system
Given the following post, respond as if you're commenting on Reddit:
human
Does Helium and other light gases leave Earth's atmosphere, or does it form the uttermost layer of it?
gpt
Short answer: our atmosphere is steadily evaporating. Lighter gases like Hydrogen and Helium slowly escape into deep space from the superior layers of Earth's atmosphere.


Long answer: Below 100 km (from sea level) air density is high enough to keep particles in a fluid molecular regime (mean-free path much shorter than the scale height) so collision is what dominates this scenario. When you go above 100 km however density becomes so low that there are almost no more collisions and particles start to follow a free molecular regime in which their trajectory becomes ballistic (rather than collision-dominated). You need a statistical approach to the problem and to do so we use the Maxwell-Boltzmann distribution. In simple words the distribution tells you how many particles are there with a selected value of absolute speed. You are most likely to find slow particles than fast particles obviously. Moreover, since you also need to consider potential energy due to gravity, you will find less particles with a certain speed at higher altitudes since one molecule travelling upwards loses kinetic energy and gains potential energy. However probability says that still are there very few particles which are really fast and are located in the uppermost layers of the atmosphere. If those particles happen to have a speed greater than the escape velocity from Earth they leave the planet for there is nothing above them to stop their motion. Luckily for us, important gases like Oxigen and Nitrogen are too heavy to evaporate due to diffusive segregation of species, which locks them in the lower layers of the atmosphere, making it almost impossible for them to escape. This is because the more a particle weights, the more energy it will need to reach a certain speed (E = 1/2 * m * v^2) so the probability to find a high-energy heavy-particle which can make it's way through the entire air layer without collisions (that would change it's energy) and still have enough speed to escape is very very low.


If you need more detailed informations or the mathematical explanation feel free to send me a PM.