How to increase air density on a planet?

Question

The question is about what changes would have to be made to an Earth-like planet, in order to increase the density of air and hence make lifting gases more effective. If that is scientifically possible at all, that is.

Examples of lifting gases with capacity at Earth-sea-level:

• Helium with a lifting capacity of approx. 1.1145kg per m3
• Hydrogen with a lifting capacity of approx. 1.203kg per m3

Addendum:

Earth-like means that extreme changes in climate and/or composition of the planet are out of scope, as they would presumably make it impossible for most of our Earth flora and fauna to exist. Rather I am looking for changes that go in symbiosis with our own eco system (e.g. the fact that air on Earth-sea-level is more dense than in the Earth-Himalayas)

Answer 1

The obvious approach is to replace nitrogen with a denser, inert gas. Since nitrogen is the only inert non-noble elemental gas, the noble gases seem the obvious choice. There are, however, a few difficulties. The densities at STP for the various gasses are as follows:

Nitrogen 1.25 kg/m^3

Helium .178

Neon .90

Argon 1.78

Krypton 3.73

Xenon 5.89

Radon 9.96

Helium and neon are both lighter than nitrogen, so they won't do any good. Radon has a half-life of about 180 days, so we can forget that. That leaves argon, krypton, and xenon.

It may come as a surprise, but xenon makes an excellent general anesthetic, so let's not adopt an atmosphere which renders us unconscious.

Argon and krypton are both useful contenders, as long as you keep in mind that both act as anesthetics at high pressure, about 5 atmospheres for krypton and 10-15 atmospheres for argon.

Iserni has brought up the problem of spontaneous stratification, and in principle this is true. However, weather (wind) provides a powerful mixing mechanism. Whether this would be adequate is beyond my powers of analysis.

Answer 2

Density = Mass / Volumn. To get higher density you either need to increase the mass or decrease the volume.

Decreasing Volume You could increase the pull of gravity comparative to the earths. This should decrease how far above the surface the atmosphere reaches.

Artificially restrict the atmosphere. Energy shield or some other barrier.

Increasing Mass Inject more gasses into the atmosphere. Higher temperature planets would have less surface water and more water vapor and gas. More volcanic planets would have greater release of gases from the planets core. Less life? Plants and animals recycle different gases from the atmosphere and then lock them first in their bodies and then into the earth. Fewer living organisms means less gas in the atmosphere.

Note There are just the basic principles. I did not do the math on any of this. I do not know how much any of these factors need to change in order to make a perceptible difference on lifting gases.

Answer 3

Not easily. Density is the ratio of air mass to volume, and volume is inversely proportional to pressure.

If you increased the pressure, that would have side effects on living organisms; modifying the temperature would have similarly undesirable effects in a very short time.

Supposing the gravity gradient was reasonably flat, changes in gravity would affect density, but they'd also affect whatever needed to be buoyed so as to neutralize any benefit.

One way to go might be to introduce some kind of unobtainium gas, with the following properties:

• not reactive, or very little (except maybe in circumstances which could add to the story? - a little like the miglign lifting gas of The Wooden Spaceships by Bob Shaw, even if that's set in a different Universe with different physical laws).
• very dense
• transparent to visible wavelengths

What would this entail? You would get more buoyancy, at the expense of lowered partial pressure of oxygen, so the air would be less breathable. You'd need to proportionately increase the oxygen content of the air.

The gas, being dense and hence quite heavy, would tend to pool in lowlands, so that digging e.g. a well would become very difficult unless ventilation was provided; any hole in the ground would become a death trap in still air.

Even with reasonable recirculation, the gas distribution would not be linear; you would get a sort of layer - say two hundred meters thick? - of "heavy air" where you get high buoyancy, then the air would revert quickly to normal; the airships would "float" on an ocean of heavy air, much as seaships float on an ocean of water. You'd get interesting optic effects too, if the gas' refraction index was sufficiently different from that of normal air. In the right circumstances, you could "see" over a hill by looking at the "sky" immediately above it.

With time - a very long time - you'd also get parapatric speciation, and have heavy-air creatures better adapted to life in the lowlands, and light-air creatures adapted to live on the hills. Speciation would be slow to come by since the differences between the two habitats would be small.

There would also be other effects due to e.g. changed convection parameters - depending on the gaseous unobtainium thermal dilation and capacity, it could be too easy, or very difficult, to get good airflow out of a chimney in the lowlands.

There might also be some other effect I have overlooked and which could nonetheless make this whole exercise pointless, because it would interfere with life or civilization in some way.