The Lunar Regolith

Regolith is a terrestrial term, also used for the Moon. It has been defined as " a general term for the layer or mantle of fragmental and unconsolidated rock material, whether residual or transported and of highly varied character, that nearly everywhere forms the surface of the land and overlies or covers bedrock. It includes rock debris of all kinds, [including] volcanic ash. .. " (Bates and Jackson, 1980). All the lunar landings and all photographic investigations show , that the entire lunar surface consists of a regolith layer that completely covers the underlying bedrock, except perhaps on some very steep-sided crater walls and lava channels, where there may be exposed bedrock. The regoliths developed on the Earth are produced by processes that are uniquely terrestrial—the presence of oxygen, the influences of wind and water, and the activities of life. In contrast, on the airless, lifeless Moon, the lunar regolith results from uniquely different processes—the continuous impact of large and small meteoroids and the steady bombardment of the lunar surface by charged atomic particles from the sun and the stars. Exposed rocks on the lunar surface are covered with impact craters whose diameters range from more than 1000 km to less than 1 µm. The corresponding impacting objects range from asteroids tens of kilometers in diameter to particles of cosmic dust a few hundred angstroms across, a range of more than 12 orders of magnitude. Despite the size disparity, the effects on bedrock are similar at both ends of the size spectrum—excavation of a crater, accompanied by shattering, pulverization, melting, mixing, and dispersal of the original coherent bedrock to new locations in and around the crater. The moment that fresh bedrock is exposed on the Moon (e.g., by the eruption of a lava flow), meteoroid bombardment begins to destroy it. As the impacts continue, the original bedrock is covered by a fragmental layer of broken, melted, and otherwise altered debris from innumerable superimposed craters. This layer is the lunar regolith. Studies of returned samples have shown that the bulk of this lunar regolith (informally called the lunar soil) consists of particles <1 cm in size although larger cobbles and boulders, some as much as several meters across, are commonly found at the surface. Because the impact cratering events produce shock overpressures and heat, much of the pulverized material is melted and welded together to produce breccias (fragmental rocks) and impact …