Sputtering and Other Non-thermal Escape Processes

There are quite a few processes that can remove particles from an atmosphere that are temperature independent. In general these processes are driven by sun through both solar radiation and the solar wind. Many of them, like charge exchange, depend on the chemistry of the particles involved and therefore they go beyond the scope of this model and this page. Sputtering doesn’t and hence I will describe it.

In sputtering an energetic particle (ion or neutral) enters the atmosphere and strikes an atom or molecule in the atmosphere imparting a large amount of energy to it. This atom or molecule then often has sufficient energy to escape if it can become directed in the right direction, and it doesn’t run into too many other particles on its way out. If it runs into just a few particles it can boost them to velocities greater than the escape velocity as well. Technically it is this multiple collision process that is called sputtering while a single collision process is called knock-on.

These processes are illustrated in the applet below. It begins with 15 large particles bouncing at low energies in the box. At random intervals new particles of half the mass of the original population come in from above at high velocities. In this system it doesn’t take very long for all of the original population to be removed from the system even though the settings for the large particles and the box are identical to that for the simulations of Jean’s escape. In that system, even with collisions the large particles take a very long time to escape.

As with sputtering and hydrodynamic escape, in order to be able to escape a particle really needs to be in the exosphere when it attains the escape velocity in order to successfully leave the system. However, impacts are more likely to occur further down in the atmosphere where it is denser. For this reason only a small number of the collisions that occur will actually result in the loss of a particle. Like with hydrodynamic escape, the mass dependence of this process is essentially linear. Hence it can result in the removal of heavier particles from the atmosphere.

In the model these processes are simply implemented as the loss of a fraction the exosphere and the layers immediately below it as a function of particle mass and solar influx which is an adjustable parameter of the model.