High Pressure World distinctions from High Gravity World?

Odd question. The tropes that deal with high gravity worlds talk about aliens or human colonies (given a long enough time frame or genetic engineering) evolving to be shorter in stature to compensate for the more intense gravity ("space dwarves" as it were, sometimes very literally like in the Warhammer 40k universe). Would something similar apply to a world with a denser atmosphere than Earth but not necessarily denser gravity? Obviously an extreme example, especially given the host of other problems involved with evolving there like not instantly being atomized by the oven temperatures, but Venus has a gravity ten percent less than that on Earth but a CO 2 dense layer of gas 92 atmospheres of pressure- basically as dense as several 1000 meters under the ocean. So if life evolves on less utterly hellish world that still has a dense atmosphere could we assume that the same general tropes regarding high gravity worlders apply? Or are there different sorts of complications to consider?

I'm not confident in this, but my thought at the moment is that I might expect life to develop towards the thin (in order to reduce the column of atmosphere bearing down on one) and the streamlined (in order to reduce atmospheric resistance when moving).

These tropes seem to be unaware of non-gravity based constraints on body size and the possibility of animals adapting to increased gravity w/o changing the body size.

As far as I know, the main limiting factors for body size are predation (smaller animals have more predators), temperature (cold favours larger sizes), food quality (a small body cannot effectively digest low quality food) and food quantity (larger bodies need more food). It's hard to say what effects gravity has because it doesn't vary a lot, but the fact that the largest present-day mammals aren't much larger than the largest dinosaurs despite weighing considerably less underwater argues against the notion that it would make much of a difference. I guess the truth is that large land mammals would go extinct in a high gravity word and be displaced by similarly shaped birds (which have air sacs and thus weigh less)

I don't think atmospheric density has any effect on land animals at all, unless it is dense enough that buoyancy begins to matter. At best, it would alter their speed as there is more air resistance.

Flight is a somewhat different matter - the ratio of air density/gravitational acceleration has a strong effect on flight performance. I don't have the studies at hand but I remember that even a minimal increase in this ratio would noticeably increase the frequency and maximum size of flighted species. And these are based on empirical considerations not just theory, since unlike gravity air density has varied over time in Earth's atmosphere.

Edited by SeptimusHeap on Mar 15th 2022 at 9:49:32 AM

On the other hand, I don't think that we have land animals today of anything like the size of the largest land-animals in pre-history.

Indeed, it's only really in the higher-density and thus higher-buoyancy medium of the ocean in which we do find mammals to match the largest dinosaurs.

That said, I think that this may be due to changes in atmospheric composition: I seem to recall that Earth of old had rather more oxygen, which allowed for rather larger organisms.

That's indeed true, but it's more a hinderance for insects since their respiratory systems don't scale to large sizes. I think the main reason why there aren't any land animals as large as the dinosaurs of yonder is because mammals lack air sacs and thus their skeletons and respiration cannot support very large sizes, whereas birds which do cannot evolve to such sizes as they would have to evolve to intermediate sizes first - where ecologic niches are occupied by mammals.

In fact, by this logic a denser atmosphere could support larger animals as there would be more oxygen in a given batch of air/more oxygen molecules pressing against a given surface area.

You may be right about mammals and birds, indeed; I'll confess that I'm not sure.

Hmm... Conversely, there's also greater pressure, I daresay due to the increased mass of the air-column above.

(Hence my thought of narrower animals: decreasing the upwards-facing profile should decrease the force applied (due to narrowing the air column). Perhaps a planet of tall, willowy beings?)

I don't think that would make much of a difference; animals are mostly water and water is incompressible. There are specific adaptations for pressure but only for pressures at ocean bottoms - we wouldn't be talking of "atmosphere" by that point.

The pressure isn't really a "converse" - the function of the lung depends on how many oxygen molecules are impacting its surface area, and for a given temperature that increases with the pressure.

I'm not really thinking of the lungs, myself—I'm just thinking of physically bearing up (e.g. joints) under a significantly greater atmospheric pressure.

The OP mentioned an example atmosphere of 92atm, which is... a lot.

About the same as water at 1km depth.

Speaking of, if your world has humans in it you may want to consider that many atmospheric components become toxic at high pressure. In fact, even hydrogen at such pressures would be toxic. An Earth-like atmosphere at 92atm would kill you faster than pure hydrogen cyanide.

Isn't it a question of pressure differential? So as long as internal pressure equals external pressure, the animal is fine? I think the primary effects of a high pressure atmosphere will be indirect in terms of the kind of environmental constraints it imposes.

