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There's a more scientific way to explain it but heres a simple one; If you have a balloon filled with helium where does it want to go? Up because its lighter than air so earths gravity is having a lesser effect on it. SO if its not in the confines of the balloon it will keep going until it reaches the outer edges of the atmosphere, at this point it will slowly 'leak' off into space and it will do so first as its lighter than most other gases floating around in the upper atmosphere

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12y ago
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13y ago

No planet can hold any gas. Everything escapes, the only question is how fast.

Atmosphere is lost faster, when:

gas is lighter

temperature is higher,

gravity is lower,

planet has smaller size.

Potential energy of helium atom near the surface is

P = -mgRe = -μ/Na gRe

Exponential factor in Boltzmann distribution is

exp(-P/kT) = exp(μ/Na gRe / kT) = exp(μ/(RT) gRe)

Assuming T= 300 K we have

μ/RT gRe = 0.004/(8.3 300) 9.8 6,370,000 = 100

So once per exp(-100) ~ 10^-43 attempts at escaping helium atom manages to do so. Probabilty 10^-34 is very small, but it sharply depends on temperature. Throw in 1000K and you have p ~ 10^-13, which means rather quick escape.

Conclusion:

Planet Earth is unable to hold lighter gases, namely hydrogen H2, HD, D2, and helium He4 and He3. Heavier gases like nitrogen and oxygen are safe.

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12y ago

Helium is lower density than the air, thus rises by buoyancy. As it rises the air pressure outside decreases and the ballon expands from the pressue inside.

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11y ago

they've left because of their decomposition by ultra-violets rays

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11y ago

they dont escape due to earth's gravity

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12y ago

Yes

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Q: Why did hydrogen and helium leave earth's atmosphere?
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