English

Etymology

Irregular formation from etyl grc ῥῆγος + -lith (from λίθος).

Pronunciation

• a UK /ˈɹɛgəlɪθ/

Noun

1. The layer of loose rock, resting on the bedrock, that constitutes the surface of most land (and the surface of the moon etc.).

Translations

Regolith (Greek: "blanket rock") 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, some asteroids, and other planets. The term was first defined by George P. Merrill in 1897 who stated, "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."

On the Earth

On Earth, the regolith is composed of four major subdivisions;

• Soil or pedolith
• Alluvium or recent unlithified transported cover
• 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.

In some instances, for example in the cratons, thin veneers of unconsolidated alluvium, colluvium or debris may be considered part of the regolith, especially if considerably younger than the basement or bedrock.

The origins of regolith on Earth are 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.

On the Moon

Nearly the entire lunar surface is covered with regolith, bedrock being exposed only on very steep-sided crater walls and the occasional lava channel. This regolith has been formed over the last 4.6 billion years by the impact of large and small meteoroids and the steady bombardment of micrometeoroids and solar and galactic charged particles breaking down surface rocks.

The impact of micrometeoroids, sometimes travelling faster than 60,000 mph (30 km/s), generates enough heat to melt or partially vaporize dust particles. This melting and refreezing welds particles together into glassy, jagged-edged agglutinates.

The regolith is generally about 4-5 meters thick in mare areas and 10-15 m in 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 centimeter 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 micrometres in diameter.

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 (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 Apollo program the astronauts often found it necessary to use a hammer to drive a core sampling tool into it.

On 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 vallies 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.

On 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 we have of an asteroidal regolith. The recent Japanese Hayabusa mission also returned spectacular and surprising images of an asteroidal regolith on an asteroid so small it was thought that gravity was too low to develop and maintain a regolith.

On Titan

• Helium-3
• Soil
• Sand

External links

• Lunar Regolith and Fragmental Breccias

References