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So far I had no luck trying to find the visible absorption spectrum of CO₂ anywhere, all I get is the far infrared absorption spectrum and stuff like that.

If you just search "what color is liquid CO₂" it simply says everywhere that "it's colorless" but... That's also said a lot about other liquids such as water, but we all know that's not true. Water is not colorless at all, it just appears colorless in small quantities since it's a very transparent liquid, and one can only appreciate its slightly blue coloration in very large quantities such like in pools, lakes, oceans, etc. Water is not just blue because of Rayleigh scattering and the sky's reflection, it is also and mainly blue because it does in fact absorb more red and green light than it absorbs blue light, just like any other blue thing.

That being said, I don't think liquid CO₂ is colorless, I think it's just appears colorless in small quantities, but in very large quantities some color is perceivable, exactly just like it happens with water. But I might be totally wrong since that belief is based on absolutely nothing, that's simply how I guess it is since I have absolutely no idea and it's hard to imagine a liquid that totally ignores visible light lol

I had this question unanswered for a looong time, would love to finally get an answer.

Thanks in advance!

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    $\begingroup$ If the atmospheric reflection has little to no input, wouldn't this best be asked in the chemistry or physics stack? $\endgroup$
    – rek
    Commented Jul 6, 2021 at 17:58
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    $\begingroup$ This is an interesting question. There could be a difference as a liquid. We can look at water vs. liquid iron and there's an obvious difference in what they do or would look like from space - but there's that pesky Oxy atom in both water and CO2. I once read a paper that explained that all atmospheric gases scatter (Rayleigh) in the blue spectrum. They vary a bit, but it's basically blue. Therefore, as a gas, CO2 would also scatter blue. I'm comfortable recommending (but not answering) that liquid CO2 would also look blue from space based on that limited insight. $\endgroup$
    – JBH
    Commented Jul 6, 2021 at 18:56
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    $\begingroup$ Being a greenhouse gas that absorbs low reds and infra-reds and microwaves, would that not make it appear "blue-green" in large quantities? chem.purdue.edu/jmol/cchem/color.html $\endgroup$
    – JonSG
    Commented Jul 6, 2021 at 19:24
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    $\begingroup$ I'm not so sure now. I think we need better quality spectra in the visible range. I will try to do a deeper search later when I get the time. As an example of the problem - methane does absorb red light despite being shown not to in the link I provided and the deleted answer. And is the reason why Neptune appears blue. $\endgroup$
    – Slarty
    Commented Jul 7, 2021 at 9:44
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    $\begingroup$ @JBH yes I can see it. I was concerned that the spectra might not be accurate enough, but I think they are. I asked a question here: physics.stackexchange.com/questions/650055/… $\endgroup$
    – Slarty
    Commented Jul 7, 2021 at 22:55

8 Answers 8

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Blue

enter image description here

CO2 has the same transparency as H2O over the visible light range (380 to 700 nanometers).

At 3 atoms, CO2 also has about the same particle size (232 picometers) as H2O (282 picometers). Rayleigh scattering is largely dependent on particle size. CO2 is 17% smaller, which is squared in the scattering equation, giving you a slightly more intense scattering (2%) of all visible frequencies, including blue.

Based on these two data points, liquid CO2 oceans will look like H2O oceans in the visible light range. Which is to say, a kind of blue.

In the infrared range, things will be different. Water is black in infrared, where a CO2 ocean will continue to be largely transparent.

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As best I can tell, liquid CO2 is clear: it doesn't absorb any wavelengths in the visible light spectrum. In large quantities it would likely be subject to Rayleigh scattering, giving it a bluish color similar to a water ocean, which I suppose could be altered by particulates (Mie scattering).

No one that I can find has published the index of refractivity of liquid CO2, likely because it's not a particularly common substance. I have seen people note that the index of refractivity of gaseous CO2 increases with pressure, and my guess would be that liquid CO2 (at the pressures it exists) would be noticeably more refractive than water.