Up to a point. At very high pressures, the internal pressure of molecules - which are being compressed by the surrounding molecules - begins to matter and changes their properties. That is why nitrogen narcosis and similar phenomena exist.

Hmm... Okay, fair.

I do still stand at least by my earlier assertion of animals being more streamlined, to deal with the higher air resistance. And indeed, the example of the creatures of the ocean would arguably seem to support that idea.

As to chemistry, indeed, that is a challenge. However, it's one that I don't know enough about to really comment on, save to suggest pressure suits for any human colonists out in the open.

One of the hazards of high pressure atmospheres is the same hazard scuba divers face if they descend too far into the sea.

https://en.wikipedia.org/wiki/Nitrogen_narcosis

For the purposes of this thread I suppose I am talking about two different worlds- one with a thicker atmosphere survivable by humans (say, 3.5 atms) and one with an atmosphere of whatever thickness you like beyond that that hosts life of a non-terrestrial sort. I am interesting in your thoughts on both when it comes to how such a world might shape the life on it. Wonderful discussion so far, so thank you.

An atmosphere with a pressure of 3.5 atms would lead to life similar to Earth's but with far more and larger winged species - the amount of lift provided by a wing scales linearly with atmospheric pressure. What's the composition of this atmosphere?

Well, full disclosure, I am working on a science fiction story about a time when humanity has colonized and terraformed the Solar System, including Venus, using a variety of methods but the main one being harvesting enormous amounts of hydrogen gas from the outer system and the Sun (I found the idea put forward by Isaac Arthur of essentially shooting lasers out the Sun to cause a temperature change and induce predictable solar flares at regular locations and intervals so we can generate and collect the levels of hydrogen needed for this and a variety of other future projects at a rate well beyond what the Sun pumps out normally). Combine that with the CO 2 atmosphere and you get a obscene amount of water (enough to create shallow seas covering 80% of the planet- shallow oceans also being particularly ideal if you want to create a rich aquatic environment full of, for example, plant life that produces oxygen). The process would also leave behind a large amount of graphite which could then be shipped up into orbit and form a planetary ring, one method for keeping the temperatures relatively within Earth norms.

The process also leaves behind a thick layer of nitrogen, three times what our atmosphere is on Earth. I have not crunched the numbers, mostly for the sake of having a terraformed environment, but I have overall assumed that adding rainforests, animals, and of course humans has added another half atmosphere of pressure to the whole thing.

I am also introducing some extremophile species that were engineered for a pre-terraforming Venus environment, specifically created to take in gases and elements of the planet and convert them into elements and gases suited for a Earth like planet. This also includes sentient beings, bioengineered cousins of humans if I feel I can justify it to my own satisfaction (constant hellfire and pressures that could crush a man like a soda can aren't insignificant obstacles to overcome in evolution or biological engineering, obviously). Since one of Venus' names was originally the Morning Star and given the environment, this ecology of lifeforms is generally referred to as the Luciferans, as is specifically the sentient 'shepherds' and 'gardeners' of the other lifeforms.

The Luciferans were not expected to survive the shift from hellworld to paradise. They did. There is no small amount of tension between them and the rest of the human race on the planet.

All of this said, those are ideas I'm cooking with for my specific setting, but I am also more broadly interested in the discussion on alien evolution in different environments. I would be grateful for any input on my personal story or that broader topic.

Ignoring energy requirements for a moment, this kind of process would generate a quantity of water equivalent to 28% of Earth's. The math on nitrogen seems correct to me, but I note that mild nitrogen narcosis would set in under such conditions^note (93bar*(1-96.5/100)-1atm)/(9.807m/s/s*1000kg/m3)~23m which is "mild narcosis" territory. I don't think you can engineer species that can survive in Venus's present-day atmosphere: The water activity and strength of carbon-carbon bonds just isn't enough.

An additional thought about physics and chemistry: acoustics. The speed of sound through a medium is related to its pressure and density, so a high-pressure atmosphere should have a higher speed of sound. (Sounds might also be more or less audible at a distance; I'm less sure about this part and it depends on the actual composition and viscosity of the air as well.)

In addition, the difference between deflagration and detonation is whether the oxidising reaction propagates faster than the speed of sound in the surrounding environment, and the speed of sound is based on pressure and density. On a world with a dense-enough atmosphere, insufficiently energetic explosives will just burn.

Only if it has the same density as low pressure air. Speed of sound scales with the square root of the temperature divided through the square root of the molecular mass; pressure and density doesn't enter into it.