Assumptions - atmospheric pressure is 3 atm, but partial pressure of oxygen is comparable to earth, the rest is mostly nitrogen. Planet is tidally locked and insignificantly heavier. (for simplicity reasons, I assume that humans are able to adjust to that pressure, it is generally below the amount that causes nitrogen narcosis)

OK, so what would be consequences for any story happening in such setting?

1) changes in the air transport, airships would become much more practical than on Earth, piston aircrafts would need much shorter runaway (low quality airstrips would become much more acceptable), but anything fast (jets) would have troubles in getting enough oxygen without getting clearly too much friction.

2) Vehicles would really have to be aerodynamic and better slow because of heavy drag (unless there is some good reason to burn fuel more generously). Do I get correctly that a car designed for Earth would for the same speed use 3 times more fuel, while the same fuel efficiency would get while driving $\frac{Earth speed}{\sqrt{3}}$

3) If anyone uses bike on such planet, it would be a recumbent bike.

4) Maximum range of guns - divide by 3 (?) effective presumably not so much reduced because that part of effective gun range is defined by aiming problems, which would not be affected.

5) Reduced effective range of any grenade, bomb etc in to 1/3 of that on Earth (?)

6) More native, airborne creatures. Earth chicken would turn in to a formidable flier. ;) (any design including insect-like, flying fish-like)

7) Milder climate than otherwise because of heat retention and better distribution.

8) Winds with greater force (so boost for any sail ships or wind turbines). But shouldn't the speed actually go down?

9) Water boils in something like 126C degrees.

10) Higher boiling point means less evaporation, less evaporation means less rainfall (how to get any real numbers?)

11) Harder to set fire to anything, as huge quantity of denser air would take this heat away

12) Breathing in cold air would be much more harder, as there would more heat lost with each breath

Anything to correct/add? Any idea how instead of "more"/"less" get some science based numbers?

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    $\begingroup$ Usually I see folks use partial pressure when talking about a mixture of gasses. (as opposed to talking about molarity) $\endgroup$
    – Seeds
    Commented Sep 6, 2016 at 21:30
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    $\begingroup$ tidally locking the planet seems to have greater impacts than the pressure increase IMHO. Same gravity and same composition would simply mean more of the atmosphere, but would the planet be able to maintain it across a larger time scale? There's a few interesting questions on if the planet is feasible before getting to your other questions. $\endgroup$
    – Twelfth
    Commented Sep 6, 2016 at 21:31
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    $\begingroup$ 11) Harder to set fire to anything, as huge quantity of denser air would take this heat away -- Seems to me the increased oxygen presence would have the opposite effect here. $\endgroup$
    – Twelfth
    Commented Sep 6, 2016 at 21:31
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    $\begingroup$ but anything fast (jets) would have troubles in getting enough oxygen without getting clearly too much friction. - not so true, significant amount of trust is created by heating air which do not participates in burning process, I would call it most, but do not know numbers, but guess for your planet it will be not a stop problem. $\endgroup$
    – MolbOrg
    Commented Sep 7, 2016 at 17:00

3 Answers 3


Oxygen toxicity could be a problem. This isn't a straightforward issue. It depends on the length of exposure and the organs of the body affected. For some preliminary research, try the Wikipedia entry on oxygen toxicity and Oxygen Toxicity: a brief history of oxygen in diving.

A number of the research studies involved pure oxygen at short duration. There is the possibility of harm at prolonged exposure, for example, people living and working in a dense atmosphere.

There is no clear understanding on the exact biochemical processes involved with CNS oxygen toxicity, nor is there a clear consensus on what the "safe" upper PO2 limit should be. Convulsions have occurred in divers breathing an inspired PO2 as low as 1.2 ATA, but such cases usually involve extenuating circumstances (such as medical conditions in the divers which pre-dispose them to convulsions).

The above quote comes from Diving Physics and "Fizzology" which discusses partial pressure adverse effects for various breathing gases including oxygen.

When immersed underwater, most divers regard a PO2 of 1.4 ATA as a safe upper limit during periods of physical exertion, and 1.6 ATA during periods of rest.

Where 1 ATA equals surface atmospheric pressure.

  • $\begingroup$ Hey on the bright side, giant insects might evolve! Now everyone can enjoy having to worry about dog sized spiders and pigeon sized dragonflies. $\endgroup$
    – Skye
    Commented Sep 7, 2016 at 12:55
  • $\begingroup$ Ah! That really cheers me up. Spiders and dragonflies of that size aren't a real worry, it's the horse sized scorpions and several metres of centipedes that of concern. You're right, arthropoda would get a real boost in high oxygen atmospheres. $\endgroup$
    – a4android
    Commented Sep 7, 2016 at 14:29
  • $\begingroup$ He mentioned the oxygen partial pressure remains the same though? $\endgroup$
    – Neuryte
    Commented Nov 13, 2018 at 6:13

For number eight, an increased density atmosphere would decrease the thermal contrast between the poles of a tidally locked planet. This in turn would mean less forcing and lower wind speed. This would be a bit of a bummer for wind-carts, but on the bright side, the habitable belt of your planet would extend further out on either side!

If you wanted to lower the thermal contrasts even more you could have the planet rotating slowly on a synchronous orbit with the sun or primary. If a tidally locked planet rotates fast it has a strong Coriolis effect which means the creation of an equatorial jet. This jet lowers the heat redistribution efficiency of the atmosphere, meaning higher thermal contrasts among the hemispheres. A planet that rotates more slowly has a negligible Coriolis effect and thus no equatorial jet. The air circulation would be a simpler anti-stellar to stellar flow.

Just wondering, what is your planet tidally locked to? Is it a sun or another planet? If by any chance it is tidally locked to a red dwarf, just remember that even old red dwarves flare occasionally and creatures that spend more time in the air are more vulnerable to solar flares.


The planet's tidal locking means that the day side would be too hot, and the night side would be too cold, forcing humans to live within the twilight strip. However, the atmosphere's density will help decrease the temperature range, expanding the habitable zone out. Megafauna will be more capable of surviving, due to oxygen more easily absorbing into lungs, and thus, the organisms will be a good amount larger than their Earthly counterparts. If they were to survive unaided, humans would need to live at higher altitudes, and due to the atmosphere being approximately 6.7% oxygen, 92.3% nitrogen, and 1% other, we can assume that its molar mass is 28.41 g/mol. The scale height would be about 7.5 km, assuming a mean temperature of 250 K, and we could use that to find where human life would be possible.

At that altitude, atmospheric pressure would drop to 1.10 atm, and that's not enough oxygen for human survival. Humans would need special equipment to be able to safely explore the surface, at least until evolution adapts their bodies to the increased pressure.


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