How to explain an ocean planet having lots of greenhouse gases

Question

How would I explain an ocean planet having a large amount of greenhouse gasses, I do not mean akin to Venus, I just mean enough to increase the size of the habitable zone by a sufficent amount. Enough so that it would likely not be in the habitable zone without them.

So, more extreme than Earth's current greenhouse effect. But not as extreme as something like Venus or a Hycean planet.

How would I scientifically explain this for a planet covered entirely in oceans? I want to find a reasonable scientific answer, as I know volcanos cannot be the answer as they would all be hidden deep underwater. And I don't think aerobic bacteria would be enough to form a suffiently CO2 heavy atmosphere. Would there be other greenhouse gases that I could find a reason to be high? Such as ozone?

Does my idea simply not work out? Please help me find an explanation for this.

Notes: I do not want to use industry to increase greenhouse effects, I want it to naturally have this amount of greenhouse gases before human arrival. I also want it to be relatively easy for humans to breathe on the planet when they do eventually go there, so no toxic levels of gases, if possible .

Answer 1

H₂O

Water vapor is already a crucial greenhouse gas on Earth, responsible for as much as half of its absorption of sunlight. And you've got tons of water to work with from the evaporation of your planet's oceans. While one might think an ocean planet might develop a thick cloud layer that reflects a significant amount of sunlight, cloud formation relies on condensation nuclei of dust and other particulates, and without any land to create dust I'm not sure if there would actually be any more cloud cover. Then the extremely humid, but not-quite-condensing, atmosphere can do its part to warm the planet.

CO₂

And I don't think aerobic bacteria would be enough to form a suffiently CO2 heavy atmosphere.

This is backwards. Carbon dioxide readily formed in any cool enough environment during the solar system's formation, and it's relatively nonreactive so it tends to stay on any planet with enough gravity to hold onto it (hence we see lots on Mars and Venus). Billions of years ago, Earth's atmosphere was rich with CO₂, and only after photosynthesizing organisms emerged did oxygen mostly replace it. Since you're asking how to dramatically warm this planet, I must assume it's quite far from its star — so photosynthesis is fairly weak and perhaps what plant life exists hasn't been enough to drop CO₂ levels to near-zero (though hopefully it's low enough that people can breathe there).

Albedo

Not a greenhouse gas, but this'll help a bit — oceans are dark and generally absorb more light than landmasses. So even if your planet was otherwise identical to Earth, one would expect it to be slightly warmer per the blackbody equations.

Answer 2

Warm Shallow(ish) Oceans and Relatively Active Geology

Assuming your talking about a planet capable of sustaining an Earth like atmosphere with a similar carbon cycle your problem is that oceans are actually massive carbon and methane sinks. However methane in particular is a very effective green house gas with current estimates putting the amount of methane stored as hydrates on the floor of of the worlds oceans at about 20,000 trillion cubic meters! The release of even a small % of that total would have a significant warming impact on Earths climate.

The thing is on Earth methane hydrates only tend to form and importantly accumulate below depths of 300 meters (usually deeper) and in areas where methane is released by geological faults in the sub sea crust combines with water molecules. And since the large majority of Earth's oceans are way deeper and colder than that they can act as sinks or huge amounts of methane.

So if you assume an active geology with faults in the crust releasing methane into the ocean in areas where the water is shallow and warm (sunken continental landmasses, chains of high volcanic sea mounts and mud volcanoes which release a lot of methane) and you could get significant quantities of methane rising to the surface and being released into the atmosphere.

I would also suggest that maybe you make your planet the 'moon' of a larger world (it doesn't have to be a gas giant) just so that you get an extra 'kick' to your worlds tectonic stress levels generated by gravitational forces similar to that experienced by some of Jupiter's moons but on a smaller scale.

Answer 3

There should not be too much of a problem with assuming that a planet has a dense carbon dioxide / nitrogen atmosphere and lots of water (as Earth did originally). And no reason why this should not be further from the Sun than Earth.

The hard bit is getting oxygen in sufficiently high quantities and appropriate levels of carbon dioxide for warming. This is believed to have happened on Earth in a series of stages with photosynthetic bacteria consuming the carbon dioxide and producing oxygen as a waste product.

The oxygen then combined with everything on the Surface that could be oxidized like iron, but the situation became unstable at various points. This was long before animals evolved to consume the oxygen and produce carbon dioxide and on several occasions carbon dioxide levels fell too low leaving insufficient in the atmosphere to warm the planet resulting in multiple incidences of "Snowball Earth" where glaciation came right down to the tropics.

Between about 575 - 433 million years ago multicellular animals evolved and respiration helped stabilize the composition of the atmosphere as we know it today, mostly nitrogen 21% oxygen and small amounts of carbon dioxide and other gases.

The problem of a world even further out with even more carbon dioxide in its atmosphere would be to negotiate all of this instability to end up with sufficient oxygen as well as sufficient carbon dioxide and avoid an unrecoverable runaway snowball planet.

The exact details of what happened on Earth are still not fully understood and agreed on in detail, so the details of what might have happen on your world would remain obscure (for better or worse).

Note I assume humans arrive from elsewhere as they would not evolve on an all water world.

Answer 4

Methanogens

Methane is a rather potent greenhouse gas -- it can trap over 100 times more heat than CO₂ on a raw percentage basis. However, it's much shorter lived in the atmosphere, so you'll see varying comparisons about how a ton of methane compares to a ton of carbon dioxide for warming potential. (Hence why we mostly hear about CO₂.)

But if you have a constant supply of methane, how short lived the effect isn't relevant. And since it's so effective at trapping heat, you don't need much of it to have an effect -- levels which are effective at warming the planet are far, far below those which have any combustion potential, and are also far below those which have any appreciable direct biological effect.

Methanogens are microorganism which produce methane. They're typically found in carbon rich, anaerobic environments ... such as marine sediments. On Earth they aren't the significant factor in global warming (at least aren't currently*), but for a fictitious ocean planet, one might hypothesize that they're much more active in converting the oceanic dead fall, and produce enough methane to warm up the planet significantly (especially when combined with the water vapor and carbon dioxide which is also in the atmosphere and doing their greenhouse thing).

*) Some researchers have proposed that at least one mass extinction might be blamed on climate change from methane-producing microorganisms.

You could also posit that there isn't a runaway greenhouse effect because your ocean planet has reached a stable equilibrium due to the particularities of your methanogens: as temperatures rise, your alien methanogens don't do quite so well, which reduces the amount of methane they produce. (Or warmer weather and more methane in the atmosphere results in growth of a different microorganism which eats the methane.) And since methane is removed from the atmosphere relatively quickly, things return to "normal" relatively quickly.

Note that if you want to get fancy, there are other compounds which could also work. CFCs and the like are also potent greenhouse gasses. For example, tetrafluoromethane is several thousands times better than CO₂, and also sticks around much longer. So if there's a microorganism which produces tetrafluoromethane (or similar compounds) - even at a low levels - that can greatly contribute to warming.