Assume an exoplanet like a colder (roughly habitable temperatures throughout) Venus with liquid water and a layer of supercritical carbon dioxide at the bottom of it's ocean basins. What would happen to that water? Would the water:

  • A: Form a separate ocean on top of the supercritical CO2 with minimal mixing because of CO2's higher molecular weight? (44.009 g·mol^−1 vs 18.015 g·mol^−1)
  • B: Form a separate ocean below the supercritical CO2 because of it's higher density? (79.08 to 996.16 kg/m3 vs 1000 to 1050 kg/m3)
  • C: Dissolve into the supercritical CO2 to form a solution? How much water could the CO2 dissolve by volume?
  • D: Would the situation never occur in the first place, since the water ocean would have to form before the supercritical CO2, and after that the gaseous CO2 would never be able to reach the ocean depths in enough quantity to form a supercritical CO2 layer?

This answer Seems to imply that it would be B, but I'm not sure if that accounts for the water dissolving and the CO2's supercriticality. Water and Carbon Dioxide phase diagrams

  • $\begingroup$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. $\endgroup$
    – Community Bot
    May 23 at 20:27
  • 1
    $\begingroup$ I think you can safely rule out A. Molecular weight doesn't factor into whether something can float, only the density (ice cubes in water). You might have to take into account the pressure at the bottom of a water ocean to see whether supercritical CO2 can exist down there. It might get down there via subduction of plates, though not sure about Venus' tectonic activity. $\endgroup$
    – BMF
    May 23 at 20:27
  • $\begingroup$ it doesn't work the water is WAAAAYYYYY denser than the CO2, the density of a gas is far to low compared to a solid. there is two to three orders of magnitude difference in the density of CO2 from liquid to gas. this is why increasing the temperature of a sealed container of water just enough to turn in to into a gas turns it into a bomb. $\endgroup$
    – John
    May 23 at 20:58
  • $\begingroup$ Given the number of supercritical dryers in existence, water will definitely dissolve into the carbon dioxide, at least to some extent. $\endgroup$
    – camelccc
    May 23 at 21:58
  • $\begingroup$ Perhaps look up Lake Nyos disaster. A deep lake that CO2 entered from the bottom and saturated the lake. It depends on where the CO2 is entering from. $\endgroup$ May 24 at 0:37

1 Answer 1


The CO2 evaporates and some of the water freezes.

The water's thermal mass dwarfs the CO2's, and it's denser. The CO2 rushes upwards and goes subcritical. The solid CO2 fraction of what results sublimes off.

The effect is very much like dry ice in water. It's cold afterwards, and tastes pleasantly sour.

  • $\begingroup$ So option D then. I'll expand the supercritical depth so there can be a supercritical fog above the water ocean. Hopefully It won't be too dark. $\endgroup$
    – duke-of-k
    Jun 12 at 22:08

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