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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. [1] 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.

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. [2]

Lunar soil is composed of various types of particles including rock fragments, mono-mineralic fragments, and various kinds of glasses including agglutinate particles and volcanic and impact spherules. The agglutinates form at the lunar surface by micrometeorite impacts that cause small-scale melting which fuses adjacent materials together with tiny specks of metallic iron (Fe 0 ) embedded in each dust particle’s glassy shell.

Over time, material is mixed both vertically and horizontally (a process known as "gardening") by impact processes. However, the contribution of material from great distances is relatively minor, such that the soil composition at any given location largely reflects the local bedrock composition.

The significance of acquiring appropriate knowledge of lunar soil properties is great. The potential for construction of structures, [3] ground transportation networks, and waste disposal systems, to name a few examples, will depend on real-world experimental data obtained from testing lunar soil samples. The load-carrying capability of the soil is an important parameter in the design of such structures on Earth.

Due to myriad meteorite impacts (with velocities in the range of 20 km/s), the lunar surface is covered with a thin layer of dust. The dust is electrically charged and sticks to any surface it comes in contact with. The density of lunar regolith is about 1.5 g/cm 3 . [4] The soil becomes very dense beneath the top layer of regolith. Other factors which may affect the properties of lunar soil include large temperature differentials , the presence of a hard vacuum , and the absence of a significant lunar magnetic field (thereby allowing charged solar wind particles to continuously hit the surface of the Moon).