In the series of stories that I'm writing, I have a star system with one of its outer planets being a world with a thick carbon dioxide atmosphere, similar to venus, but it's far enough out to be room temperature (and, at those pressures, for it to have oceans of liquid CO2)

But at those pressures and temperatures water still remains liquid. So you'd have both water and CO2 oceans. Would the oceans be a mixture of CO2 and water? Or would the heavier CO2 sink below the water and form something like a salad dressing where it separates out into layers? Or would the super criticality of the CO2 mean it's actually less dense than water and so would float above the water oceans?

  • 1
    $\begingroup$ On one hand the question speaks of "oceans" of carbon dioxide, meaning liquid CO2; on the other hand it speaks of supercritical CO2... Which is it? (And liquid CO2 is much less dense than liquid water at any temperature which could be called room temperature -- relative density between 0.6 at 30 °C and 0.78 at 20 °C.) $\endgroup$
    – AlexP
    Commented Feb 16, 2023 at 1:41
  • $\begingroup$ I'm not super familiar with the chemistry and the equilibrium between the two, but wouldn't the CO2 and Water combine to form an Acid instead of forming two distinct layers? That or you just have Ice and liquid CO2 $\endgroup$
    – Shadowzee
    Commented Feb 16, 2023 at 2:03
  • $\begingroup$ like what @Shadowzee mentioned, CO2 and H2O form H2CO3 which is carbonic acid. Incidentally, that's what makes your fizzy drink fizzy, so it'd probably be a sort of clear homogenous acidic liquid. But! CO2 is generally considered hydrophobic so it'd be something like oil, with boundary layers of carbonic acid. $\endgroup$
    – Harry Mu
    Commented Feb 16, 2023 at 2:05
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    $\begingroup$ @Shadowzee: CO2 and H2O do have a range of temperatures and pressures where they are both liquid; for example, at 20 °C and 59 atm they are both liquid. Only a limited amount of CO2 can dissolve in water (forming carbonic acid); that's about 1.5 grams of CO2 per liter of water at normal temperature and pressure. With increased pressure, more CO2 can be dissolved in water but still only a limited amount -- they are never fully miscible the way water and alcohol are. The point being that it depends on how much CO2 is there to dissolve: it may be that all of it is dissolved, or only part of it. $\endgroup$
    – AlexP
    Commented Feb 16, 2023 at 2:17

1 Answer 1


There are too many variables to give a definite answer. It primarily depends on the pressure, temperature and the proportion of carbon dioxide. At 20 degrees C and 100 atmospheres pressure both carbon dioxide and water should be liquids.

Although some carbon dioxide would dissolve in the water this would be limited and two phases should be present providing your “salad dressing” ocean (all other things being equal).

At this temperature and pressure liquid carbon dioxide has a density of 856 kg/cubic metre so the carbon dioxide should float above the water.

Using this calculator https://www.peacesoftware.de/einigewerte/calc_co2.php7 it appears that at temperatures where carbon dioxide and water are both liquids that carbon dioxide would always be the upper phase as carbon dioxide is only heavier than water below zero where pure water would generally have already frozen.

That said all other things are unlikely to be equal. There are all sorts of complicating factors:

Interaction with rocks and other materials in the ocean are likely to introduce significant impurities such as dissolved salts. If these are present in high concentration then it is possible that the water phase might still be liquid at -15 degrees C. At this temperature carbon dioxide is actually heavier than pure water. And although probably not heavier than cold salt water, the difference might be small enough to allow convection and other effects* to create a suspension of the two phase to some extent.

Freezing could also be interesting. If the temperature decreased sufficiently to freeze the top of the water layer, it might become less dense than the carbon dioxide layer above and float up through it to the ocean surface.

*Mixing to form suspensions of one phase in another are bound to occur to some extent caused by currents, tides, weather, rain, winds, seasonal temperature variation and the surfactant action of impurities etc.

  • $\begingroup$ "At 20 degrees C and 100 atmospheres pressure" carbon dioxide is a supercritical fluid, not a liquid. The critical point of CO2 is about 31 °C and 72.8 atm. $\endgroup$
    – AlexP
    Commented Feb 16, 2023 at 13:18
  • $\begingroup$ I was using this: files.mtstatic.com/site_4334/8794/… $\endgroup$
    – Slarty
    Commented Feb 16, 2023 at 16:51
  • $\begingroup$ It think it has to be above the critical temperature and critical pressure to be a truly supercritical fluid. If it's only above its critical pressure (and not above its critical temperature) its classified a compressible liquid. $\endgroup$
    – Slarty
    Commented Feb 16, 2023 at 17:05

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