Regolith (/ˈrɛɡəlɪθ/) is a layer of loose, heterogeneous superficial deposits covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestrial planets and moons.
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.
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).
Regolith on Earth originates from 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.
On Earth, the presence of regolith is one of the important factors for most life, since few plants can grow on or within solid rock and animals 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.
Regolith covers almost the entire lunar surface, bedrock protruding only on very steep-sided crater walls and the occasional lava channel. This regolith has formed over the last 4.6 billion years from the impact of large and small meteoroids, from the steady bombardment of micrometeoroids and from solar and galactic charged particles breaking down surface rocks.
The impact of micrometeoroids, sometimes travelling faster than 96,000 km/h (60,000 mph), generates enough heat to melt or partially vaporize dust particles. This melting and refreezing welds particles together into glassy, jagged-edged agglutinates, reminiscent of tektites found on Earth.
The regolith is generally from 4 to 5 m thick in 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 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 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.
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 the Apollo landings 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 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.
Asteroids have regoliths developed by meteoroid impact. The final images taken by the NEAR Shoemaker spacecraft of the surface of Eros are the best images of the regolith of an asteroid. The recent Japanese Hayabusa mission also returned clear images of regolith on an asteroid so small it was thought that gravity was too low to develop and maintain a regolith. The asteroid 21 Lutetia has a layer of regolith near its north pole, which flows in landslides associated with variations in albedo.
Saturn's largest moon Titan 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 similarity with regolith on other bodies, although traditionally (and etymologically) the term had been applied only when the loose layer was composed of mineral grains like quartz or plagioclase 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 they behave differently from regolith, the grains melting and fusing with only small changes in pressure or temperature. However, Titan is so cold that ice behaves like rock. Thus there is an ice-regolith complete with erosion and aeolian and/or sedimentary processes.
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 in pebbles. The pebbles, which may be made of water ice, are somewhat rounded, which may indicate the action of fluids on them.
(101429) 1998 VF31 is a sub-kilometer asteroid that orbits near Mars's L5 Lagrangian point, on average trailing 60° behind it. Its orbit is highly stable, and was originally thought to be spectroscopically similar to 5261 Eureka, suggesting they may both be primordial Martian asteroids.Spectroscopic observations through 2007 indicate that it has a large proportion of metal and achondrites on its surface (either with or without a mesosiderite contribution); which could also indicate that the surface regolith has undergone space weathering. These observations also reveal differences in the spectra with 5261 Eureka, suggesting they may not be related to each other after all.Badlands
Badlands are a type of dry terrain where softer sedimentary rocks and clay-rich soils have been extensively eroded by wind and water. They are characterized by steep slopes, minimal vegetation, lack of a substantial regolith, and high drainage density. They can resemble malpaís, a terrain of volcanic rock. Canyons, ravines, gullies, buttes, mesas, hoodoos and other such geologic forms are common in badlands. They are often difficult to navigate by foot. Badlands often have a spectacular color display that alternates from dark black/blue coal stria to bright clays to red scoria.Bauxite
Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO(OH)), mixed with the two iron oxides goethite (FeO(OH)) and haematite (Fe2O3), the aluminium clay mineral kaolinite (Al2Si2O5(OH)) and small amounts of anatase (TiO2) and ilmenite (FeTiO3 or FeO.TiO2).In 1821 the French geologist Pierre Berthier discovered bauxite near the village of Les Baux in Provence, southern France.Bedrock
Bedrock in geology is the lithified rock that lies under loose softer material called regolith within the surface of the Earth's crust or other terrestrial planets.Eucrite
Eucrites are achondritic stony meteorites, many of which originate from the surface of the asteroid 4 Vesta and as such are part of the HED meteorite clan. They are the most common achondrite group with well over 100 distinct finds at present.
Eucrites consist of basaltic rock from the crust of 4 Vesta or a similar parent body. They are mostly composed of Ca-poor pyroxene, pigeonite, and Ca-rich plagioclase (anorthite).Based on differences of chemical composition and features of the component crystals, they are subdivided into several groups:
Non-cumulate eucrites Are the most common variety and can be subdivided further:
Main series eucrites formed near the surface and are mostly, though not exclusively, regolith breccias lithified under the pressure of overlying newer deposits.
Stannern trend eucrites are a rare variety.
Nuevo Laredo trend eucrites are thought to come from deeper layers of 4 Vesta's crust, and are a transition group towards the cumulate eucrites.
Cumulate eucrites are rare types with oriented crystals, thought to have solidified in magma chambers deep within Vesta's crust.
Polymict eucrites are regolith breccias consisting of mostly eucrite fragments and less than one part in ten of diogenite, an arbitrary dividing line from the howardites, which are related in structure. They are comparably rare.Geology of the Moon
The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "lunar science") is quite different from that of Earth. The Moon lacks a significant atmosphere, which eliminates erosion due to weather; it does not have any known form of plate tectonics, it has a lower gravity, and because of its small size, it cooled more rapidly. The complex geomorphology of the lunar surface has been formed by a combination of processes, especially impact cratering and volcanism. The Moon is a differentiated body, with a crust, mantle, and core.
