# Is 50% CO2, 50% H2O ocean possible? If not possible, what other proportions are OK?

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

### 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:

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.

• 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. Commented Mar 3, 2021 at 11:16
• That would not be an ocean, but atmosphere. The question was about an ocean (with surface) Commented Mar 3, 2021 at 12:34
• @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.
– Ash
Commented Mar 3, 2021 at 16:13
• @Ash I don't think a supercritical fluid can have surface Commented Mar 3, 2021 at 17:34

This is the solubility of $$CO_2$$ in water

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

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.

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