Radiation formation of a non-volatile crust.

Ion irradiation of the outer meters of a cometary surface can produce new molecular species in the solid state. Because of the interfaces with the interplanetary vacuum these species segregate in an irreversible way into a nonvolatile residue and new very volatile species. The latter are ejected directly or lost when the comet enters the inner solar system. Therefore, a comet exposed to background particle radiations in the Oort cloud obtains an outer web of nonvolatile material (~10 sup 2 gm/cm sup 2) which will lead to the formation of a substantial 'crust'. When a new comet enters the inner solar system there will be early activity, initial fissures in the crust and the break-off of unstable pieces of the crust, due to warming of subsurface species. If this comet enters a period orbit in the inner solar system the remaining mantle should be continuously hardened, due primarily to thermal processing. There will also be permanently active regions on such a comet which were initially shaded from the cosmic ray radiation when the comet was in the Oort cloud or which subsequently lost their crust.