Monte Carlo model of sputtering and other ejection processes within a regolith

The ejection of atoms and molecules from a surface due to a flux of energetic particles has been studied for a variety of target materials and radiation types. We simulate these processes within a porous regolith using a Monte Carlo model. In doing so we assume that the surfaces of regolith grains behave like the surfaces of laboratory samples, where the results of an ejection event have been found to depend upon the energetic-particle angle of incidence and intrinsic properties of the target material. Total sputtering yields, sputter-product exit-angle distributions, and energy distributions are calculated for application to Solar System objects with emphasis on sodium ejection from the lunar surface. We find that the regolith yield is typically less than that from a laboratory surface made of the same material and depends on the sticking probability. The sputter-product exit-angle distribution from a porous regolith is found to be little altered by variation of the primary sputtering parameters, with the exception of nearly specular sputtering. The regolith grain shape has little effect on the total sputtering yield. The results are in rough agreement with an earlier analytic model [Johnson, R.E., 1989. Application of laboratory data to the sputtering of a planetary regolith. Icarus 78, 206-210]. The experimental results of Hapke and Cassidy [1978. Is the Moon really as smooth as a billiard ball? Remarks concerning recent models of sputter-fractionation on the lunar surface. Geophys. Res. Lett. 5 (4), 297-300] require a strongly forward-directed sputtering process, which may apply to a silicate with a low sputtering yield but not to the sputtering of ices by heavy ions.