Regolith
Regolith (Greek: "blanket rock") is a layer of loose, heterogeneous material covering solid rock. Regolith is present on Earth, the Moon, some asteroids, and other planets. The term was first defined by G. 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."[1]
On the Earth
On Earth, the regolith is considered to be "everything between fresh rock and fresh air", and is composed of four major subdivisions;
- Soil or pedolith
- Alluvium or recent unlithified transported cover
- Saprolith, generally divided into the
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.
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 is 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, generates enough heat to melt or partially vaporize dust particles. This melting and refreezing welds particles together into glassy, jagged-edged agglutinates.[2]
The regolith is generally about 4-5 meters thick in mare areas and 10-15 m in older highland regions.[3] 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 soil, that which is composed of grains one centimeter in diameter or less. "Lunar dust" generally connotes even finer materials, the fraction which is less than 100 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 there were concerns that the regolith would not support the weight of the lunar module and that the module might sink beneath the surface. In fact, the regolith was found to be quite firm, often requiring a hammer to drive a core sampling tool into it.
On asteroids
Asteroids also 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.
External links
References
- ^ Merrill, G. P. (1897) Rocks, rock-weathering and soils, New York: MacMillan Company, 411p.
- ^
Mangels, John (2007-02-15). "Coping with a lunar dust-up". The Seattle Times. Retrieved 2007-02-16.
{{cite news}}: Check date values in:|date=(help) - ^ Heiken et al (1991) Lunar Sourcebook, a user's guide to the Moon. New York: Cambridge University Press. 736p. ISBN 0521334446