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So I've created a cold water-world super-earth where the majority of the planet is covered in ice caps, but has an equatorial water ocean that moves north and south with the seasons. Meanwhile, much closer to the poles, it grows cold enough that, combined with the high atmospheric pressure of the planet, liquid CO2 lakes, rivers, and seas form on top of the ice sheet, complete with CO2 rain.

Is this realistically possible? Or is the thick atmosphere required for CO2 to exist as a liquid circulate heat too much for the necessary temperature gradient to form?

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    $\begingroup$ The more unrealistic part might be the ocean moving north and south, unless it has a very long year. $\endgroup$
    – Christopher James Huff
    Oct 11, 2022 at 20:45

2 Answers 2

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You just need to compare the phase diagrams of water and CO2, and search for combinations of pressure and temperature (P,T) where both are liquid.

Phase diagram for water:

enter image description here

Phase diagram for CO2:

enter image description here

It looks like around 100 bars of pressure and above, both CO2 and water are liquid roughly between 0° and 25° Celsius: water is liquid between 0° and 300° Celsius, while CO2 is liquid between -50° and 25°, even though the closer you get to 25°, the closer you are to the critical point, so you wouldn't be able to distinguish between liquid and gaseous CO2.

Additionally, it's very likely that the two would mix, so you won't have water bodies and CO2 bodies as separated entities, but rather "carbonated water" everywhere. That would add to their being solid or liquid, as their solutions have different melting/boiling points.

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  • $\begingroup$ Surprisingly realistic in the end. The mixing should (afaik) lead to a bigger overlap of liquid temp/pressure gradients, so the center ocean can even be of okay size. Near the poles the Water should freeze out of the CO2 so imho still realistic. If I read the weight correctly however, Water Ice would be far more buoyant in a CO2 sea $\endgroup$
    – Hobbamok
    Oct 11, 2022 at 14:42
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    $\begingroup$ @Hobbamok Yes. Density of liquid CO2 is about 1.1 compared to liquid water about 1, and ice about 0.91 and solid CO2 about 1.6. So the dry ice on the bottom, the liquid CO2 on that, the water above that, and the water ice on the top. Mixing of liquid water and liquid CO2 would be pretty interesting. And changes in this mix when pressure or temperature changed would also be pretty interesting. $\endgroup$
    – BillOnne
    Oct 11, 2022 at 15:26
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    $\begingroup$ It looks to me like around 10 bar with a temperature gradient from -60 at the poles to 10 at the equator might be even closer to the original thought. Then there would be liquid water and gaseous CO2 at the equator, and frozen water and liquid CO2 at the poles. Although I guess there would still be solutions in both places. $\endgroup$
    – Mark Foskey
    Oct 12, 2022 at 2:05
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    $\begingroup$ High pressure liquid water and CO2 won't mix fully: youtube $\endgroup$
    – Juraj
    Oct 12, 2022 at 12:08
  • $\begingroup$ @Juraj thanks, very cool and insightful link $\endgroup$
    – Hobbamok
    Oct 24, 2022 at 9:25
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This is almost possible on Earth. Dry ice sublimates at ~ -80C and temperatures like this are commonly seen in Antarctica (deep ice cracks go all the way to -100C.

We do have an impressive difference of about 120-130C between the poles and the equator. What we don't have is enough partial pressure of CO2.

On the other hand, even 1/10 bar of CO2 will create a strong greenhouse effect, leading to equalization between equatorial and polar temperatures. See e.g. Venus. You can get your solar irradiation as low as you want and seasons as harsh as you want, but you won't get enough temperature difference in order to allow both liquid water AND solid CO2 on the same planet.

Edit: This answer assumes the wrong question - liquid water plus dry ice polar caps. Not sure why I got it wrong.

As the answer from L.Dutch suggests, at high enough partial pressure of CO2 water and CO2 mix freely and have a continuum of melting/freezing and evaporating/condensing temperatures.

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    $\begingroup$ Sure, parts of Antarctica occasionally get cold enough for dry ice to form -- but that's a long step from having a lake of liquid carbon dioxide, which requires enough pressure to let it melt instead of sublimating. $\endgroup$
    – Zeiss Ikon
    Oct 11, 2022 at 16:09
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    $\begingroup$ @ZeissIkon Lakes of carbon dioxide sometimes do form on the seafloor, where the pressure is high enough. They don't get very large or stick around very long, though, as they tend to dissolve into the surrounding water. $\endgroup$
    – Logan R. Kearsley
    Oct 12, 2022 at 14:34
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    $\begingroup$ @LoganR.Kearsley: chunks of more-or-less solid clathrate hydrate containing CO2 or CH4 or both are apparently stable enough to accumulate on the ocean floor, and apparently "Lakes of liquid CO2 in the deep sea" already naturally occur, stabilized by such clathrates. $\endgroup$
    – David Cary
    Oct 14, 2022 at 22:59

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