Radiation Formation of a Non-Volatile Crust

Ion irradiation of the outer meters of a cometary surface produces new molecular species in the solid state. Because of the vacuum interfaces these segregate in an irreversible way into a non-volatile residue and new very volatile species, which are lost directly or lost when the comet enters the inner solar system. It is, therefore, likely that a comet exposed to background radiations in the Oort cloud would obtain an outer web of nonvolatile material which will lead to the formation of a substantial 'crust' (~10^2 gm/cm^2). Except for fizzures and break-off of pieces due to warming of subsurface gases, this mantel should be continuously hardened for a periodic comet due, primarily, to thermal processing. There will also be active regions which were shaded from the cosmic ray radiation.