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Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some , and other terrestrial planets and moons.
Regolith can vary from being essentially absent to hundreds of metres in thickness. Its age can vary from instantaneous (for an ash fall or alluvium just deposited) to hundreds of millions of years old (regolith of Precambrian age occurs in parts of Australia, (1991). 9781852930745, Belhaven Press. ISBN 9781852930745 though this may have been buried and subsequently exhumed. (2025). 9780643099968, Csiro Publishing. ISBN 9780643099968)
Regolith on Earth originates from weathering and biological processes. The uppermost part of the regolith, which typically contains significant organic matter, is more conventionally referred to as soil. The presence of regolith is one of the important factors for most life, since few can grow on or within solid rock and would be unable to burrow or build shelter without loose material.
Regolith is also important to engineers constructing buildings, roads and other civil works. The mechanical properties of regolith vary considerably and need to be documented if the construction is to withstand the rigors of use.
Regolith may host mineral deposits, such as mineral sands, calcrete uranium, and lateritic nickel deposits. Understanding regolith properties, especially geochemical composition, is critical to geochemical and geophysical exploration for mineral deposits beneath it.L. K. Kauranne, R. Salminen, & K. Eriksson 1992 Regolith Exploration Geochemistry in Arctic and Temperate Terrains. ElsevierC. R. M. Butt 1992 Regolith Exploration Geochemistry in Tropical and Subtropical Terrains. Elsevier The regolith is also an important source of construction material, including sand, gravel, crushed stone, lime, and gypsum.
The regolith is the zone through which aquifers are recharged and through which aquifer discharge occurs. Many aquifers, such as alluvial aquifers, occur entirely within regolith. The composition of the regolith can also strongly influence water composition through the presence of salts and acid-generating materials.
The impact of micrometeoroids, sometimes travelling faster than , generates enough heat to melt or partially vaporize dust particles. This melting and refreezing welds particles together into glassy, jagged-edged agglutinates, reminiscent of found on Earth.
The regolith is generally from 4 to 5 m thick in Lunar mare areas and from 10 to 15 m in the older highland regions. Below this true regolith is a region of blocky and fractured bedrock created by larger impacts, which is often referred to as the "megaregolith".
The density of regolith at the Apollo 15 landing site () averages approximately 1.35 g/cm3 for the top 30 cm, and it is approximately 1.85g/cm3 at a depth of 60 cm.
The term lunar soil is often used interchangeably with "lunar regolith" but typically refers to the finer fraction of regolith, that which is composed of grains one centimetre in diameter or less. Some have argued that the term "soil" is not correct in reference to the Moon because soil is defined as having organic matter content, whereas the Moon has none. However, standard usage among lunar scientists is to ignore that distinction. "Lunar dust" generally connotes even finer materials than lunar soil, the fraction which is less than 30 micrometers in diameter. The average chemical composition of regolith might be estimated from the relative concentration of elements in lunar soil.
The physical and optical properties of lunar regolith are altered through a process known as space weathering, which darkens the regolith over time, causing ray system to fade and disappear.
During the early phases of the Project Apollo Moon landing program, Thomas Gold of Cornell University and part of President's Science Advisory Committee raised a concern that the thick dust layer at the top of the regolith would not support the weight of the lunar module and that the module might sink beneath the surface. However, Joseph Veverka (also of Cornell) pointed out that Gold had miscalculated the depth of the overlying dust, which was only a couple of centimeters thick. Indeed, the regolith was found to be quite firm by the robotic Surveyor program spacecraft that preceded Apollo, and during the Apollo landings the astronauts often found it necessary to use a hammer to drive a core sample tool into it.
The sand is believed to move only slowly in the Martian winds due to the very low density of the atmosphere in the present epoch. In the past, liquid water flowing in gullies and river valleys may have shaped the Martian regolith. Mars researchers are studying whether groundwater sapping is shaping the Martian regolith in the present epoch and whether carbon dioxide hydrates exist on Mars and play a role. It is believed that large quantities of water and carbon dioxide ices remain frozen within the regolith in the equatorial parts of Mars and on its surface at higher latitudes.
The Huygens probe used a penetrometer on landing to characterize the mechanical properties of the local regolith. The surface itself was reported to be a clay-like "material which might have a thin crust followed by a region of relative uniform consistency." Subsequent data analysis suggests that surface consistency readings were likely caused by Huygens displacing a large pebble as it landed and that the surface is better described as a 'sand' made of ice grains. Titan probe's pebble 'bash-down', BBC News, April 10, 2005. The images taken after the probe's landing show a flat plain covered in pebbles. The pebbles, which may be made of water ice, are somewhat rounded, which may indicate the action of fluids on them. New Images from the Huygens Probe: Shorelines and Channels, But an Apparently Dry Surface , Emily Lakdawalla, 2005-01-15, verified 2005-03-28
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