# Can you create a small atmosphere with giant walls?

Can you create an atmosphere on a celestial satellite by using giant walls? Perhaps assisted with some means of technology? In a story I am creating I want to create something different from the normal bubble structures you see in some syfy novels.

The walls would also be on the moon.

Edit: What if there was a form of centralized gravity? As if there was a device that centered gravity into one place, but not large enough for the whole planet and not as strong as earth gravity?

• I assume that the walls of Larry Niven's Ringworld are just too darn big..... Apr 4, 2018 at 23:39
• It was calculated, with a ring world, walls of 1000km high would hold the atmosphere in without needing a roof. Apr 5, 2018 at 3:30

## 2 Answers

I assume that by specifying "walls" you mean "walls without a roof." (If not, ignore everything I say below!)

The answer is "yes," but it may be less useful than you think. Basically, when you have a gas surrounding a planet, the gas has a characteristic decrease in density with altitude. The rate at which density decreases depends on the molecular weight of the gas (higher MW gasses thin out more quickly with altitude) and temperature (cooler gasses thin out more quickly with altitude). But there is no altitude where the atmosphere is 100%-gone, and there's always a slow leakage into space.

In general, air pressure drops as the negative exponential of the height. On Earth, pressure halves every 18,000 feet (more or less). On the Moon with its lower gravity, the halving distance would be around 100,000 feet (20 miles). As a consequence, the lunar atmosphere extends a lot further into space for any given density. (Which is partly why the Moon has so much more trouble holding onto it.)

The more massive the planet, the slower the leakage. If it wasn't for extraneous effects (solar wind, solar UV) the Earth's atmosphere would be essentially permanently trapped. The Moon's, not so much. If you magically gave the Moon an atmosphere resembling Earth's, it would lose it fairly quickly. (I can't find a number for this, but a couple of early-Moon scenarios show it losing a thick atmosphere in ca. 10 MYears.)

Here's the kicker: Walls, by themselves, don't do much to hold in an atmosphere unless they're tall enough that the air density at the top is low enough that leakage is tolerable. If you want an Earth-surface atmosphere at the lunar surface, to get to .001 atmosphere, you'd need to go up 10 "halving-heights" or 200 miles. Using a very simple model, I get the time in hours to halve the air pressure in an open tube on the Moon to be roughly h/(1400*p) where h is the tube's height in miles and p is the pressure at the top of the tube in atmospheres. (I made some linearizing assumptions, so there's probably at least a factor of two error in the formula, but probably less than a factor of ten.)

That says that a 200 mile tube (h=200, p=.001) would lose half its air in 150 hours. A 300 mile tube would lose half its air in 1700 hours. A 400 mile tube would lose half its air in 20,000 hours, and so forth. To get to 100 years, you need about 500 mile-high walls.

You might do better sticking with domes...

• I don't see a variable here for the width of the tube - surely height alone is not the determining factor? Apr 4, 2018 at 17:13
• @jdunlop basically yes, it is. A tube twice as wide will hold four times as much atmosphere... but also leak it at four times the rate. So the time to halving stays the same. That's assuming the width is constant, of course. Apr 4, 2018 at 17:16
• Huh. Math is neat. Apr 4, 2018 at 17:27

Yes you can - there is actually a theory that if we create a large enough spinning stanford torus, with walls high enough on the sides of the ring, you could contain an atmosphere without the need for a roof. It would need to be extraordinarily large though.

The issue with the moon (or another celestial body) is gravity is probably not strong enough to hold the atmosphere in unless the walls are extraordinarily high, especially as the spin of the body makes the problem worse, as for instance on a small asteroid.

If the gravity on the moon is only one-sixth of that of Earths, one would expect the wall height at least to be six times greater than that of what is needed on earth to do the same thing. If you imagine the height of the atmosphere on Earth - you can imagine these are very high walls.

It would be much easier by the way to create underground caverns for your story. The moon already has underground lava tubes from its early formation - this would be much more economical and you would not have the need to build walls, and you could contain atmosphere much easier.

• Thank you for your answer flox, it was helpful. I will keep your answer in mind as I wait for some more answers to come in. Apr 4, 2018 at 15:23
• Spinning makes the problem easier, not worse.The artificial gravity works on the atmosphere too, compressing it against the inside of the ring more than its own mass would do. Also worth noting that a Stanford Torus is an enclosed atmosphere. You're describing a Ringworld, made famous by the books of the same name by Larry Niven. The rim walls on his Ringworld were 1,600 km tall. Apr 4, 2018 at 16:12