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The ocean in question is mostly made ammonia, with some traces of ices (mostly water ice) at the bottom of the ocean. It also does have dissolved alcohols and salts though but these can be ignored if they complicate the calculations (so a pure ammonia ocean approximation should be fine). The ambient temperature is 225ºK, and the pressure is 4 atmospheres. The star is of G5V type and the planet has a semi-major axis of 1.2 AU.

In addition, please assume how this would look to human eyes, so I'm assuming that the visible light absorption spectrum of visible light of ammonia will be most relevant. Finally, I'm most interested in the color of the sea in a regular afternoon, but bonus points if you can give any changes in color during the night or sunset/sunrise. So, what color would this ammonia ocean look to humans?

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  • $\begingroup$ That depends of the color of the illuminant, doesn't it? If the illuminant is red, the ocean has no choice but to be red... Just like the water sea of Earth shows different colors at different times of the day and under different skies. $\endgroup$
    – AlexP
    Commented May 1 at 23:22
  • $\begingroup$ I don't exactly know, but I can provide information on the time of day and star also. $\endgroup$
    – Neil Iyer
    Commented May 1 at 23:23
  • $\begingroup$ Note that the dissolved salts, unless they are truly negligible, may dramatically change the color of the solution. Unlike dissolved salts in water... $\endgroup$
    – AlexP
    Commented May 1 at 23:32
  • $\begingroup$ I understand that, but this is an estimate in a story after all - it doesn't need to be absolutely perfect, but if anyones interested, the primary salts are ammonium salts like ammonium chloride, but regular ones are still present too. $\endgroup$
    – Neil Iyer
    Commented May 1 at 23:35

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Keep in mind the vapor pressure of ammonia at 225 K is 0.47 bar, so in equilibrium more than 10% of your atmosphere is ammonia. The color as perceived by a human will be approximately "oh my god the smell it burns!"

That said, looking at spectra for liquid ammonia from DOI https://doi.org/10.1039/F19888400657 and other sources, there's a bit of an absorption peak around 650 nm, but not much else going on (and it's a pretty weak peak); otherwise absorption drops gradually with increasing wavelength throughout the visual spectrum. For a bulk liquid, which has apparent color primarily from backscattering from a light source minus absorption, you'd thus expect less red returned to the viewer than other colors both from 650 nm, but more overall in the longer wavelength colors than towards blue. Overall, the absorption is so low that you're going to be dominated by any dissolved salts (by a large factor!) and then by surface reflections (like water), but probably in bulk a little bit towards the brown/orange/yellow side compared to water, when illuminated from a broadband white source.

From a distance, I'd be surprised if you can tell the difference between water and ammonia by the naked eye, given the dominant impact of the illumination source. From up close, I'd be surprised if you could tell the color at all over your eyes watering from the smell.

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    $\begingroup$ I should have specified this, but the humans (to survive) probably will be wearing some sort of oxygen tank and spacesuit... $\endgroup$
    – Neil Iyer
    Commented May 2 at 2:53
  • $\begingroup$ Good news! At -50 degrees Celsius, there's a reasonable chance an unsuited human would freeze to death before they notice the smell. $\endgroup$
    – codeMonkey
    Commented May 2 at 17:44
  • $\begingroup$ @codeMonkey, at -50 C, an unprotected human is going to develop near-immediate frostbite, but actually dying (or even losing consciousness) will take a while. $\endgroup$
    – Mark
    Commented May 2 at 23:30

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