Regolith is a layer of loose, heterogeneous material 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.
Etymology The term regolith combines two Greek words:
rhegos, "blanket", and lithos, "rock". 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.
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.
The impact of micrometeoroids, sometimes travelling faster than 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 tektites found on Earth.
The regolith is generally from 4 to 5 metres thick in mare areas and from 10 to 15 metres
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 averages approximately 1.35 g/cm3 for the top 30cm, and it is approximately
1.85g/cm3 at a depth of 60cm.
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. 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 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
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
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. Titan
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 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. The idea of an ice-regolith complete with
erosion and aeolian and/or sedimentary processes is unique 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 in pebbles. The pebbles, which may
be made of water ice, are somewhat rounded, which may indicate the action of fluids on
them. Earth
Earth's regolith comprises 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, 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 to hundreds of millions of years old.
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 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. See also
Notes
External links Cooperative Research Centre for Landscape,
Environments, and Mineral Exploration The Regolith Glossary: Surficial Geology,
Soils and Landscapes, Richard A Eggleton, Editor