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I've been collaborating with someone on a new version of a planetary classification system for our sci-fi settings and I had an idea recently. From looking at a phase diagram for nitrogen, it should be possible for a world to exist which could have liquid nitrogen on its surface in equilibrium with a nitrogen atmosphere. I decided to try and make one in Space Engine to put in a star system for my worldbuilding project but found myself wondering something.

Would a nitrogen sea look white from orbit? Liquid nitrogen is described as colorless. Barring any contaminants, would that look white? I found plenty of info online for some other exotic oceans, but not for this.

Possible Appearance of a Liquid Nitrogen Sea World

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    $\begingroup$ Unlikely to be pure nitrogen so the impurities matter. Any oxygen would make it pale blue. Also incidentally note - no ice bergs as solid nitrogen would sink. $\endgroup$ – Slarty Jun 25 at 10:10
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    $\begingroup$ @Slarty Dissolved oxygen seems rather unlikely, unless there is some kind of psychrophillic life involved. $\endgroup$ – Logan R. Kearsley Jun 25 at 16:05
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As HDE 226868 noted, it is rather difficult to find data on the transmission / absorption spectrum of LN2.

However, we can make some guesses at what it should look like over large depths, based on comparing its structure with that of water.

Water strongly absorbs red and infrared wavelengths because it has lots of degrees of rotational and vibrational freedom, with molecular energy level gaps in the red to infrared range.

Molecular nitrogen, in comparison, has only one degree of vibrational freedom, and a much stronger, stiffer bond. Thus, while liquid nitrogen and liquid water are difficult to distinguish in the quantities in which liquid nitrogen is typically produced on Earth, both being colorless, we can expect nitrogen to remain effectively colorless, with minimal visible absorption, at much greater depths. The color of an ocean would thus be approximately the color of whatever is under it, and whatever is reflecting off of it.

There remains one other coloring effect, though: Rayleigh scattering, which is what makes the sky blue. A thick atmosphere of pure nitrogen would be just as blue as our thick atmosphere of mostly-nitrogen-and-some-oxygen, and you could expect the ocean to take on some of that color by reflection. On top of that, Rayleigh scattering also occurs in liquids, so extreme depths of liquid nitrogen would appear blue in transmission--thus, ocean on the limbs of the planet should be tinted lightly blue, and a deep ocean which reflects light back up from the floor will have the color of bright features of the floor shifted towards red, while the bulk of the ocean surface will be slightly blue in compensation.

The most likely color of that floor is white, due to precipitation of nitrogen ice under pressure. So, a world with a shallow nitrogen ocean should look mostly white, with blue tinges from the nitrogen atmosphere on the limbs, while a world with a deeper nitrogen ocean (possible on smaller worlds where pressure increases more slowly with depth) should shift more towards the unsaturated (whitish) blue due to liquid Rayleigh scattering, with orange-red patches where the seafloor is highly reflective.

Of course, if there is anything dissolved or suspended in that ocean, the color could change drastically. LN2 isn't a very good solvent, but the ultimate color of a world with an LN2 ocean could be determined by whatever life ends up developing in it, if any, and then it could be pretty much any color you want; such life would have to use some other intracellular biosolvent, but that just means the nitrogen ocean will be to it much like what the nitrogen atmosphere is to us--a medium that it moves through, and uses for temperature regulation, but which is otherwise biologically inert.

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    $\begingroup$ * Rayleigh scattering (cities in North Carolina do not scatter light in any unusual way) $\endgroup$ – UTF-8 Jun 25 at 11:42
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    $\begingroup$ @UTF-8 Neither does John William Strutt $\endgroup$ – user253751 Jun 25 at 14:37
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    $\begingroup$ @UTF-8 Thanks, fixed. $\endgroup$ – Logan R. Kearsley Jun 25 at 15:25
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    $\begingroup$ Ah right, scattering and reflection would shift things a bit toward blue. I should have remembered that. $\endgroup$ – Sam D. Jones Jun 25 at 17:10
  • $\begingroup$ Nice answer - you patched up all the spots mine fell short on (+1 earlier). Any idea how nitrogen differs from water in its rotational modes? $\endgroup$ – HDE 226868 Jun 25 at 22:04
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In its flyby of Pluto in 2016, the New Horizons spacecraft determined that the dwarf planet is at least partially covered by the frozen remnants of a liquid nitrogen sea. In its solid form, the ice looks nearly pure white, with impurities due to water ice:

Image showing liquid nitrogen on Pluto
Image credit: NASA/JHUAPL/SwRI

Liquid nitrogen, on the other hand, is colorless, not white. Therefore, its color will depend on how it absorbs incoming light. For example, water (which often appears colorless) is blue in oceans because it absorbs redder colors and reflects bluer colors. It's not easy to find optical absorption spectra for liquid nitrogen, unfortunately (far-infrared and ultraviolet spectra, though, are plentiful!), so we have to do some guessing.

I would be surprised if the optical properties of liquid nitrogen were drastically different than water, though, particularly given that both are naturally colorless (though it's possible I'm wrong). Plus, unlike water, nitrogen ice is denser than liquid nitrogen, meaning that if any formed, it would sink, rather than float. We would then have a clear/maybe-slightly-blue liquid above a whiteish ice, presumably leading to a whitish-bluish color.

Any planet cold enough to have a liquid nitrogen sea would likely be far from the star, making it favorable for nitrogen ice to form, too - particularly given that liquid nitrogen can only exist in a small range of temperatures.

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one possibility is this could be a tidally locked world orbiting a red dwarf or a white dwarf.

the night side could have oceans of nitrogen, and the day side an atmosphere of Nitrogen.

and it would have white colored skies mostly, with the color of the water reflecting white.

good choice of game by the way.

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    $\begingroup$ An atmosphere would transfer heat to the dark side by convection as is the case with the Arctic and Antarctic on Earth which have 6 month nights but the nitrogen in the atmosphere does not freeze. $\endgroup$ – Slarty Jun 25 at 10:09
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    $\begingroup$ @Slarty bartfast Well then someone was slacking on that part of the design, weren't they? $\endgroup$ – user253751 Jun 25 at 14:37
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    $\begingroup$ Yes, but I claim mitigating circumstances. They didn't order enough fjords to make it pay $\endgroup$ – Slarty Jun 25 at 14:56

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