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Can a planet have its whole surface covered with a 50% carbon dioxide and 50% water ocean (the substances being diluted in each other)?

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Yes - with temperatures and pressures so extreme they blur the definition of "Ocean" (and "liquid")

So there's this crazy thing called "a supercritical fluid", which behaves as both a liquid and a gas when sufficient temperature and pressure has been reached. (So - it's partially an ocean - depending on how strict your definition of "liquid" is).

Not too much of a stretch - it's in the "liquid" part of the triple point diagram:
enter image description here

From the linked wikipedia page, carbon dioxide reaches it at 304K at 7.38 MPA. Water reaches it at 647K at 22.064MPA. So to get both supercritical, you're looking at a planet around 700K, 30MPA. Basically get venus and triple the pressure.

According to wikipedia, any two supercritical fluids can generally be mixed together into a fully homogeneous solution. So long as you pick a point of temperature and pressure that's supercritical for both CO2 and H20, you can mix the two into a perfectly dissolved fluid.

At 700K, 30MPA, CO2 is 219mg/cc, water is about 320mg/cc. A approx 2:3 mix by volume will be a approx 50:50 mix by weight.

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  • $\begingroup$ That much pressure will be tricky. A hot Neptune type planet might fit the bill... something that formed in the outer regions of a planetary system and then migrated inwards, but not too far inwards. $\endgroup$ Commented Mar 3, 2021 at 11:16
  • $\begingroup$ That would not be an ocean, but atmosphere. The question was about an ocean (with surface) $\endgroup$
    – Anixx
    Commented Mar 3, 2021 at 12:34
  • $\begingroup$ @Anixx Depends on your definition of Ocean really. But If you need a surface to make something an ocean by that definition, just add an atmosphere of something that doesn't go supercritical at these temperatures and pressures. I rounded the pressure up a bit in the answer so it would mix all the way down, but if you pull it down to about 23MPA but keep temperature at 700K, I reckon gaseous sulfuric acid (h2so4) would make an atmosphere above the supercritical water/co2 mix that would hold it in the supercritical state, leaving a nice surface at the threshold. $\endgroup$
    – Ash
    Commented Mar 3, 2021 at 16:13
  • $\begingroup$ @Ash I don't think a supercritical fluid can have surface $\endgroup$
    – Anixx
    Commented Mar 3, 2021 at 17:34
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This is the solubility of $CO_2$ in water

enter image description here

As you see, it is in g/kg. A 50% concentration doesn't look plausible. Moreover, in Earth like conditions $CO_2$ goes directly from solid to gas.

To have it liquid you would need higher pressures, as seen in the $CO_2$ phase diagram

enter image description here

Based on this paper, in the range -29 C, +22 C and 15 to 60 atm, the solubility of $CO_2$ in water goes from 0.02% to 0.10%. This can be expected, considering that $CO_2$ is non polar while water is.

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  • $\begingroup$ Oh I see. So, 3% CO2 is possible. What about the opposite, the most is CO2 but a few percent is water? $\endgroup$
    – Anixx
    Commented Mar 3, 2021 at 6:06
  • $\begingroup$ And, 10 bars is not a big problem either. $\endgroup$
    – Anixx
    Commented Mar 3, 2021 at 6:06
  • $\begingroup$ @Anixx, 3 grams in 1 kg is 0.3% $\endgroup$
    – L.Dutch
    Commented Mar 3, 2021 at 6:07
  • $\begingroup$ What happens when the both CO2 and H2O are liquid? Will they mix? Or will they be like oil and water? $\endgroup$
    – Anixx
    Commented Mar 3, 2021 at 6:08
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    $\begingroup$ @nzaman I think it is incorrect to compare gas volume to liquid volume. $\endgroup$
    – Anixx
    Commented Mar 3, 2021 at 6:17

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