Lunar soil - AbsoluteAstronomy.com

Lunar soil is the fine fraction of the regolith

Regolith

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, some asteroids, and other terrestrial planets and moons.-Etymology:...

found on the surface of the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

. Its properties can differ significantly from those of terrestrial soil

Soil

Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

. The physical properties of lunar soil are primarily the result of mechanical disintegration of basaltic and anorthositic rock, caused by continuous meteoric impact and bombardment by interstellar charged atomic particles over billions of years. The process is largely one of mechanical erosion in which the particles are ground to finer and finer size over time. This situation contrasts fundamentally to terrestrial soil formation, mediated by the presence of molecular oxygen (O2), humidity, atmospheric wind, and a robust array of contributing biological processes. Some have argued that the term "soil

Soil

" is not correct in reference to the Moon because on the Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, soil is defined as having organic

Organic matter

Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...

content, whereas the Moon has none. However, standard usage among lunar scientists is to ignore that distinction.

The term lunar soil is often used interchangeably with "lunar regolith" but typically refers to only the finer fraction of regolith, that which is composed of grains one cm in diameter or less. Lunar dust generally connotes even finer materials than lunar soil. There is no official definition of what size fraction constitutes "dust", some place the cutoff at less than 50 micrometres in diameter, other at less than 10.

Soil formation processes

The major processes involved in the formation of lunar soil are:

Comminution
Comminution
Comminution is the process in which solid materials are reduced in size, by crushing, grinding and other processes. It occurs naturally during faulting in the upper part of the crust and is an important operation in mineral processing, ceramics, electronics and other fields. Within industrial uses,...

: mechanical breaking of rocks and minerals into smaller particles by meteorite and micrometeorite impact;
Agglutination
Agglutination
In contemporary linguistics, agglutination usually refers to the kind of morphological derivation in which there is a one-to-one correspondence between affixes and syntactical categories. Languages that use agglutination widely are called agglutinative languages...

: welding of mineral and rock fragments together by micrometeorite-impact-produced glass; and
Solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

spallatation and implantation: sputtering caused by impacts of ions and high energy particles

These processes not only form lunar soil, they also continue to change the physical and optical properties of the soil over time; this process is known as space weathering

Space weathering

Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...

.

In addition, fire fountaining, whereby volcanic lava is lofted and cools into small glass beads before falling back to the surface, can create small but important deposits in some locations, such as the orange soil found at Shorty Crater in the Taurus-Littrow valley on Apollo 17

Apollo 17

Apollo 17 was the eleventh and final manned mission in the American Apollo space program. Launched at 12:33 a.m. EST on December 7, 1972, with a three-member crew consisting of Commander Eugene Cernan, Command Module Pilot Ronald Evans, and Lunar Module Pilot Harrison Schmitt, Apollo 17 remains the...

, and the green glass found at Hadley-Apennine found on Apollo 15

Apollo 15

Apollo 15 was the ninth manned mission in the American Apollo space program, the fourth to land on the Moon and the eighth successful manned mission. It was the first of what were termed "J missions", long duration stays on the Moon with a greater focus on science than had been possible on previous...

. Deposits of volcanic beads are also thought to be the origin of Dark Mantle Deposits (DMD) in other locations around the Moon.

Mineralogy and composition

The lunar soil is composed of various types of particles including rocks fragments, mono-mineralic fragments, and various kinds of glasses including agglutinate particles and volcanic and impact spherules. The agglutinates form at the lunar surface by micrometeorite impacts that cause small-scale melting which fuses adjacent materials together.

Over time, material is mixed both vertically and horizontally (a process known as "gardening") by impact processes. However, the contribution of material from great distances is relatively minor, such that the soil composition at any given location largely reflects the local bedrock composition.

There are two profound differences in the chemistry of lunar regolith and soil from terrestrial materials. The first is that the Moon is very dry. As a result, those minerals with water as part of their structure such as clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

, mica

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

, and amphiboles are totally absent from the Moon. The second difference is that lunar regolith and crust are chemically reduced, rather than being significantly oxidized like the Earth's crust. In the case of the regolith, this is due in part to the constant bombardment of the lunar surface with hydrogen (H) from the solar wind. One consequence is that iron on the Moon is found in the metallic 0 and +2 oxidation state, whereas on Earth iron is found primarily in the +2 and +3 oxidation state.

Properties

The significance of acquiring appropriate knowledge of lunar soil properties is great. The potential for construction of structures, ground transportation networks, and waste disposal systems, to name a few examples, will depend on real-world experimental data obtained from testing lunar soil samples. The load-carrying capability of the soil is an important parameter in the design of such structures on Earth.

