Definition for Thermal escape
From Biology Forums Dictionary
Atmospheric escape is the loss of planetary atmospheric gases to outer space.
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.
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.
This is why Hydrogen escapes from a given atmosphere more easily than Carbon dioxide. Also, if the planet has a higher mass, the escape velocity is greater, and fewer particles will escape. This is why the Gas giant planets have significant amounts of hydrogen and Helium, which escape on Earth. Distance from the star also plays a part; a close planet has a hotter atmosphere, with a faster range of velocities, and more chance of escape. A distant body has a cooler atmosphere, with a slower range of velocities, and less chance of escape. This helps Titan, which is small compared to Earth but further from the Sun, keep its atmosphere.
However, while it has not been observed, it is theorized that an atmosphere with a high enough pressure and temperature can undergo a 'blow-off'. In this situation molecules basically just flow off into space. Here it is possible to lose heavier molecules that would not normally be lost.