Question: How high does atmosphere go

As high as the sky … no, just kidding. We live at the bottom of an ocean of air, but while the watery ocean has a well defined top surface, the atmosphere just dwindles away.

As most of us realize, at sea level the atmosphere exerts a certain pressure–about 1 kilogram per square cm, or about 15 pounds per square inch. It comes from the weight of the air piled up on top of us. Atop a mountain 5 kilometers high, half the atmosphere is below you, so the weight on top is only half as much, and the pressure and density are also only half of what they are as at sea level. (The value given as 5 km is approximate and also depends on temperature.) Another 5 kilometers, half as much again–that is 1/4 the pressure and density of sea level. And so forth.

At this rate, 20 such halvings bring us to 100 kilometers and a millionth of the density, (actually it’s about twice that). Air still contains 12 million million molecules in each cubic centimeter, but up to this height, its composition has remained about the same–78% nitrogen, 21% oxygen, nearly 1% of argon, some 0.1% carbon dioxide and other gases. From here on, however, collisions between molecules are less frequent, and different gases tend to settle at different rates, the ones with heavier molecules at the bottom, with a smaller “halving distance.” Both oxygen and nitrogen form molecules of two atoms, but oxygen molecules tend to get split up into separate atoms, and these (being lighter than the molecules) extend further up. Nitrogen dominates to 200 kilometers, but then up to 600 kilometers it’s atomic oxygen, and after that–would you believe helium? Down at the ground its concentration is insignificant, but being lightest, it outlasts the others. Still higher, the main component is hydrogen, even lighter than helium.

By then, the density is so low that atoms and molecules rarely collide, they just rise above the atmosphere like tossed stones, then fall back. But even at 600 kilometers some atmosphere remains–ultra-violet photographs from the Moon have seen a hydrogen “geocorona” glow extending to 3-4 Earth radii, gradually fading away. And satellite orbits at 400 kilometers are still degraded by air resistance. Of course, all this concerns the “neutral” atmosphere, held by gravity. Free electrons and ions (atoms missing electrons) are held by the Earth’s magnetic field to even greater distances, though their density (at least near Earth) is smaller.

So, how far does the atmosphere go? Depends on how you measure it!

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