“…Surface darkening. Whether visible surface darkening, caused by ion bombardment on lunar fines, mineral powders, and glass beads, is a real or anomalous phenomena has long been debated [Rosenberg and Wehner, 1964;Hapke, 1966Hapke, , 1968Hapke, , 1973KenKnight et al, 1967;Nash, 1967;Lord, 1968;Dybwad, 1971;Yin et al, 1972;Hapke et al, 1975;Hapke and Cassidy, 1978;Pillinger and Jull, 1978;Benedix, 1993]. Experimenters who derive a null result have suggested that the darkening is not necessarily due to the irradiation but to poor vacuum, hydrocarbon contamination, or vitrification.…”