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 asteroids, and other terrestrial planets and moons.
Etymology
The term regolith combines two Greek words: (), 'blanket', and (), 'rock'. The American geologist George P. Merrill first defined the term in 1897, writing:
Earth
Earth's regolith 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 lava flows that are interbedded with unconsolidated material duricrust, 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- and water-deposited salts. biota and organic 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 age occurs in parts of Australia, though this may have been buried and subsequently exhumed.)
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 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 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. 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
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. Regolith production by rock erosion can lead to fillet buildup around lunar rocks.
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 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 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.