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WRITTEN BY: John P. Rafferty
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Regolith, a region of loose unconsolidated rock and dust that sits atop a layer of bedrock. On Earth, regolith also includes soil, which is a biologically active medium and a key component in plant growth. Regolith serves as a source of other geologic resources, such as aluminum, iron, clays, diamonds, and rare earth elements. It also appears on the surfaces of the Moon, other planets, and asteroids; however, the material found on other celestial bodies explored so far does not contain soil. The word is the Greek term for “blanket rock.”
On Earth, regolith is largely a product of weathering. Bedrock may be exposed to water or other compounds that percolate through the soil, or it may occur as an outcrop (that is, a deposit of rock exposed at Earth’s surface). These chemicals can alter the rock’s mineral content over time, breaking down some material into smaller components and separating it from the bedrock layer. Bedrock can also become regolith as a result of mechanical weathering, a process that breaks the rock into smaller pieces through the application of a force, such as thermal expansion, freeze-thaw cycles, or scouring by particles carried by wind and water. Plant roots can also assist the weathering process by penetrating and widening cracks already present in the rock.
On the Moon, regolith occurs as a mixture of powdery dust and broken rock. Lunar regolith is formed by the impact of meteorites on the body’s surface. The force of the collision melts some of the impacted regolith to form objects known as agglutinates and heaves debris (ejecta) outward from the point of impact. Regolith development on asteroids also follows the lunar pattern. On Mars, sand has been shown to make up a significant portion of the regolith, whereas on Saturn’s moon Titan, regolith is composed of water ice and hydrocarbon ice.
John P. Rafferty
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Mercury as seen by the Messenger probe, Jan. 14, 2008. This image shows half of the hemisphere missed by Mariner 10 in 1974–75 and was snapped by Messenger’s Wide Angle Camera when it was about 27,000 km (17,000 miles) from the planet.
Mercury: The atmosphere
…an insulating debris layer, or regolith, about 10–20 cm (4–8 inches) thick, made of organic compounds that also arrived on Mercury in cometary and asteroidal impacts.…
The familiar near side of Earth’s Moon, photographed on December 7, 1992, by the Galileo spacecraft on its way to Jupiter. Two primary kinds of terrain are visible—the lighter areas, which constitute the heavily cratered and very old highlands, and the darker, roughly circular plains, traditionally called maria, which are relatively young lava-filled impact basins.Among the maria are (left to right) the crescent-shaped Oceanus Procellarum near the left limb, the large, almost perfectly circular Mare Imbrium, or Imbrium Basin (with the crater Copernicus a bright dot at its lower margin), Mare Serenitatis immediately to the right of Imbrium, Mare Tranquillitatis to the lower right of Serenitatis, and Mare Crisium, isolated near the right limb. Another bright crater, Tycho, stands out at the bottom left of the image.
…to fine dust, is called regolith. Before the first unmanned spacecraft landings on the Moon in the 1960s, some astronomers feared that the surface would be so pulverized that the machines might sink in. These missions—and the manned landings that followed—revealed that the regolith was only somewhat compressible and was…
Phobos, the inner and larger of the two moons of Mars, in a composite of photographs taken by the Viking 1 orbiter in October 1978 from a distance of about 600 km (370 miles). The most prominent feature is the impact crater Stickney, which is almost half as wide as the moon itself. Also visible are linear grooves that appear to be related to Stickney and chains of small craters.
…with a very dark gray regolith (unconsolidated rocky debris) that reflects only about 6 percent of the light falling on it—about one-half that of the Moon’s surface. This fact and the satellite’s low mean density (1.9 grams per cubic cm) are consistent with the composition of carbonaceous chondrite meteorites, suggesting…
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MORE ABOUT Regolith
6 REFERENCES FOUND IN BRITANNICA ARTICLES
In Mercury: The atmosphere
In Moon: Distinctive features
In Moon: Apollo to the present
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A false-colour close-up of the asteroid Eros shows dust and fragments of rocky debris inside a large crater. The NEAR Shoemaker spacecraft took the image from about 50 km (30 miles) above the asteroid’s surface. The regolith of the redder areas has been chemically altered by small impacts and exposure to the solar wind, whereas the regolith of the bluer areas has been less “weathered.”
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