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As With Water, It Depends

CO2 is colorless to the human eye due to the mechanics of particle physics. It's liquid form is therefore a reasonable analog to H2O.

H2O has its color depending on circumstances. According to the National Oceanic and Atmospheric Administration, the 'default' color of H2O is 'blue,' but the effect that makes this so is fairly weak. The dominant coloration of water (and CO2) oceans is dependent on the size and nature of the body and what else is contained within it.

Particles in suspension in a body of clear liquid will easily overwhelm the coloration of those bodies. This is how oceans can appear green, red, black, and with the right pollutants any color you darn well please.

A thing to note with CO2 (From this fact sheet) is that the conditions which cause it to take liquid form also tend to cause a portion of it to form a solid - dry ice. Dry Ice is distinctly white. Depending on the conditions you are envisioning this liquid ocean of CO2 to exist within, it seems likely that from long distance these oceans would appear mostly white, as the ice portion of the water would overwhelm all other factors.

Various degrees of turbidity (depending on wave action, or even organic behavior for example) would further mess with this. For shallows, under a no-ice scenario, the coloration of the sea bed would also play a powerful role.

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    $\begingroup$ dry ice is white for the same reason ice and snow are white because of the surface texture or internal facets, not because the material itself is white. liquid CO2 is clear $\endgroup$
    – John
    Commented Jul 21, 2021 at 19:37
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    $\begingroup$ Sure, but the OP asks about a natural phenomenon - so the ice that does form does so under the same conditions as snow, etc. So it would appear white. The OP is asking about visual appearance, not about the pure chemistry. Water is also clear, but it almost never appears that way in large, naturally formed bodies. $\endgroup$
    – William Walker III
    Commented Jul 21, 2021 at 19:39
  • $\begingroup$ The question is about liquid co2 not solid, so the structurally induced whiteness of the solid state is irrelevant. there is no reason to suspect ice in the ocean at all. Don't confuse the conditions on earth for what the conditions on another body will be. $\endgroup$
    – John
    Commented Jul 21, 2021 at 19:42
  • $\begingroup$ @John You'll note that, by and large, that's precisely what I did. The only reason I bring up the ice thing at all is because that's where the source (FAO in this case) leads. It seems to me that you're keen to pick a fight, and I'm really not understanding why - can you reframe your objection? $\endgroup$
    – William Walker III
    Commented Jul 21, 2021 at 19:51
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Building on Ted Wrigley answer.

Our eye can "see" wavelengths between 380 and about 750 nanometers.

Wavelengths and visible spectrum

What makes water (H20) blue is that, on our visible spectrum, the absorption coefficient of the wavelengths which our eyes identify as "blue" is close to none, reflecting the rest of it (just like you pointed out).

Visible light absorption spectrum of pure water

I looked around CO2 absorption spectrums. Most of them either used a wavelength domain from ultraviolet to microwave, or solely focused on infrared. The ones where i got some answers were:

You should notice there that there is no absorption in the visible spectrum, making it basically clear, and if there is, it is so minimal that might not make a difference to the human eye.

Transmission spectra of some atmospheric gases.

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    $\begingroup$ It looks like it allows light through above about 4.5um? Something like this: i.sstatic.net/QWUu1.png $\endgroup$
    – AncientSwordRage
    Commented Jul 21, 2021 at 9:37
  • $\begingroup$ I believe the visible spectrum is between 0.4 and 0.75 um $\endgroup$
    – GFA
    Commented Jul 21, 2021 at 9:42
  • $\begingroup$ So some of the blue light is not allowed through... wouldn't that make it not-blue in colour? $\endgroup$
    – AncientSwordRage
    Commented Jul 21, 2021 at 10:15
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    $\begingroup$ Visible spectrum in nanometers (nm): between 380 and 750 nm. Multiplied by 0.001 (conversion rate of nm to um) we have Visible spectrum in micrometers (um): between 0.38 and 0.75 You are checking for values outside the visible spectrum, more specifically, between 4 and 7 um. $\endgroup$
    – GFA
    Commented Jul 21, 2021 at 10:29
  • $\begingroup$ OH! The conversion from UM to NM really really through me off, especially as the 'interesting' stuff seems to happen around 4 and 7 um in the above chart. Is it worth highlighting on the final image the bits we care about? Also why does the 0.4-0.7um part of the chart last not seem to reflect the sharp gradient on the first chart? $\endgroup$
    – AncientSwordRage
    Commented Jul 21, 2021 at 10:34
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Liquid CO2 does not absorb light in the visible spectrum.

