Regolith

Regolith

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Encyclopedia
Regolith is a layer of loose, heterogeneous material covering solid rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

. It includes dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

, soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

, broken rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

, and other related materials and is present on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, some asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s, and other terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

s and moons
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

.

Etymology


The term regolith combines two Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 words: rhegos (ῥῆγος), "blanket", and lithos (λίθος), "rock". George P. Merrill
George Perkins Merrill
George Perkins Merrill was an American geologist.He was educated at the University of Maine , took a post-graduate courses of study and was assistant in chemistry at Wesleyan University, Connecticut , and subsequently studied at Johns Hopkins...

 first defined the term in 1897, writing:

Earth


Earth's regolith comprises the following subdivisions and components:
  • soil
    Soil
    Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

     or pedolith
  • alluvium
    Alluvium
    Alluvium is loose, unconsolidated soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel...

     and other transported cover, including that transported by aeolian, glacial, marine, and gravity flow processes.
  • saprolith, generally divided into the
    • upper saprolite
      Saprolite
      Saprolite is a chemically weathered rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock surface. In most outcrops its color comes from ferric compounds...

      : completely oxidised bedrock
      Bedrock
      In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

    • lower saprolite: chemically reduced partially weathered rocks
    • saprock: fractured bedrock with weathering restricted to fracture margins.
  • volcanic ash
    Volcanic ash
    Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...

     and lavas
  • duricrust
    Duricrust
    Duricrust is a hard layer on or near the surface of soil. Duricrusts can range in thickness from a few millimeters or centimeters to several meters....

    , formed by cementation of soils, saprolith and transported material by clays, silicates, iron oxides and oxyhydroxides, carbonates and sulfates, as well as less common agents, into indurated layers resistant to weathering and erosion.
  • groundwater
    Groundwater
    Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...

    - and water-deposited salts.
  • biota
    Biota (ecology)
    Biota are the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biota of the Earth lives in the biosphere.-See...

     and organic
    Organic matter
    Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...

     components derived from it.


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
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

 age occurs in parts of Australia).

Regolith on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 originates from weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

 and biological processes
Biota (ecology)
Biota are the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biota of the Earth lives in the biosphere.-See...

; 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
Dirt
Dirt is unclean matter, especially when in contact with a person's clothes, skin or possessions when they are said to become dirty. Common types of dirt include:* dust — a general powder of organic or mineral matter...

, dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

, gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...

, sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

, and (when wet) mud
Mud
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...

.

On Earth, the presence of regolith is one of the important factors for most life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

, since few plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s can grow on or within solid rock and animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s 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 many mineral deposits, for example 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. 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.


Moon


Nearly the entire lunar
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 surface is covered with regolith, bedrock
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

 being exposed only on very steep-sided crater walls and the occasional lava channel
Lava channel
A lava channel is a stream of fluid lava contained within marginal zones of static lava or levees. The initial channel may not contain levees per se, until the parental flow solidifies over what develops into the channel and creates simple levees. This initial levee allows for the building of a...

. This regolith has been formed over the last 4.6 billion years by the impact of large and small meteoroid
Meteoroid
A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite...

s and the steady bombardment of micrometeoroid
Micrometeoroid
A micrometeoroid is a tiny meteoroid; a small particle of rock in space, usually weighing less than a gram. A micrometeor or micrometeorite is such a particle that enters the Earth's atmosphere or falls to Earth.-Scientific interest:...

s and solar and galactic charged particles breaking down surface rocks.

The impact of micrometeoroids, sometimes travelling faster than 60,000 mph (96,000 km/h), generates enough heat to melt or partially vaporize dust particles. This melting and refreezing welds particles together into glassy, jagged-edged agglutinates, reminiscent of tektite
Tektite
Tektites are natural glass rocks up to a few centimeters in size, which most scientists argue were formed by the impact of large meteorites on Earth's surface. Tektites are typically black or olive-green, and their shape varies from rounded to irregular.Tektites are among the "driest" rocks, with...

s found on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

.

The regolith is generally from 4 to 5 meters thick in mare
Lunar mare
The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...

 areas and from 10 to 15 meters 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 term lunar soil
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...

 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
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

" is not correct in reference to the Moon because soil is defined as having organic
Organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...

 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
Space weathering
Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...

, which darkens the regolith over time, causing crater rays
Ray system
A ray system comprises radial streaks of fine ejecta thrown out during the formation of an impact crater, looking a bit like many thin spokes coming from the hub of a wheel. The rays can extend for lengths up to several times the diameter of their originating crater, and are often accompanied by...

 to fade and disappear.

During the early phases of the Apollo
Project Apollo
The Apollo program was the spaceflight effort carried out by the United States' National Aeronautics and Space Administration , that landed the first humans on Earth's Moon. Conceived during the Presidency of Dwight D. Eisenhower, Apollo began in earnest after President John F...

 Moon landing program, Thomas Gold
Thomas Gold
Thomas Gold was an Austrian-born astrophysicist, a professor of astronomy at Cornell University, a member of the U.S. National Academy of Sciences, and a Fellow of the Royal Society . Gold was one of three young Cambridge scientists who in the 1950s proposed the now mostly abandoned 'steady...

 of Cornell University
Cornell University
Cornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...

 and part of President's Science Advisory Committee
President's Science Advisory Committee
In 1951 President of the United States Harry S. Truman established the Science Advisory Committee as part of the Office of Defense Mobilization . As a direct response to the launches of the Soviet artificial satellites, Sputnik 1 and Sputnik 2, on October 4 and November 3, 1957, the Science...

 raised a concern that the thick dust layer at the top of the regolith would not support the weight of the lunar module
Apollo Lunar Module
The Apollo Lunar Module was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman to carry a crew of two from lunar orbit to the surface and back...

