What light gas can I have in the upper atmosphere?

Question

I want to have an upper atmosphere composed of a gas which is lighter than "air" so that my human occupants of a planet can harvest it and use it to fill blimps and hot air balloons.

I'm planning on having my "normal air" as Nitrogen Oxygen as I see no need to change it from Earth-like. I'd rather not use have a Hydrogen layer (as it explodes and would ignite both balloon and planet).

Is it possible to have a layer of Helium in a planet's upper atmosphere (which then the humans could use some dodgy hydrogen balloons to reach and harvest).

If not Helium what other gases are viable?

Answer 1

There are three problems with a lightweight upper atmosphere.

1. There aren't that many lightweight gases. Off the top of my head, your options for lighter-than-nitrogen/oxygen (molecular weight 29) are neon (MW: 20), hydrogen fluoride (MW: 20), methane (MW: 16), ammonia (MW: 17), water vapor (MW: 18), helium (MW: 4), and hydrogen (MW: 2). Of these, methane and hydrogen are flammable, water vapor will tend to condense, ammonia is toxic, and hydrogen fluoride is highly reactive. For the purposes of ballooning, a mid-weight gas like neon has only half the lifting power of hydrogen or helium.

2. A lightweight upper atmosphere will tend to escape into space. With hydrogen and helium in particular, a sizable fraction of the molecules at upper-atmosphere temperatures will have velocities above escape velocity. Something heavier like neon or ammonia will tend to stick around, but will run into problem 3:

3. The atmosphere is pretty well mixed. An exceptionally lightweight gas like hydrogen or helium will be noticeably more abundant in the upper atmosphere, but will tend to escape. Mid-weight gases like neon, on the other hand, will be mixed in by ordinary atmospheric circulation.

Answer 2

Helium is the obvious candidate, it's only slightly heavier than hydrogen. It's not flammable or explosive, and it works well in balloons.

There are three fundamental problems though which makes the idea unrealistic:

Where does it come from

Where would all that helium come from? The chemical makeup of the planet would be very different from our own.

Why does it stay separate

You would expect atmospheric mixing to constantly churn the atmosphere. Oxygen, Nitrogen and Carbon Dioxide all have very different densities but they still mix fairly evenly in our atmosphere.

Why doesn't it escape into space

Lighter gasses escape more easily, you would expect Hydrogen and Helium (especially in the upper atmosphere) to be stripped away into space by atmospheric escape.

A possible solution

There is one scenario that might just work although it's still rather implausible. That is if the planet was a moon in low orbit around a gas giant. The gas giant would be the source of the helium and hydrogen, replenishing that which was lost and causing escaped gas to remain in a ring to be gathered in again as the moon orbits.

That doesn't solve the atmospheric mixing problem but you can hand-wave that to a certain extent.

Answer 3

You can get past the problem of He escaping into space only if the planet's core is rich in radioactive isotopes that undergo alpha decay. That would produce a supply of He to replenish what's lost to space.

Then you might actually have, instead of an upper-atmosphere layer, certain mineshafts that go deep enough to tap into this natural source of He.