Our understanding of the lunar (and asteroidal) regolith has evolved in fits and starts over the last five or six decades. Before Zond 3 (1959), observations of the Moon were restricted to Earth-based observations. From 1959 onwards, the era of spacecraft exploration began along with a brief interval of human exploration. Data from spacecraft as well as the samples returned by the Apollo and Luna missions, and the surface experiments conducted by Apollo and Lunokhod provided a detailed picture of the structure and origin of the lunar regolith. At that point in time, the regolith was viewed as a fragmental layer covering the surface, primarily formed by impact events fracturing surface material and redistributing it into localized layers around craters. Space weathering then modified the fragmental material by a combination of micrometeor melting and vaporization and radiation effects. The regolith was viewed as consisting of range of particles sizes from microns to meters with a thin, low-density surface that gradually became dense with depth. While there is a more or less uniform background of small rocks (
Over the last decade, results from a series of robotic missions, particularly LRO, as well as continued observation from Earth, modeling and theoretical studies, allowed our understanding of the regolith to take a giant leap. We now recognize that the regolith is a more complicated and heterogeneous system with significant variations in its properties. Perhaps most amazing is the recognition that the regolith is a catalyst for the formation of H2O and OH and acts as an ephemeral and long-term storage reservoir for volatiles. The distribution of large m-scale rocks varies across the surface in some cases such rocks appear abundant in the subsurface while the surface exhibits few rocks. Areas around craters have had the regolith disturbed by the impact events, such that is remains colder than the surrounding regions, despite appearing visually normal. The surface is coated by a few cm thick layer having very high porosity (70-90%), below that depth the density increases rapid to a depth of ~10 cm. Temperatures in permanently shadowed craters can drop to 40K and the thermal conductivity of the regolith varies by over a factor of 100. The contribution of thermal fatigue to breaking down rocks may be an important factor in the generation of the regolith small-size fraction. Overturn and gardening of the surface appear to be occurring at a rate almost 100 times greater than previous estimates.
These various data sets indicate that the processes of regolith formation and evolution are more complicated than the early view of a more or less simple fragmental layer overlying mare basalt or highlands megaregolith, that gracefully ages with time in a more or less uniform manner.