Reading:
Definition: The exosphere of an atmosphere is where the mean free path for collisions is MORE than a scaleheight.
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Diffusion - out of the regolith - important for light and inert gases - H, He, Ar (primordial? from time of formation?)
Photo-Sputtering, Adsorption and/or Thermal Evaporation
Photons hit the surface and excite atoms, "shaking" them off the surface - or heating them up
Meteoritic Bombardment -> Impact Vaporization
Micro-meteoritic flux at Earth is 120 tons / day! That's 40 +-20 million kg / year.
Ion Sputtering - Ion Implantation
Solar wind (H+, He2+, O6+, Fe8+, C4+...) impact Mercury and the Moon at 300 km/s. Ions are implanted into the surface layers - and then are sputtered off again or evaporated. Sputtering is just what it sounds like - energetic ions bombard the surface and knock off a spray of atoms at lower energies.
Photo-Ionization
e.g. Na + photon -> Na+ + e- which, being charged, are swept away in the solar wind.
Charge Exchange
e.g. solar wind H+ + Na -> Na+ + H* the Na+ is swept away, the H* is an energetic neutral hydrogen atom, it keeps its original 300 km/s
Radiation Pressure
e.g. photon hits Na atom and bounces off - delivering momentum. Na atom moves away from the Sun.
Jeans (Thermal) Escape
When a gas atom has more speed than the escape speed, it escapes. In any distribution of atoms or molecules in a gas, there will be a fraction that is greater than the escape speed of the Moon, Mercury - or whatever object.
In a gas that is in thermal equilibrium (lots of collisions to distribute the energy) they have a Maxwell-Boltzmann distribution of energies - and speeds.
There is a most probable speed - sometimes called thermal speed - which is
Vo = (2kT/m)1/2
where m = molecular mass = <A> mp (<A> = molecular mass in a.m.u., mp = mass of proton)
The fraction with speeds greater than V is....
Fig
6-3
So, an atom or molecule is more likely to escape - V > Vescape - if
The Galilean satellites are embedded in the magnetosphere of Jupiter
The atmospheres of Callisto, Ganymede and Europa come from the bombardment of their icy surfaces by energetic particles. Ion implantation, ion sputtering, ionization, charge-exchange - the works!
... which leads us to Io next week. And Pluto and Triton.