wavelengths

for reference. source

Effects of Temperature on the Near-Infrared Absorption Spectra of Molecules in the Condensed States. I. Carbon Dioxide1. Waggener et al. Journal of Physical Chemistry 1967

Liquid carbon dioxide, in cells of several centimeters, does not absorb measurably in the wavelength region from 0.3 to 1.2 u- Its spectrum in the region between 1.2 and 2.5 u is shown in Figure 2, and the principal bands are identified.

Visible light is 0.39 to 0.76 so falls in that range. CO2 does absorb in the infrared and ultraviolet but not in visible light. The liquid CO2 ocean would be colorless.

I see it mooted that liquid CO2 would be blue like water because of Rayleigh scattering as happens in the sky. Water is not blue from Rayleigh scattering. It is blue because it absorbs in the red frequencies, and so with enough depth blue is what remains to scatter back to us.

http://www.webexhibits.org/causesofcolor/5B.html

Water’s intrinsically blue color is easy to see when the water is sufficiently deep, such as in the Caribbean and Mediterranean Seas, and in Colorado mountain lakes. Pure water and ice have a pale blue color, which is most noticeable at tropical white-sand beaches or in ice caves in glaciers. (Green colors are usually derived from algae.) The blueness of the water is neither due to light scattering (which gives the sky its blue color) nor dissolved impurities (such as copper). Because the absorption that gives water its color is in the red end of the visible spectrum, one sees blue, the complementary color of orange, when observing light that has passed through several meters of water. Snow and ice has the same intense blue color, scattered back from deep holes in fresh snow.

Of course if there were suspended particles in the CO2 ocean those might participate in Rayleigh scattering. Any sort of impurities could impart color because the CO2 will be optically clear.

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    $\begingroup$ WOW! That's an amazing answer thanks! I can't believe you actually found SOMETHING saying anything about the visible absorption spectrum of liquid CO2, that's great. $\endgroup$
    – DeMooniC
    Commented Jul 27, 2021 at 3:57
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Because of the intense pressure required to support the existence of liquid CO2, there would have to be a thick atmosphere, which makes me think that you wouldn't be able to see the liquid CO2 ocean from space. Even on a planet with gravity that would create such surface pressures there would still be a ridiculously thick atmosphere. Furthermore the concentrations of CO2 and other greenhouse gasses in the upper atmosphere would have an insane greenhouse effect that would make the planet insanely hot, perhaps so hot that it physically would not allow for liquid CO2 for very long. It would have to be an exoplanet, perhaps even some kind of 'planet like interstellar object'.

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  • $\begingroup$ Yeah I was thinking of exoplanets. You don't really need that much pressure to get CO2 oceans if the planet is cold enough, with 20 bars and -40°C you are good to go. Those conditions are not that hard to achieve if the planet is orbiting a bit far from a low mass red dwarf or if it is a moon of a cold gas giant, greenhouse effects wouldn't be that big of a deal and maybe, (correct me if I'm wrong) the atmosphere would still be a bit transparent at 20 bars? I don't know, chances are not in most of cases. What about something like looking to the ocean from a high mountain for example? $\endgroup$
    – DeMooniC
    Commented Jul 24, 2021 at 3:05
  • $\begingroup$ Pressure is a small factor in whether the atmosphere would be transparent or not. The only hope you have for explaining it is possibly saying that it is only composed of molecules which are very transparent. Things like atomized liquid water are transparent but can easily refract light (fog). So maybe it would be best to say that the planet's atmosphere is some times transparent? like earth. $\endgroup$
    – Westly Durkee
    Commented Aug 10, 2021 at 23:41
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Okay, I'm not very sure if my answer is correct, but I think that it the color would differ if in different depths, like water, and if, for example, the depth of the water/liquid CO₂ was, let's say, 1,000 feet, then I would say that the color of the water/liquid CO₂ would be maybe a dark navy blue, seen in space, of course, because of the depth. If the depth was at around 50 feet, then I'd say the water/liquid CO₂ would be a azure blue (again, seen in space), but again, I'm not very sure if my answer is correct. I would agree with you that liquid CO₂, just like water, is transparent, or having no color, in small quantities.

