Petrology of lunar soils

The moon is covered with a thin layer of unconsolidated debris called the lunar regolith. A typical sample of this regolith could be characterized as a gray, poorly sorted pebble‐ or cobble‐bearing silty sand having a bulk density of about 1.5 g/cm³. It consists of lithic and mineral fragments mostly from the local underlying bedrock and glass formed as melt by meteorite impacts. Most investigators conclude that regolith samples are formed primarily by impact‐cratering processes, although some may contain a minor pyroclastic component. The lunar regolith sampled by man commonly is stratified, the layers being interpreted as ejecta from primary or secondary impact craters. Physical and chemical indices of exposure at or near the lunar surface indicate that these layers, some sampled at depths of several meters, were once at the lunar surface. Each layer, which has distinct physical, petrographic, and chemical characteristics controlled by source areas for soil particles and the time of exposure to meteorite bombardment at the lunar surface, is here referred to as a lunar soil. The upper few millimeters of each soil layer, when exposed to the lunar environment, is subjected to intensive bombardment and reworking by micrometeorite impacts. These impacts break up soil particles (comminution) and melt a portion of the impacted soil. The melt, mixed with soil particles, forms irregular clusters called agglutinates. Micrometeorite bombardment of the surface soil layer continues until the layer is buried by fresh ejecta or is destroyed by a larger impact.