Due to myriad meteorite impacts (with velocities in the range of 20 km/s), the lunar surface is covered with a thin layer of dust. The dust is electrically charged and sticks to any surface it comes in contact with. The soil becomes very dense beneath the top layer of regolith.

Other factors which may affect the properties of lunar soil include large temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

differentials

Differential thermal analysis

Differential thermal analysis is a thermoanalytic technique, similar to differential scanning calorimetry. In DTA, the material under study and an inert reference are made to undergo identical thermal cycles, while recording any temperature difference between sample and reference...

, the presence of a hard vacuum

Vacuum

In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

, and the absence of a significant lunar magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

(thereby allowing charged solar wind particles to continuously hit the surface of the moon). A weaker gravitational force and the absence of an atmosphere

Atmospheric pressure

Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

are additional factors which will affect the design of structures on the surface of the Moon.

Moon dust fountains and electrostatic levitation

There is some evidence that the Moon may have a tenuous atmosphere of moving dust particles constantly leaping up from and falling back to the Moon's surface, giving rise to a "dust atmosphere" that looks static but is composed of dust particles in constant motion. The term "Moon fountain" has been used to describe this effect by analogy with the stream of molecules of water in a fountain following a ballistic trajectory while appearing static due to the constancy of the stream. According to the model recently proposed by Timothy J. Stubbs, Richard R. Vondrak, and William M. Farrell of the Laboratory for Extraterrestrial Physics at NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Goddard Space Flight Center

Goddard Space Flight Center

The Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...

, this is caused by electrostatic levitation

Electrostatic levitation

Electrostatic levitation is the process of using an electric field to levitate a charged object and counteract the effects of gravity. It was used, for instance, in Robert Millikan's oil drop experiment and is used to suspend the gyroscopes in Gravity Probe B during launch.Due to Earnshaw's...

. On the daylit side of the Moon, solar ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

and X-ray

X-ray

X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

radiation is energetic enough to knock electron

Electron

The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s out of atoms and molecules in the lunar soil. Positive charges build up until the tiniest particles of lunar dust (measuring 1 micrometre and smaller) are repelled from the surface and lofted anywhere from metres to kilometres high, with the smallest particles reaching the highest altitudes. Eventually they fall back toward the surface where the process is repeated over and over again. On the night side, the dust is negatively charged by electrons in the solar wind

Solar wind

The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

. Indeed, the fountain model suggests that the night side would charge up to higher voltages than the day side, possibly launching dust particles to higher velocities and altitudes. This effect could be further enhanced during the portion of the Moon's orbit where it passes through Earth's magnetotail; see Magnetic field of the Moon

Magnetic field of the Moon

thumb|right|300px|Total magnetic field strength at the surface of the Moon as derived from the Lunar Prospector electron reflectometer experiment.The external magnetic field of the Moon is very weak in comparison to that of the Earth...

for more detail. On the terminator there could be significant horizontal electric fields forming between the day and night areas, resulting in horizontal dust transport - a form of "moon storm".

This effect was also predicted in 1956 by science fiction author Hal Clement

Hal Clement

Harry Clement Stubbs better known by the pen name Hal Clement, was an American science fiction writer and a leader of the hard science fiction subgenre.-Biography:...

in his short story "Dust Rag" published in Astounding Science Fiction.

Also in 1956, the American scientist Thomas Townsend Brown

Thomas Townsend Brown

Thomas Townsend Brown was an American physicist.-Early and middle years:Brown was born in Zanesville, Ohio; his parents were Lewis K. and Mary Townsend Brown. In 1921, Brown discovered what was later called the Biefeld-Brown effect while experimenting with a Coolidge X-ray tube. This is a vacuum...

appears to have predicted a similar lofting-falling cycle of photoelectrically excited lunar dust (along with controversial and as yet unproven speculations about unusual gravitational properties of this dust, an interest he maintained to the end of his life).
There is some evidence for this effect. In the early 1960s before Apollo 11

Apollo 11

In early 1969, Bill Anders accepted a job with the National Space Council effective in August 1969 and announced his retirement as an astronaut. At that point Ken Mattingly was moved from the support crew into parallel training with Anders as backup Command Module Pilot in case Apollo 11 was...

