: By what mechanism does helium escape into space? It is lighter than other gas molecules, thereby enabling it to rise to the top of the atmosphere, but it does not have antigravitational properties which enable it to "escape" into space.Simple thermodynamics.
There is a thermal distribution in any box full of gas molecules; they don't all have the same energy; this was demonstrated in the last century. What you get is a particular distribution of energies; a smooth curve called a Maxwell-Boltzmann distribution curve. A lot of gas molecules will have a lower energy; but some will have a higher energy. If the energy of individual molecules of He is high enough to overcome the gravitational threshold, then a collision with another gas molecule can give the high-energy molecule sufficient kinetic energy to leave the atmosphere and float off into space. Thus there is a continual slight leakage of light elements like Helium into space.
It sounds complex, but it's not really any more complex than steam coming off a bath; or water evaporating from the sea and forming clouds; the molecules have enough energy to leave the atmosphere and wander off.
The atmosphere also doesn't have a real ending point as such; it just becomes less and less concentrated until the number of molecules you encounter become low enough to start calling the atmosphere "space".
As to why heavier-than-air molecules don't float off into space, a very few of the highest-energy ones do; but the vast majority don't; it would be analogous to a stone floating; they're stones, they're denser, they sink.
To illustrate kinetic theory, what happens if you heat a sealed can?
The individual molecules start to acquire higher individual energies, which result in them going faster and striking the walls of the container with higher speed (which causes the molecules of the can to heat up - this is how thermal conduction works). As the total heat of the system builds up, the pressure inside the can eventually becomes high enough to rupture the can; because there are enough high-energy molecules of gas hitting the can wall to disrupt the structure of the metallic can.
For another example, look at Jupiter and Saturn; the gas planets. They receive much less energy from the Sun per unit area than the Earth does; thus the hydrogen and helium molecules don't have enough kinetic energy to escape the atmosphere. The result is that Jupiter and Saturn have huge atmospheres full of lower-energy hydrogen and helium; whereas the Earth and the inner planets like Mars, Mercury, Venus and the Moon have comparatively small atmospheres composed of heavier molecules; they have lost their light gases to space.
Hope this helps.
Gideon.
