Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials. It is found on Earth, the Moon, some asteroids, and other planets.
On Earth, regolith is an important factor that supports living organisms. In addition, it is found to host many minerals and is the zone through which aquifers are recharged and discharged. Civil engineers need to take into consideration the type of regolith present at construction sites.
The Moon's surface is almost entirely covered with regolith, formed by the impact of large and small meteoroids and particulate matter. Likewise, asteroids have regolith formed by meteoroid impact. The Martian surface is covered with sand, dust, and rocks. In addition, Saturn's moon Titan has been observed to have extensive dune fields.
The term regolith is a combination of two Greek words: rhegos, which means "blanket," and lithos, which means "rock." It was first defined by George P. Merrill in 1897. He stated, "In places this covering is made up of material originating through rock-weathering or plant growth in situ. In other instances it is of fragmental and more or less decomposed matter drifted by wind, water or ice from other sources. This entire mantle of unconsolidated material, whatever its nature or origin, it is proposed to call the regolith."
On Earth, regolith (Ollier and Pain 1996) (Taylor and Eggleton 2001) (Scott and Pain 2009). is composed of the following types of components:
In some areas of the Earth, regolith is essentially absent, whereas in others, it is hundreds of meters in thickness. It may be freshly formed, such as from an ash fall or alluvium that has just been deposited, to hundreds of millions of years old. Regolith of the Precambrian age has been found in parts of Australia (Ollier 1991).
On Earth, regolith is formed through weathering and biological processes. If it contains a significant proportion of biological compounds, it is more conventionally referred to as soil. People also call various types of earthly regolith by such names as dirt, dust, gravel, sand, and (when wet) mud.
The presence of regolith on Earth is one of the important factors that support the existence of life forms. Few plants can grow on or through solid rock, and animals would be unable to burrow or build shelter without loose material.
Regolith is also an important consideration for engineers constructing buildings, roads and other civil works. The mechanical properties of regolith vary considerably and need to be documented if the structure to be constructed is to withstand the rigors of use.
Many mineral deposits are hosted in regolith. Examples include mineral sands, calcrete uranium, and lateritic nickel deposits, among others. Elsewhere, understanding regolith properties, especially geochemical composition, is critical to geochemical and geophysical exploration for mineral deposits beneath it (Kauranne, Salminen, and Eriksson, 1992) (Butt, 1992). 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.
Nearly the entire lunar surface is covered with regolith, bedrock being exposed only on very steep-sided crater walls and the occasional lava channel. This regolith has been formed over the last 4.6 billion years by the impact of large and small meteoroids and the steady bombardment of micrometeoroids and solar and galactic charged particles breaking down surface rocks.
The impact of micrometeoroids, sometimes traveling faster than 60,000 mph (30 km/s), generates enough heat to melt or partially vaporize dust particles. This melting and refreezing welds particles together into glassy, jagged-edged agglutinates.
The regolith is generally about 4-5 meters thick in mare areas and 10-15 meters in older highland regions (Heiken, Vaniman, and French 1991). 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 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 centimeter 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 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 physical and optical properties of lunar regolith are altered through a process known as space weathering, which darkens the regolith over time, causing crater rays to fade and disappear.
During the early phases of the 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 spacecraft that preceded Apollo, and during Apollo program the astronauts often found it necessary to use a hammer to drive a core sampling tool into it.
Mars is covered with vast expanses of sand and dust and its surface is littered with rocks and boulders. The dust is occasionally picked up in vast planet-wide dust storms. Mars dust is very fine and enough remains suspended in the atmosphere to give the sky a reddish hue. The sand is believed to move relatively slowly in the Martian winds, because the atmospheric density is very low at present. It is thought that in the past, liquid water flowing in gullies and river vallies may have shaped the Martian regolith.
Mars researchers are studying whether groundwater sapping is now shaping the Martian regolith, and whether Mars has carbon dioxide hydrates that 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.
Asteroids have regolith developed by meteoroid impact. The final images taken by the NEAR Shoemaker spacecraft of the surface of 433 Eros, a near-Earth asteroid, are the best images we have of asteroidal regolith. The recent Japanese Hayabusa mission also returned spectacular and surprising images of regolith on an asteroid so small it was thought that gravity was too low to develop and maintain a regolith.
Titan, Saturn's largest moon, has been observed to have extensive fields of dunes, though the origin of the material forming the dunes is not known. The material could be small fragments of water ice eroded by flowing methane, or particulate organic matter that formed in Titan's atmosphere and rained down on the surface. Scientists are beginning to call this loose, icy material regolith because of its mechanical similarity with regolith on other bodies, although traditionally the term has been applied only when the material was composed of mineral or rock fragments. Loose blankets of ice grains were not considered regolith because when they appear on Earth in the form of snow they behave differently than regolith, the grains melting and fusing with only small changes in pressure or temperature. The idea of an ice-regolith complete with erosion and aeolian and/or sedimentary processes is new to Titan because of its thermodynamic environment.
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 analysis of the data 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. The images taken after the probe's landing show a flat plain covered with pebbles. The somewhat rounded pebbles, which may be made of water ice, suggest the action of fluids on them.
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