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Regolith (/ˈrɛɡəlɪθ/)[1] 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 asteroids, and other terrestrial planets and moons.

Etymology

The term regolith combines two Greek words: rhegos (ῥῆγος), 'blanket', and lithos (λίθος), 'rock'.[2][3][4] The American geologist George P. Merrill first defined the term in 1897, writing:

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.[5]

Earth

Alluvial gravels in Alaska

Earth's regolith[6][7][8] includes the following subdivisions and components:

  • soil or pedolith
  • alluvium and other transported cover, including that transported by aeolian, glacial, marine, and gravity flow processes.
  • "saprolith'", generally divided into the
    • upper saprolite: completely oxidised bedrock
    • lower saprolite: chemically reduced partially weathered rocks
    • saprock: fractured bedrock with weathering restricted to fracture margins.
  • volcanic ash and lavas
  • duricrust. It is formed by cementation of soils, saprolith and transported material by clays, silicates, iron oxides and oxyhydroxides, regolith combines two Greek words: rhegos (ῥῆγος), 'blanket', and lithos (λίθος), 'rock'.[2][3][4] The American geologist George P. Merrill first defined the term in 1897, writing:

    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.[5]

    Earth

    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.[5]

    EarthEarth's regolith[6][7][8] includes the following subdivisions and components:

    • soil or pedolith
    • alluvium and other transported cover, including that transported by aeolian, glacial, marine, and gravity flow processes.
    • "saprolith'", generally divided into the
      • upper saprolite: completely oxidised bedrock
      • lower saprolite: chemically reduced partially weathered rocks
      • saprock: fractured bedrock with weathering restricted to fracture margins.
    • volcanic ash and lavas
    • duricrust. It is formed by cementation of soils, saprolith and transported material by 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).[9]

      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.[10][11] 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

      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.[16]

      Titan

      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 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 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.[17] 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.[18]

      Pebbles on Titan's surface, photographed from a height of about 85 cm by the Huygens spacecraft

    • Dunes on Dunes on Titan's surface in a radar image taken by the Cassini spacecraft of a region approximately 160 by 325 kilometers (99 by 202 miles)

    See also