Our planet is surrounded by a wispy layer of gas that keeps us warm, allows the weather to happen and basically makes all of life on Earth possibile.Except that precious atmosphere of ours is leaking into outer space every second. Thankfully it is a rather slow leak, since for any object, weather it is a molecule of gas, a rocket or a cat, to break the tether of our planets gravity and escape, it has to hightail it out of here at a speed of 11 000 m/s.

It takes the energy of a ton of TNT to boost a person to that speed, and less energy for lighter objects ( 1/10 out of that for a cat for example). Other than a large asteroid impact that can eject large amounts of atmosphere into space, the only gases that regularly escape Earths atmosphere are hydrogen and helium.There are different ways hydrogen and helium molecules can wind up on a one-way trip to space.

Some escape by simply getting enough energy from the suns heat-this process is an example of thermal escape mechanisms.

One classical thermal escape mechanism is Jeans escape. In a quantity of gas, the average velocity of a molecule is determined by temperature, but the velocity of individual molecules varies continuously as they collide with one another, gaining and losing kinetic energy. The variation in kinetic energy among the molecules is described by the Maxwell distribution. The kinetic energy and mass of a molecule determine its velocity by E_{\mathit{kin}}=\frac{1}{2}mv^2

Individual molecules in the high tail of the distribution may reach escape velocity, at a level in the atmosphere where the mean free path is comparable to the scale height, and leave the atmosphere. The more massive the molecule of a gas is, the lower the average velocity of molecules of that gas at a given temperature, and the less likely it is that any of them reach escape velocity.


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