The Nature of Lunar Soil

The index properties of returned lunar soils from the United States Apollo and the Soviet Luna programs are reviewed. The scope of the paper includes lunar soil genesis, particle types, grain size distribution, grain shape distribution, specific gravity, minimum and maximum density, and relative density. The writers also present their own data for the latter three properties, as determined on three 1-gm submillimeter lunar samples taken on Apollo 14 and 15. The lunar soils are shown to be well-graded silty sands to sandy silts, composed of many different particle types (mineral fragments, glasses, agglutinates, basalts, and breccias) all produced primarily by meteorite impacts on the lunar surface. The specific gravity varies from 2.90 to 3.24 and the minimum and maximum bulk densities vary from 0.87 to 0.10 g/cm³ and from 1.51 to 1.89 g/cm³, respectively. The in situ lunar soil can have a low relative density at the surface, increasing rapidly to a very high relative density at depths greater than 10 to 20 cm.