Is a dense CO2 atmosphere compatible with an ocean

Question

As part of an ongoing effort to update a planetary classification system for sci-fi settings, I'm trying to nail down if an old idea for an exotic type of life-bearing world is actually plausible and I'm not sure how the various factors balance out. The idea is a "Wet Venus" sort of world, with much higher air pressure than Earth, made mostly of CO2, with lesser amounts of nitrogen and oxygen. Oxygen is a lifting gas in this atmosphere, helping an aerial ecology form in the upper layers of the atmosphere, leaving the surface dark under multiple cloud layers.

The trick I'm having trouble figuring out is how plausible is the CO2 remaining in the presence of an ocean of water. On the one hand, Henry's Law states that there will come a saturation point where CO2 is balanced between air and ocean. On the other hand, CO2 dissolves into an acid and most research I've seen on Earth's ancient atmosphere seems to say that the oceans got rid of the primal mostly-CO2 atmosphere. On the gripping hand (for the sci-fi fans) an acidic ocean would also make the formation of things like limestone not a thing, which would help keep CO2 in the air.

Which way does this balance out? The dense CO2 really helps make floating life doable, but is that plausible? Would having the world be more volcanic be needed?

Answer 1

I all depends on what kind of ocean planet you want

If there is any land and the oceans are relatively shallow (read more then 1% land), you will still have a functioning carbon silicate cycle. As said cycles efficiency depends on the temperature of the planet, a CO2 atmosphere is only plausible in the outer habitable zone. Though such a planet could easily become too cold while entering a snowball climate, resulting in the freeze out of the vital atmospheric CO2. This would result in a bigger version of Europa, Triton or Enceladus until the sun gets older and hot enough to remelt this high albedo world.

Ocean planers with deep oceans that still have a water-seafloor interface will be wildcards for the most part. Both a quick runaway greenhouse or a long term snowball are plausible. There might be a window of stability, but is it big enough to survive for billions of years?

Both these cases have dubious chances for surface habitability, but their seafloor should be teeming with life that cares little for the atmospere, as long as a runaway state can be avoided. The final case is pretty uninhabitable to begin with and the atmospere won't help either.

Stereopalagic ocean worlds have so much water that it solidifies into exotic ices deep down, forming a second mantle. Ganymede, Titan and Callisto are cases of this in the solar system.

The ability of the ocean to dissolve CO2 increases with decreasing temperature. Thus, at low surface temperatures the ocean can bind an increasing amount of CO2 which removes the important greenhouse gas from the atmosphere where it is needed to retain habitable conditions on the surface. For higher temperatures on the other hand, less CO2 can be dissolved in the ocean and starts to accumulate in the atmosphere which results in the build-up of a massive CO2 dominated atmosphere. This greatly contributes to the greenhouse effect, further increasing the surface temperature.

So, ultimately there can be be wet Venuses, but they won't be very stable and snowball or greenhouse worlds are more more likely to result form worlds that have the potential to become the kind planet you're after.

Sources:

https://www.nature.com/articles/s41467-020-19896-2

https://www.google.com/url?q=https://core.ac.uk/download/pdf/85212581.pdf&sa=U&ved=2ahUKEwiZ0euC1c7wAhVsg_0HHcRCAyoQFjAMegQICxAB&usg=AOvVaw3ZmboXJZZSb0jZODp1Z6ic