WHEN Duke et al.1 measured the size of lunar soil returned by Apollo 11, they found a size distribution similar to that of glacial till and other terrestrial soil, except that the lunar soil is noticeably deficient in material finer than 15 µm. They feel that a partial explanation for this deficiency is that it is caused by melting and consequent consolidation into clumps of finer particles on the Moon. We wish to suggest another partial explanation, derived from wear theory—it is that the finer particles are missing in lunar soil because materials in a lunar environment have a higher surface energy.