, Surveyor 7

Surveyor 7

Surveyor 7 was the seventh and last lunar lander of the American unmanned Surveyor program sent to explore the surface of the Moon.*Launched January 7, 1968; landed January 10, 1968*Weight on landing: 305.7 kg...

and several subsequent Surveyor spacecraft that soft-landed on the Moon returned photographs showing an unmistakable twilight glow low over the lunar horizon persisting after the Sun had set. Moreover, the distant horizon between land and sky did not look razor-sharp, as would have been expected in a vacuum where there was no atmospheric haze. Apollo 17 astronauts orbiting the Moon in 1972 repeatedly saw and sketched what they variously called "bands," "streamers" or "twilight rays" for about 10 seconds before lunar sunrise or lunar sunset. Such rays were also reported by astronauts aboard Apollo 8, 10, and 15. These may have been similar to crepuscular rays on Earth.

Apollo 17 also placed an experiment on the Moon's surface called LEAM

Leam

Leam is a hamlet in the English county of Derbyshire. There are a number of buildings, which once formed a single estate.Leam is due south of Hathersage, close to Grindleford. There are several inhabitants....

, short for Lunar Ejecta and Meteorites. It was designed to look for dust kicked up by small meteoroids hitting the Moon's surface. It had three sensors that could record the speed, energy, and direction of tiny particles: one each pointing up, east, and west. LEAM saw a large number of particles every morning, mostly coming from the east or west—rather than above or below—and mostly slower than speeds expected for lunar ejecta. Also, a few hours after every lunar sunrise, the experiment's temperature rocketed so high—near that of boiling water—that LEAM had to be turned off because it was overheating. It is speculated that this could have been a result of electrically-charged moondust sticking to LEAM, darkening its surface so the experiment package absorbed rather than reflected sunlight.

It's even possible that these storms have been spotted from Earth: For centuries, there have been reports of strange glowing lights on the Moon, known as "Transient lunar phenomenon

Transient lunar phenomenon

A transient lunar phenomenon , or lunar transient phenomenon , is a short-lived light, color, or change in appearance on the lunar surface....

" or TLPs. Some TLPs have been observed as momentary flashes—now generally accepted to be visible evidence of meteoroids impacting the lunar surface. But others have appeared as amorphous reddish or whitish glows or even as dusky hazy regions that change shape or disappear over seconds or minutes. These may have been a result of sunlight reflecting off of suspended lunar dust.

Harmful effects of lunar dust

There are concerns that the dust found on the lunar surface could cause harmful effects on any manned outpost technology and crew members:

Abrasive nature of the dust particles may rub and wear down surfaces through friction;
Negative effect on coatings used on gaskets to seal equipment from space, optical lenses that include solar panels and windows as well as wiring;
Possible damage to an astronaut's lungs, nervous, and cardiovascular systems.

The principles of astronautical hygiene should be used to assess the risks of exposure to lunar dust during exploration on the Moon's surface and thereby determine the most appropriate measures to control exposure. These would include for example, removing the spacesuit in a three stage airlock, vacuuming the suit before removal, using local exhaust ventilation with a high efficiency particulate filter to remove any dust in the space craft's atmosphere etc (Ref: Dr J R Cain presentation "The application of astronautical hygiene to protect the health of astronauts", UK Space Biomedicine Association Conference 2009, Downing College, University of Cambridge).

The harmful properties of the lunar dust are not well known. However, based on studies of dust found on Earth, it is expected that exposure to lunar dust will result in greater risks to health both from direct exposure (acute) and if exposure is over time (chronic). This is because lunar dust is more chemically reactive and has larger surface areas composed of sharper jagged edges than Earth dust (Ref: Dr John R Cain, "Moon dust - a danger to lunar explorers" , Spaceflight, Vol 52, February 2010, pp60 – 65). If the chemical reactive particles are deposited in the lungs, they may cause respiratory disease. Long-term exposure to the dust may cause a more serious respiratory disease similar to silicosis. During lunar exploration, the astronaut's spacesuits will become contaminated with lunar dust. The dust will be released into the atmosphere when the suits are removed. The methods used to mitigate exposure will include providing high air recirculation rates in the airlock, the use of a "Double Shell Spacesuit", the use of dust shields, the use of high grade magnetic separation and the use of solar flux to sinter and melt the regolith (Ref: Dr John R Cain, "Lunar dust: the hazard and astronaut exposure risks", Earth, Moon, Planets DOI 10.1007/s11038-010-9365-0 October 2010).

External links

Geotechnical Properties of Lunar Soil
Parameters of Lunar Soils Lunar and Planetary Institute
Discovered After 40 Years: Moon Dust Hazard Influenced By Sun's Elevation

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