Lunar regolith


Lunar regolith is the unconsolidated material found on the surface of the Moon and in the Moon's tenuous atmosphere. Lunar soil typically refers to only the finer fraction of lunar regolith, which is composed of grains 1 cm in diameter or less, but is often used interchangeably. Lunar soil differs substantially in properties from terrestrial soil. Lunar dust is even finer regolith than lunar soil, with grain sizes less than one millimeter.
Lunar regolith is primarily the result of mechanical weathering. Continual meteoric impacts and bombardment by solar and interstellar charged atomic particles of the lunar surface over billions of years ground the basaltic and anorthositic rock, the regolith of the Moon, into progressively finer material. This situation contrasts fundamentally to terrestrial soil formation, mediated by the presence of molecular oxygen, humidity, atmospheric wind, and a robust array of contributing biological processes.
As the Moon's fine surface layer, lunar regolith is picked up by even weak natural phenomena active at the Moon's surface, allowing it to be part of the Moon's scant atmosphere. It is easily disturbed and poses a significant hazard to exposed equipment and human health. The fine lunar regolith is made of sharp and very adhesive particles, with a distinct gunpowder taste and smell. Potentially, lunar regolith could be prospected as a lunar resource, particularly for lunar in situ utilization, such as a lunar building material and regolith for growing plants on the Moon.

Formation processes

The major processes involved in the formation of lunar regolith are:
These processes continue to change the physical and optical properties of the dirt over time, and it is known as space weathering.
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 dirt found at Shorty Crater in the Taurus-Littrow valley by Apollo 17, and the green glass found at Hadley–Apennine by Apollo 15. Deposits of volcanic beads are also thought to be the origin of Dark Mantle Deposits in other locations around the Moon.

Circulation

There is some evidence that the Moon has a tenuous layer 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 a model proposed in 2005 by the Laboratory for Extraterrestrial Physics at NASA's Goddard Space Flight Center, this is caused by electrostatic levitation. On the daylit side of the Moon, solar hard ultraviolet and X-ray radiation is energetic enough to knock electrons out of atoms and molecules in the lunar regolith. Positive charges build up until the tiniest particles of lunar dust 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. On the night side, the dust is negatively charged by electrons from the solar wind. Indeed, the fountain model suggests that the night side would achieve greater electrical tension differences than the day side, possibly launching dust particles to even higher altitudes. This effect could be further enhanced during the portion of the Moon's orbit where it passes through Earth's magnetotail, part of the magnetic field of the Moon. 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 anticipated in 1956 by science fiction author Hal Clement in his short story "Dust Rag", published in Astounding Science Fiction.
There is some evidence for this effect. In the early 1960s, Surveyor 7 and several prior 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, contrary to the expectation of airless conditions with no atmospheric haze, the distant horizon between land and sky did not look razor-sharp. 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 might have been similar to crepuscular rays on Earth.
Apollo 17 also placed an experiment on the Moon's surface called LEAM, 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. In addition, the experiment's temperature increased to near 100 degrees Celsius a few hours after each lunar sunrise, so the unit had to be turned off temporarily 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. However, scientists were unable to make a definite determination of the source of the problem, as LEAM operated only briefly before the Apollo program ended.
It is 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 phenomena" 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 from suspended lunar dust.

Anthropogenic spread

While the Moon has a faint atmosphere, traffic and impacts of human activity on the Moon could cause clouds of lunar regolith to spread far across the Moon, and possibly contaminate the original state of the Moon and its special scientific content.

Physical properties

Due to a myriad of meteorite impacts, the lunar surface is covered with a thin layer of dust. The dust is electrically charged and sticks to any surface with which it comes in contact.
The density of lunar regolith is about 1.5 g/cm3 and increases with depth.
Other factors which may affect the properties of lunar regolith include large temperature differentials, the presence of a hard vacuum, and the absence of a significant lunar magnetic field, thereby allowing charged solar wind particles to continuously hit the surface of the Moon.

Chemical constituents

98-99% of the composition of lunar rocks and soil consists of seven elements: Oxygen, Silicon, Aluminum, Calcium, Iron, Magnesium, and Titanium. Nearly all of the remaining 1-2% is Manganese, Sodium, Potassium, and Phosphorus.

Mineralogy and composition

The composition of Lunar regolith reflects the composition of the parent rocks it overlies. Over time, material is mixed both vertically and horizontally by impact processes. While mare and highland regolith have distinct compositions, their mineral inventories are very similar, rather expressing a difference of ratio of mineral phases. The primary minerals identified in Lunar regolith are plagioclase, olivine, augite, orthopyroxene, pigeonite, ilmenite, chromite, quartz, cristobalite, and whitlockite. Glass is abundant in the Lunar regolith and forms as a result of impact melting. Ice is an important mineral in permanently shaded craters. Lunar regolith is divided into highland and mare on the basis of their composition, and further divided into high-, low-, and very low-titanium on the basis of their ilmenite content.
The contribution of material from external sources is relatively minor, such that the dirt composition at any given location largely reflects the local bedrock composition. Lunar regolith reportedly tastes and smells of spent gunpowder.
Lunar regolith is composed of various types of particles including rock fragments, mono-mineralic fragments, and various kinds of glasses including agglutinate particles, volcanic and impact spherules. The agglutinates form at the lunar surface by micrometeorite impacts that cause small-scale melting which fuses adjacent materials together with tiny specks of elemental iron embedded in each dust particle's glassy shell.
There are two primary differences in the chemistry of lunar regolith and dirt 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, mica, and amphiboles are absent from the Moon's surface. 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 protons from the solar wind. One consequence is that iron on the Moon is found in the elemental and cationic oxidation states, whereas on Earth iron is found primarily in the +2 and +3 oxidation states.