# How to increase air density on a planet?

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

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)

• I think your helium figure has too many 1's. Jun 22, 2015 at 13:56
• you would need (more) heavy gases in your atmosphere. Increasing the pressure would not help, since you would have to increase the pressure of your lifting gas (or compensate with a more rigid, and thus heavier, shell) Jun 22, 2015 at 14:05
• @Frostfyre, it's a decimal marker Jun 22, 2015 at 14:28
• I wasn't sure. Being American, we use a dot (.) for decimals, but the hydrogen figure met our comma format. Jun 22, 2015 at 14:46
• @Burki Increasing the pressure of the atmosphere will increase the buoyancy of a lifting gas linearly. Doubling the pressure of the atmosphere will double the lifting power. This is because the buoyancy is related to the difference in densities between the air and the lifting gas. By doubling the pressure you double the densities of both and thereby double the difference between them. en.wikipedia.org/wiki/Lifting_gas Jun 23, 2015 at 4:52

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

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.

• Now I need some help with atmospheres. What geographic phenomena and/or meteorologic events would increase the pressure into these ranges? Jun 23, 2015 at 8:51
• You could get somewhat higher pressures inside a very deep caldera, and/or with a huge temperature inversion above say 400-500 feet. The air below would be still tolerably warm but the pressure some percents higher than normal. Not too healthy since you get less air renewal. Two centuries ago in London this sort of thing killed people. Jun 23, 2015 at 10:38
• Stratification would mainly be a problem in enclosed spaces. Houses would need fans, for example. Deep wells and mineshafts even more so. It would be a very far reaching modification. Jun 23, 2015 at 10:41
• @dot_Sp0T - "What geographic phenomena and/or meteorologic events would increase the pressure into these ranges?" None. 4 atmospheres of pressure is 60 psi. Jun 23, 2015 at 12:08

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.

• simply increasing gravitational pull would not increase the lifting capacity. The density of the lifting gas would have to increase, too. Jun 22, 2015 at 14:42
• We would have to do the math. The density of the lifting gas might increase at a different rate, and either increase or decrease its lifting capacity overall. My gut feeling is increase. But I do not have the background knowledge to double check this. Jun 22, 2015 at 14:46
• Thinking about it again, you might have a point... i was assuming that there would also be a containign structure (a balloon, zeppelin or such), that would also become heavier, thus cancelling potential effects (at least to some degree), but the OP never asked for that. Jun 22, 2015 at 15:05
• Gravity does not influence buoyancy. Doubling gravity would double a ship or airship weight, and simultaneously double the weight of the displaced fluid. Net result is no change. Unless the gravity/pressure gradient is REALLY steep, but then you have worse side effects (see Barlennan's towed ship on Clement's world of Mesklin) Jun 22, 2015 at 16:20

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.

• Yes, going with unobtainium (e.g. Aerium) would be one, and presumably the easiest, way to go. It is also the way I originally intended to go and probably still will. This question is more or less part of me trying to find a way to not having to introduce a new element/invent something in order to make everything work :) Jun 23, 2015 at 6:25
• You could go with an unspecified halomethane, or a sneaky "similar to halomethanes but denser ;-)". Halons are very stable gases, even if not without side effects. Or sulfur hexafluoride. The problem with halons is that they aren't friendly to fire, and fluorides may decompose and produce hydrogen fluoride, which is a beast of hell and you most definitely don't want in an atmosphere. Radon would be good if it wasn't radioactive, and a gaseous alpha emitter is a nasty customer. Jun 23, 2015 at 7:00