Geological studies of the Moon are based on a combination of Earth-based telescope observations, measurements from orbiting spacecraft, lunar samples, and geophysical data. Six locations were sampled directly during the crewed Apollo program landings from 1969 to 1972, which returned 380.96 kilograms (839.9 lb) of lunar rock and lunar soil to Earth. In addition, three robotic Soviet Luna spacecraft returned another 326 grams (11.5 oz) from 1970 to 1976. The Moon is the only extraterrestrial body for which we have samples with a known geologic context. A handful of lunar meteorites have been recognized on Earth, though their source craters on the Moon are unknown. A substantial portion of the lunar surface has not been explored, and a number of geological questions remain unanswered.Howardite
Howardites are achondritic stony meteorites that originate from the surface of the asteroid 4 Vesta, and as such are part of the HED meteorite clan. There are about 200 distinct members known.Laterite
Laterite is a soil and rock type rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by intensive and prolonged weathering of the underlying parent rock. Tropical weathering (laterization) is a prolonged process of chemical weathering which produces a wide variety in the thickness, grade, chemistry and ore mineralogy of the resulting soils. The majority of the land area containing laterites is between the tropics of Cancer and Capricorn.
Laterite has commonly been referred to as a soil type as well as being a rock type. This and further variation in the modes of conceptualizing about laterite (e.g. also as a complete weathering profile or theory about weathering) has led to calls for the term to be abandoned altogether. At least a few researchers specializing in regolith development have considered that hopeless confusion has evolved around the name. There is no likelihood, however, that the name will ever be abandoned; for material that looks highly similar to the Indian laterite occurs abundantly worldwide, and it is reasonable to call such material laterite.Historically, laterite was cut into brick-like shapes and used in monument-building. After 1000 CE, construction at Angkor Wat and other southeast Asian sites changed to rectangular temple enclosures made of laterite, brick, and stone. Since the mid-1970s, some trial sections of bituminous-surfaced, low-volume roads have used laterite in place of stone as a base course. Thick laterite layers are porous and slightly permeable, so the layers can function as aquifers in rural areas. Locally available laterites have been used in an acid solution, followed by precipitation to remove phosphorus and heavy metals at sewage-treatment facilities.
Laterites are a source of aluminium ore; the ore exists largely in clay minerals and the hydroxides, gibbsite, boehmite, and diaspore, which resembles the composition of bauxite. In Northern Ireland they once provided a major source of iron and aluminium ores. Laterite ores also were the early major source of nickel.List of lunar meteorites
This is a list of lunar meteorites. That is, meteorites that have been identified as having originated from Earth's Moon.Lunar regolith simulant
A lunar regolith simulant is a terrestrial material synthesized in order to approximate the chemical, mechanical, or 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 materials handling, excavation, transportation, and uses of lunar regolith. Samples of actual lunar regolith are too scarce, and too small, for such research.Lunar resources
The Moon bears substantial natural resources which could be exploited in the future. Potential lunar resources may encompass processable materials such as volatiles and minerals, along with geologic structures such as lava tubes that together, might enable lunar habitation. The use of resources on the Moon may provide a means of reducing the cost and risk of lunar exploration and beyond.Insights about lunar resources gained from orbit and sample-return missions have greatly enhanced the understanding of the potential for in situ resource utilization (ISRU) at the Moon, but that knowledge is not yet sufficient to fully justify the commitment of large financial resources to implement an ISRU-based campaign. The determination of resource availability will drive the selection of sites for human settlement.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.Martian regolith simulant
Martian regolith simulant (or Martian soil simulant) is a terrestrial material that is used to simulate the chemical and mechanical properties of Martian regolith for research, experiments and prototype testing of activities related to Martian regolith such as dust mitigation of transportation equipment, advanced life support systems and in-situ resource utilization.Martian soil
Martian soil is the fine regolith found on the surface of Mars. Its properties can differ significantly from those of terrestrial soil, including its toxicity due to the presence of perchlorates. The term Martian soil typically refers to the finer fraction of regolith. On Earth, the term "soil" usually includes organic content. In contrast, planetary scientists adopt a functional definition of soil to distinguish it from rocks. Rocks generally refer to 10 cm scale and larger materials (e.g., fragments, breccia, and exposed outcrops) with high thermal inertia, with areal fractions consistent with the Viking Infrared Thermal Mapper (IRTM) data, and immobile under current aeolian conditions. Consequently, rocks classify as grains exceeding the size of cobbles on the Wentworth scale.