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  • $\begingroup$ And also, could you please not delete my answer? I took about 30 minutes writing this, and researching this. Thanks. $\endgroup$
    – Hermione Granger
    Commented Jul 19, 2021 at 22:50
  • $\begingroup$ someone deleted your answer? I was going to ask if you could supply supporting evidence for it, that may be why somebody took that action. It would certainly help your answer. $\endgroup$
    – BMF
    Commented Jul 19, 2021 at 23:53
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    $\begingroup$ What is your answer based on? 'The "VISIBLE ABSORPTION SPECTRUM OF LIQUID CO2" is 100% needed for anyone to win the bounty along with a somewhat detailed explanation.' $\endgroup$
    – rek
    Commented Jul 20, 2021 at 1:14
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Blue

Liquid carbon dioxide mainly exists when carbon dioxide is dissolved in water.

To exist in liquid state would requiere to dissolve in water, as pure water has a blue tint, this is the color that the Carbon dioxide ocean would acquire.

https://www.lenntech.com/hazardous-substances/carbon-dioxide.htm#:~:text=Although%20carbon%20dioxide%20mainly%20consists,soluble%2C%20when%20pressure%20is%20maintained.

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    $\begingroup$ Pure Carbon dioxide can exist in liquid form above 5.1Atm and between specific temperatures. A water ocean with dissolved Carbon dioxide is not what the OP asked about. $\endgroup$
    – Monty Wild ♦
    Commented Jul 20, 2021 at 0:12
  • $\begingroup$ @MontyWild sadly can't exist as an ocean in pure form since reaching a point of presure at an specific depth it will became solid. To achieve an ocean format it will need to be combined with another element. For this example I used water, you can combined with other element to achieve another color, or even change the wavelenght the start to adapt the color. $\endgroup$
    – Tridam
    Commented Jul 20, 2021 at 15:37
  • $\begingroup$ @Tridam But that applies to all liquids including water. Water would also solidify because of the pressure if an ocean gets too deep.How is CO2 different? Would it solidify sooner than water with less depth needed? Look, here is a screenshot showing the data of a proceudrally generated planet in the space sim "Space Engine": i.ibb.co/DtgtY07/image.png This is a procedurally generated, very massive greenhouse-heated 16°C terrestrial Moon of a gas giant with 2.83Km deep almost 100% pure CO2 seas. There is says the ocean floor is made of rock, this is based on real data. (1/2) $\endgroup$
    – DeMooniC
    Commented Jul 20, 2021 at 18:35
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    $\begingroup$ @Tridam If you don't trust the simulation, you can easily check with a CO2 phase diagram that for liquid CO2 at 16°C to solidify you would need oceans that get so deep that the pressure reaches around 5000 bars, at 2.83km of depth, as you canse check in the screenshot, the pressure "only" gets to 900 bars (2/2) $\endgroup$
    – DeMooniC
    Commented Jul 20, 2021 at 18:39
  • $\begingroup$ @DeMooniC I will check it as soon I reach home, this graph could help you define the depth of your ocean. qph.fs.quoracdn.net/… $\endgroup$
    – Tridam
    Commented Jul 20, 2021 at 21:23

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