Lunar regolith

Lunar soil is the fine fraction of the regolith found on the surface of the moon. Its properties can differ significantly from those of terrestrial soil. The physical properties of lunar soil are primarily the result of mechanical disintegration of basaltic and anorthositic rock, caused by continuous meteoric impact and bombardment by interstellar charged atomic particles over years. The process is largely one of mechanical weathering in which the particles are ground to finer and finer size over time. This situation contrasts fundamentally to terrestrial soil formation, mediated by the presence of molecular oxygen (O₂), humidity, atmospheric wind, and a robust array of contributing biological processes. Some have argued that the term "soil" is not correct in reference to the Moon because on the Earth, soil is defined as having organic content, whereas the Moon has none. However, standard usage among lunar scientists is to ignore that distinction.

The term lunar soil is often used interchangeably with "lunar regolith" but typically refers to only the finer fraction of regolith, that which is composed of grains one centimeter in diameter or less.Lunar dust generally connotes even finer materials than lunar soil. There is no official definition of what size fraction constitutes "dust", some place the cutoff at less than 50 micrometres in diameter, others at less than 10.

The major processes involved in the formation of lunar soil are:

These processes not only form lunar soil, they also continue to change the physical and optical properties of the soil over time; this process is known as space weathering. In addition, fire fountaining, whereby volcanic lava is lofted and cools into small glass beads before falling back to the surface, can create small but important deposits in some locations, such as the orange soil found at Shorty Crater in the Taurus-Littrow valley by Apollo 17, and the green glass found at Hadley-Apennine by Apollo 15. Deposits of volcanic beads are also thought to be the origin of Dark Mantle Deposits (DMD) in other locations around the Moon.

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