The lunar regolith: Chemistry, mineralogy, and petrology

The lunar regolith, a several meter thick layer of unconsolidated debris, forms the interface between the moon and its space environment. The regolith forms from lithic sources by the destructional processes of comminution and the constructional processes of agglutinate formation. In this manner a steady state soil can develop and will remain in a type of dynamic equilibrium until the system is disturbed by burial or mixing with new soil. Mixing takes place mainly on a local scale (vertically through meters and horizontally over several kilometers). Lateral transport is a relatively inefficient process, and therefore most of the lunar soil components were derived locally. There is no evidence for preferential transport of the finer soil fractions relative to the coarser fractions. The truly ‘exotic’ (derived at distances of tens of kilometers) component is generally in low abundance (≪1%). Therefore remotely sensed data should provide a reasonably accurate picture of bedrock geology. The chemical composition of the regolith varies with grain size. The <10 µm fraction is chemically fractionated from the bulk soil. In most cases the <10 µm fraction is enriched in Al 2 O 3 , CaO, Na 2 O, K 2 O, light rare earth elements, and Th and depleted in MgO, FeO, MnO, and Sc. These systematics are largely a result of simple comminution, with feldspar and friable mesostasis from local source rocks concentrating in the finest fraction. There is evidence that agglutinate glass forms preferentially by fusion of the finest soil fractions. Therefore agglutinate glass, like the <10 µm soil fraction, appears to be fractionated from bulk soil compositions. Highland soils are distinct from mare soils both chemically and petrologically. Soil samples collected near mare‐highland contacts are mixtures of the two lithologies. Mineral chemical systematics of monomineralic fragments in the lunar soil provide important clues to soil provenance. They can, in some cases, sort out the highland/mare contributions and, in special cases, can even further pinpoint a lithic source. In theory, lunar regolith cores can serve as solar probes, recording the sun's activity over billions of years. In practice, this has generally not been the case. The formation and evolution of most cores is still poorly understood. For most, we have not established how they formed, for example, how many depositional and erosional events were involved, what the time span involved in the accretion of the sampled soil column was, and how long that soil column has remained in situ. Without this information it is difficult, if not impossible, to read the sun's record. More interdisciplinary studies on cores must be carried out before they can be used as solar probes.