 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
Surveyor program
The Surveyor Program was a NASA program that, from 1966 through 1968, sent seven robotic spacecraft to the surface of the Moon. Its primary goal was to demonstrate the feasibility of soft landings on the Moon...

 spacecraft that preceded Apollo, and during the Apollo landings the astronauts often found it necessary to use a hammer
Hammer
A hammer is a tool meant to deliver an impact to an object. The most common uses are for driving nails, fitting parts, forging metal and breaking up objects. Hammers are often designed for a specific purpose, and vary widely in their shape and structure. The usual features are a handle and a head,...

 to drive a core sampling
Core sample
A core sample is a cylindrical section of a naturally occurring substance. Most core samples are obtained by drilling with special drills into the substance, for example sediment or rock, with a hollow steel tube called a core drill. The hole made for the core sample is called the "core hole". A...

 tool into it.

Mars


Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 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 storm
Dust storm
A dust / sand storm is a meteorological phenomenon common in arid and semi-arid regions. Dust storms arise when a gust front or other strong wind blows loose sand and dirt from a dry surface. Particles are transported by saltation and suspension, causing soil to move from one place and deposition...

s. 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 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
Groundwater sapping
Groundwater sapping is the geomorphic process in which groundwater exits a bank or hillslope laterally as seeps and springs and erodes soil from the slope. This often causes the slope to be undermined and undergo mass wasting, hence the word sapping....

 is shaping the martian regolith in the present epoch, and whether carbon dioxide hydrates
Carbon dioxide clathrate
Carbon dioxide hydrate is a Type I gas clathrate . However, there has been some experimental evidence for the development of a metastable Type II phase at temperature near the ice melting point ....

 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.

Asteroids



Asteroids have regoliths developed by meteoroid impact. The final images taken by the NEAR Shoemaker
NEAR Shoemaker
The Near Earth Asteroid Rendezvous - Shoemaker , renamed after its 1996 launch in honor of planetary scientist Eugene M. Shoemaker, was a robotic space probe designed by the Johns Hopkins University Applied Physics Laboratory for NASA to study the near-Earth asteroid Eros from close orbit over a...

 spacecraft of the surface of Eros
433 Eros
433 Eros is a near-Earth asteroid discovered in 1898, and the first asteroid to be orbited by a probe . It is an S-type asteroid approximately 34.4×11.2×11.2 km in size, the second-largest NEA after 1036 Ganymed, and belongs to the Amor group.Eros is a Mars-crosser asteroid, the first known...

 are the best images we have of an asteroidal regolith. The recent Japanese Hayabusa
Hayabusa
was an unmanned spacecraft developed by the Japan Aerospace Exploration Agency to return a sample of material from a small near-Earth asteroid named 25143 Itokawa to Earth for further analysis....

 mission also returned spectacular and surprising images of an asteroidal regolith on an asteroid so small it was thought that gravity was too low to develop and maintain a regolith. The asteroid 21 Lutetia
21 Lutetia
21 Lutetia is a large main-belt asteroid of an unusual spectral type. It measures about 100 kilometers in diameter . It was discovered in 1852 by Hermann Goldschmidt, and is named after Lutetia, the Latin name for Paris....

 has a layer of regolith near its north pole, which flows in landslides associated with variations in albedo.

Titan



Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

 is known to have extensive fields of dunes, though the origin of the material forming the dunes is not known - it could be small fragments of water ice eroded by flowing methane, or possibly 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 the mechanical
Mechanics
Mechanics is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment....

 similarity with regolith on other bodies, although traditionally (and etymologically
Etymology
Etymology is the study of the history of words, their origins, and how their form and meaning have changed over time.For languages with a long written history, etymologists make use of texts in these languages and texts about the languages to gather knowledge about how words were used during...

) the term had been applied only when the loose layer was composed of mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

 grains like quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 or plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...

 or rock fragments that were in turn composed of such minerals. Loose blankets of ice grains were not considered to be regolith because when they appear on Earth in the form of snow
Snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...

 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
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 and aeolian and/or sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

ary processes is unique to Titan because of its thermodynamic environment.

The Huygens probe
Huygens probe
The Huygens probe was an atmospheric entry probe carried to Saturn's moon Titan as part of the Cassini–Huygens mission. The probe was supplied by the European Space Agency and named after the Dutch 17th century astronomer Christiaan Huygens....

 used a penetrometer
Penetrometer
A penetrometer is a device to test the strength of a material such as soil. There are many types of penetrometer. They are usually round or cone shaped. The penetrometer is dropped on the test subject or pressed against it and the depth of the resulting hole is measured. The measurements determine...

 on landing to characterize the mechanical properties of the local regolith. The surface itself was reported to be a clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

-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 in pebbles. The pebbles, which may be made of water ice, are somewhat rounded, which may indicate the action of fluids on them.

See also


  • Helium-3
    Helium-3
    Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...

  • In-situ resource utilization
    In-Situ Resource Utilization
    In space exploration, in-situ resource utilization describes the proposed use of resources found or manufactured on other astronomical objects to further the goals of a space mission....

     
  • Lunar regolith simulant
    Lunar regolith simulant
    A Lunar regolith simulant is a terrestrial material synthesized in order to approximate the chemical, mechanical, and engineering properties of, and the mineralogy and particle size distributions of, lunar regolith. Lunar regolith simulants are used by researchers who wish to research the...


  • Soil
    Soil
    Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

  • Sand
    Sand
    Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

  • Lunarcrete
    Lunarcrete
    Lunarcrete, also known as "Mooncrete", an idea first proposed by Larry A. Beyer of the University of Pittsburgh in 1985, is a hypothetical aggregate building material, similar to concrete, formed from lunar regolith, that would cut the construction costs of building on the Moon.- Ingredients :Only...



External links