This approach enables agreement across Martian remote sensing methods that span the electromagnetic spectrum from gamma to radio waves. ‘‘Soil’’ refers to all other, typically unconsolidated, material including those sufficiently fine-grained to be mobilized by wind. Soil consequently encompasses a variety of regolith components identified at landing sites. Typical examples include: bedform armor, clasts, concretions, drift, dust, rocky fragments, and sand. The functional definition reinforces a recently proposed genetic definition of soil on terrestrial bodies (including asteroids and satellites) as an unconsolidated and chemically weathered surficial layer of fine-grained mineral or organic material exceeding centimeter scale thickness, with or without coarse elements and cemented portions.Martian dust generally connotes even finer materials than Martian soil, the fraction which is less than 30 micrometres in diameter. Disagreement over the significance of soil's definition arises due to the lack of an integrated concept of soil in the literature. The pragmatic definition "medium for plant growth" has been commonly adopted in the planetary science community but a more complex definition describes soil as "(bio)geochemically/physically altered material at the surface of a planetary body that encompasses surficial extraterrestrial telluric deposits." This definition emphasizes that soil is a body that retains information about its environmental history and that does not need the presence of life to form.Moon Diver (spacecraft)
Moon Diver is a proposed lunar mission concept by NASA's Jet Propulsion Laboratory that would employ a robotic lander and a small two-wheeled rover called Axel to investigate the exposed geological layers on the walls of a deep lunar pit.The mission was proposed in mid-2019 to NASA's Discovery Program to compete for funding and development. The finalists will be announced in January 2020. If selected, it would launch no earlier than 2025.OSIRIS-REx
The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) is a NASA asteroid study and sample-return mission. The mission's main goal is to obtain a sample of at least 60 grams (2.1 oz) from 101955 Bennu, a carbonaceous near-Earth asteroid, and return the sample to Earth for a detailed analysis. The material returned is expected to enable scientists to learn more about the formation and evolution of the Solar System, its initial stages of planet formation, and the source of organic compounds that led to the formation of life on Earth. If successful, OSIRIS-REx will be the first U.S. spacecraft to return samples from an asteroid. The Lidar instrument used aboard the OSIRIS-REx was built by Lockheed Martin, in conjunction with the Canadian Space Agency.OSIRIS-REx was launched on 8 September 2016, flew past Earth on 22 September 2017, and reached the proximity of Bennu on 3 December 2018, where it began analyzing its surface for a target sample area over the next several months. It is expected to return with its sample to Earth on 24 September 2023.The cost of the mission is approximately US$800 million not including the Atlas V launch vehicle, which is about US$183.5 million. It is the third planetary science mission selected in the New Frontiers program, after Juno and New Horizons. The principal investigator is Dante Lauretta from the University of Arizona.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. Deeply weathered profiles are widespread on the continental landmasses between latitudes 35°N and 35°S.
Conditions for the formation of deeply weathered regolith include a topographically moderate relief flat enough to prevent erosion and to allow leaching of the products of chemical weathering. A second condition is long periods of tectonic stability; tectonic activity and climate change can cause erosion. The third condition is humid tropical to temperate climate.
Poorly weathered saprolite grit aquifers are capable of producing groundwater, often suitable for livestock. Deep weathering causes the formation of many secondary and supergene ores – bauxite, iron ores, saprolitic gold, supergene copper, uranium and heavy minerals in residual accumulations.Sayh al Uhaymir 169
Sayh al Uhaymir 169 (SaU 169) is a 206 grams lunar meteorite found in the Sayh al Uhaymir region of the Sultanate of Oman in January 2002.
This stone is an impact-melt breccia with exceedingly high concentrations of thorium and other incompatible elements; phosphorus, rare-earth elements, and the three most important naturally occurring radioactive elements, potassium, thorium, and uranium have been segregated in the liquid phase when the lunar minerals crystallized. The impact that eventually sent this stone to the Earth is dated at 3.9 billion years and could be the Imbrium impact. It collided with the Earth less than 9,700 years ago.
It is complete, a light gray-greenish rounded stone, dimensions 70 mm × 43 mm × 40 mm (2.8 in × 1.7 in × 1.6 in) and mass 206 grams (7.3 oz), found on January 16, 2002, in the central desert of Oman at 20° 34.391' N and 57° 19.400' E.
According to geologist Edwin Gnos and coworkers, the meteorite's origin can be pinpointed to the vicinity of the Lalande impact crater; isotopic analysis shows a complex history of four distinct lunar impacts:
"Crystallization of the impact melt occurred at 3909 ± 13 Ma, followed by exhumation by a second impact at 2800 Ma, which raised the sample to a regolith position at unconstrained depth. A third impact at 200 Ma moved the material closer to the lunar surface, where it mixed with solar-wind–containing regolith. It was launched into space by a fourth impact at <0.34 Ma".Ultramafic rock
Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). The Earth's mantle is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks.