Lunar soil

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 continual meteoric impacts and bombardment by solar and 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 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 1 cm 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 μm in diameter, while others at less than 10 μm. The major processes involved in the formation of lunar soil are: These processes continue to change the physical and optical properties of the soil over time, and it 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. Lunar soil is composed of various types of particles including rock fragments, mono-mineralic fragments, and various kinds of glasses including agglutinate particles, 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 (Fe0) 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 external sources is relatively minor, such that the soil composition at any given location largely reflects the local bedrock composition. There are two profound differences in the chemistry of lunar regolith and soil from terrestrial materials. The first is that the Moon is very dry. As a result, those minerals with water as part of their structure (mineral hydration) such as clay, mica, and amphiboles are totally absent from the Moon. The second difference is that lunar regolith and crust are chemically reduced, rather than being significantly oxidized like the Earth's crust. In the case of the regolith, this is due in part to the constant bombardment of the lunar surface with protons (i.e. hydrogen (H) nuclei) from the solar wind. One consequence is that iron on the Moon is found in the metallic 0 and +2 oxidation states, whereas on Earth iron is found primarily in the +2 and +3 oxidation states. The significance of acquiring appropriate knowledge of lunar soil properties is great. The potential for construction of structures, 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/cm3. 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.