Physical Properties of the Lunar Surface

This chapter discusses the physical properties of the lunar surface, determined at depths varying from a few micrometers to a few tens of meters. The properties considered include geotechnical properties (those properties of a planetary surface needed to evaluate engineering problems, including the mechanical properties of soil and rock), electrical and electromagnetic properties, and the reflection and emission of radiation. The lunar surface material (regolith or “lunar soil”) is a complex mixture of five basic particle types: crystalline rock fragments, mineral fragments, breccias, agglutinates, and glasses. The relative proportion of each particle type varies from place to place and is dependent on the mineralogy of the source rocks and the geologic processes that the rocks have undergone (see Chapters 4 and 7 for more details). The lunar regolith is distinctly different from terrestrial materials. For one thing, its mineral composition is limited. Fewer than a hundred minerals have been found on the Moon, compared to several thousand on Earth (see Chapter 5). Furthermore, continuous meteoroid impact has converted the lunar surface materials into a wellgraded silty sand that has reached a “steady state” in thickness, particle size distribution, and other properties at most locations on the Moon. Hence, the limited particle types and sizes, the absence of a lunar atmosphere, and the lack of water and organic material in the regolith constrain the physical properties of the lunar surface material to relatively narrow, well-defined ranges. The primary factors that affect the physical properties are bulk density, relative density, and temperature. The physical properties of lunar soils and rocks have been measured in situ by robots and astronauts, in the laboratory on returned samples, and by remote sensing from the Earth’s surface and from lunar orbit. The results of these measurements are presented in the following sections.