# Creature design - low gravity high atmospheric density world

I would like some help in thinking through what would life look like on a planet with slightly lower gravity (around 75% Earth) and a much higher air pressure (around 3 times). The goal is having a planet with massive flying creatures.

I am thinking much larger insects. Flight would be much easier so I am thinking very big flying creatures. Huge birds, flying lizards and mammals. Animals can also simply float in the air using buoyancy (something between a helium balloon and a jellyfish)

I would also imagine very tall trees, with weaker structural properties than on earth. Think VERY tall palm trees.

Jumping would make more sense than running, and I also think the predators would either be avian or have adaptations to help them take down prey at range (spitting or missiles of some sort)

Am I missing anything obvious or making any glaring logic errors?

(Ps. The aim is for a webcomic so I am very interested in any visual considerations, interesting coloration etc)

Some notes based on questions asked:

• You better make all your aliens fire and explosion proof.... Cause oxygen and high pressure together mean only one thing.
– user81643
Jan 7, 2021 at 14:30
• @user81643 Got that right. Oxygen is only around 20% of Earth's atmosphere: most of it is actually nitrogen (which is vital for plant growth, incidentally: it has a role, even if humans can't really breathe it). It doesn't stop us from suffering some nasty forest fires at times. Step up oxygen concentration much higher, and you risk those becoming explosions instead. I'm not sure where the upper boundary is, but before you get anywhere near 100% the atmosphere would become a colossal tinderbox that would incinerate the world with a single spark, which obviously won't be a livable planet. Jan 7, 2021 at 23:17
• How would a planet with gravity lower than Earth have a stable atmosphere with pressure ten times that of Earth? Gravity is what causes this pressure. Jan 8, 2021 at 0:29
• The point of fires are a good point, what I am aiming for is oxygen availability similar than what we had 200 Million years ago: nationalgeographic.com/news/2011/8/… . So fires would be a bigger problem (and plant life will have some compensations to deal with this), but explosions shouldn't be an issue. I haven't done the math but I assume around 3% oxygen in a mostly inert atmosphere Jan 8, 2021 at 6:44
• The question of how do we get a high pressure atmosphere in a lower gravity planet. Keep in mind that gravity is a big driver, but the other driver is simply the quantity of atmospheric gas. The processes involved to form this planet would have been complex and the atmosphere may need replenishment from active volcanic activity to offset higher dissipation into space. The planet is inspired by Titan (the only example I am aware of of a low gravity, high atmosphere body en.wikipedia.org/wiki/Titan_(moon) Jan 8, 2021 at 6:59

Let me try to first figure out the surface temperature. I am going to ignore the Lambart cosine law, the regional variability, the lapse rate .. all of that, and use ideal gas equations.

We have $$P = rgH$$

P = pressure r = density (for rho) g = gravitational constant H = height of atmosphere.

also,

$$P = r/M \cdot RT$$

M = molar mass R = gas constant T = temperature

so we have,

$$rgH = r / M \cdot RT$$

or,

$$T = gHM/R$$

Now, we need a barometric height of the atmosphere (ignoring the stratification, and thus jeans escape). We can just take the height to be, say 100 km.

At this point, you have to be careful, that the stellar wind does not blow the atmosphere away.

So, we get :

$$T = 0.75 \cdot 9.8 \cdot 100 \cdot M / 8.31$$

You need to choose the M properly, for a life sustaining temperature.

So, My ideas would be :

1. Cold blooded animals to survive the low temperature with prolonged hibernation. As such, powered flight is unlikely to evolve.
2. Burst of jump, and then gliding flight
3. May be, if you introduce heavier gasses, like SO2 and such, you can have chemosynthesis, and possibly bioluminance. If you place the planet in a globular cluster, you can make a eerie night sky like this :

And, in the ground, you can have cold, craggy mountains, eroded by ammonia glaciers, and light them up by bioluminence powered by chemosynthesis. You will probably need some metal hydride catalyst in the soil too

1. The radiation of the globular cluster may not be filtered by the atmosphere, and may reach the soil. Thus hard shell may be a necessity.
2. Slowly life can develop a pattern of bio-luminance based communication, and can develop resonant cavities to produce some sort of laser.
3. Such low power laser may be used to etch naturally occuring glass, as a form of record keeping - thus developing language, and some sort of intelligence.
4. By continuously working on chemical process optimization, the life can harness quantum effects.