appear green or something similar. $\endgroup$ Commented Apr 26 at 23:49
  • $\begingroup$ Ah, right. We need a physical chemist. @Fallenspacerock According to our astronomy stack, in Uranus' atmos it's the crystals that glint green. I guess it's cold enough in the high atmosphere for them to form. Not sure that would hold true for the OP's planet. $\endgroup$ Commented Apr 26 at 23:51
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    $\begingroup$ @NeilIyer having run the numbers through the usual gas retention formulas, it looks like neon might well be retained. Unfortunately, it also looks like your world might fit the criterial to be a gas dwarf as it could conceivably retain helium and potentially molecular hydrogen in its atmosphere, which will result in a much thicker and denser atmosphere. You might need to shrink it or heat it up a bit to prevent that happening. $\endgroup$ Commented Apr 28 at 9:58

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Brown:

(The surface of Titan, as taken by the Huygens probe, image credit ESA/NASA/JPL/University of Arizona via wikimedia

The key thing here is the formation of tholins from atmospheric methane under the influence of solar radiation. Have a look at Titan for a real-world example. Titan's atmosphere isn't that different to your world's. with roughly 94% nitrogen and only ~5.6% methane. Titan is a colder world than yours and so presumably further from its sun, but there's enough light at the top of the atmosphere to form a whole range of interesting hydrocarbons and organonitrogen things from photodissociation of methane. I don't doubt that a world as warm as yours in comparison will get more than enough UV to drive formation of plenty of tholins.

What you end up with, then, is a thick orangey-browny high atmosphere haze, and orangey-browny rain that pains the surface an orangey-browny color, too. I don't think you will have much need to worry about the greeny tones that atmospheric methane might contribute to clear skies, because there won't be any clear skies. The surface probably won't be visible from space, much like Titan, and there won't be much of a view of space from the ground, either.

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Based on the data provided and the star type, you can infer the color of the sky using the following ideas:

  1. Rayleigh Scattering: With nitrogen constituting 64% of the atmosphere, it scatters blue light similar to Earth's atmosphere, suggesting a predominantly blue sky

  2. Methane Absorption: The presence of 7% methane, which absorbs longer wavelengths (red), enhances the blue color and introduces a slight greenish tint, as methane transmits blue and green colors. This is similar to the case with Uranus

  3. Liquid Ammonia: Given the temperature of 225 K and a pressure of 4 atmospheres (405.3 kPa), ammonia is in its liquid phase. According to this publication, liquid ammonia absorbs UV light near 240 nm, which should not significantly affect the visible color spectrum

  4. Star Influence: The G5V star, slightly cooler than our Sun (5660 K), emits light that peaks in the green spectrum, similar to our Sun. Thus, the color spectrum emitted by the star should not differ much from Earth's

Combining these aspects, expect a vivid blue sky with potential greenish hints on this planet, shaped by nitrogen's scattering and methane's absorption under a G5V star. I will try to update my answer later with more detailed research if possible.

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Impossible

A 4atm atmosphere with about 20% ammonia makes about 0.8 bar partial ammonia pressure. However, ammonia is liquid at 225K, and its saturated vapor pressure is far below that (about 0.4 bar).

Thus, an atmosphere with these parameters would result a very quick, very strong ammonia rain. Then you would have an atmosphere where ammonia is much lesser. There would be probably interesting ammonia-based weather on your planet, but with much lesser ammonia content.

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    $\begingroup$ Extension: probably the result would be some ammonia seas as well (not too much - maybe a hundredth of ammonia, compared to the mass of oceans of the Earth). $\endgroup$
    – Gray Sheep
    Commented Apr 27 at 22:32
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    $\begingroup$ Vapor pressure of ammonia at 225K is 0.48 bar, which is ~10% of ambient at datum level on the OPs world. Contrast with water on earth, which has a vapor pressure of 1% of ambient at datum level, and still has measurable water vapor in the atmosphere. -1 for no research, and not reading the comments on the OP. $\endgroup$ Commented Apr 28 at 9:42
  • $\begingroup$ @StarfishPrime Your own source says about 0.48bar at 225K. OP has a 4bar atmosphere with 18% NH3, that would be about 0.79bar. That is significantly more as I expected, but still far impossible. I fixed the numbers in the post. -1 for your hairsplitting and generally hostile attitude on no reason. $\endgroup$
    – Gray Sheep
    Commented Apr 29 at 10:04

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