Lunar regolith samples

Initial study of lunar soils focused on ensuring the safety of crewed spacecraft upon landing. Returned soil samples have since been analyzed to determine the origin and evolution of the Moon. Future studies will assess the suitability of lunar resources for utilization in the construction of a Moon base and as raw materials for space manufacturing.

Overview

The lunar regolith is the blanket of broken fragments, ranging from dust-sized particles to meter-sized rocks, that covers the surface of the Moon. The existence of a “soil,” or layer of small particles covering the lunar surface, was inferred prior to the first spacecraft’s landing on the Moon. At full moon, the lunar surface is observed to be bright from edge to edge, exhibiting only minimal “limb darkening” (a decrease in intensity of the light reflected near the edges of a smooth sphere). This led early observers to conclude that the uppermost surface layer of the Moon was porous on the centimeter scale, suggesting a surface dust layer. Determination of the thickness and physical properties of this dust layer was important to the success of the crewed lunar landings. Cornell astronomerThomas Gold had postulated that the Apollo Lunar Module might sink into a thick surface dust layer that could not bear the weight of the vehicle.

Early in-situ investigations of the properties of the lunar soils were conducted by lunar soft-landing spacecraft in the Soviet Luna program and the Surveyor missions launched by the National Aeronautics and Space Administration (NASA). On February 3, 1966, Luna 9, the first spacecraft to soft-land on the Moon, returned panoramic photographs of the surface and demonstrated that the soil was firm enough to support the 100-kilogram payload without noticeable effect. Surveyor spacecraft and the Luna 13, Luna 17, and Luna 21 soft-landers carried instruments that determined the lunar soil’s composition and physical properties.

The return of lunar samples to Earth for laboratory analysis offered significant advantages; earthbound instruments were not limited, as were the lightweight ones suitable for space flight. The first samples of lunar soil were returned to Earth by Apollo 11. During their stay on the lunar surface, the Apollo 11 astronauts collected twenty-two kilograms of lunar material. Of this sample, eleven kilograms were categorized as “fines” (particles smaller than one centimeter).

In addition to surface soil samples, Apollo astronauts obtained “core samples,” cylindrical samples of lunar soil taken by pushing a tube vertically into the lunar surface. Preserved layers in cores provide information on the rates of depositing and mixing of the soil. Apollo 11 astronaut Edwin E. Aldrin collected two core samples in two-centimeter-diameter tubes pushed down into the lunar surface. The first core, about ten centimeters long, contained fifty-one grams of material, while the second, measuring 13.5 centimeters, weighed sixty-five grams.

Apollo 11 lunar samples were returned to Earth on July 24, 1969, and flown to the Manned Spacecraft Center (MSC, later renamed Johnson Space Center) in Houston, Texas. There, the samples were placed in quarantine in the Lunar Receiving Laboratory (LRL) for a period of one month while biological analyses were conducted. The Lunar Sample Preliminary Analysis Team (LSPAT), consisting of MSC scientists and visiting scientists, was permitted to study samples under controlled conditions during the quarantine period. They first exposed small chips from the samples to nitrogen, oxygen, and air at various humidities to ensure that laboratory analysis conditions did not cause adverse reactions or sample deterioration. Within the LRL, samples were characterized by mineralogical and chemical techniques. In addition, experiments were performed to determine the effects of cosmic-ray exposure on the lunar material, the organic carbon content, and the noble gas concentrations. Additional experiments searched for “magnetic monopoles” (particles of isolated magnetic charge whose existence is postulated by elementary particle physicists).

Following quarantine, lunar samples were made available to about 110 scientists, selected by the Office of Space Science and Applications to perform a variety of experiments. These scientists—from twenty-one universities, two industrial facilities, three private institutions, and ten government laboratories—included twenty-seven scientists from the United Kingdom, Germany, Canada, Japan, Finland, and Switzerland. Because of the fineness of the returned material, the scientists developed new techniques and instruments to perform a variety of experiments on the lunar samples. After six months, they met to discuss their results at the Apollo 11 Lunar Science Conference, held at MSC from January 5 to January 8, 1970. The Apollo 11 Lunar Science Conference evolved into an annual meeting at which scientists from around the world report their latest results on lunar sample research and planetary science. NASA continues to allocate samples collected during the Apollo missions as new instruments or new techniques warrant further experiments.

Fines were shown to consist of a mixture of glassy materials and small crystal fragments. Within the cores, the majority of the particles ranged in size from one millimeter down to thirty micrometers. Glasses exhibited a variety of colors, from pale or colorless to gray to red, orange, green, brown, and yellow. Crystal fragments were dominated by the minerals plagioclase, clinopyroxene, ilmenite, and olivine.

Lunar material from five additional sites on the Moon was returned to Earth by the Apollo 12, 14, 15, 16, and 17 missions. Core samples up to 2.6 meters in length were obtained. More than 380 kilograms of lunar material were returned to Earth by the Apollo program, more than half of which was collected on the final two Apollo missions.

The Soviet Union’s recovery of lunar materials involved the uncrewed spacecraft in the Luna series. Luna 16, which landed on the Moon on September 20, 1970, returned a single 35-centimeter drill core containing 101 grams of soil on September 24. Luna 20 returned a similar sample in February 1972. The more advanced Luna 24 spacecraft, which landed in the Sea of Crises area of the Moon on August 18, 1976, returned a two-meter core sample.

These lunar samples have been made available to scientific investigators throughout the world. An agreement between the National Aeronautics and Space Administration (NASA) and the Soviet Academy of Sciences in 1971 provided for the exchange of Apollo and Luna samples, allowing all investigators to have access to the soils collected at the nine lunar sites sampled by either Apollo or Luna missions.

Proposals to establish a lunar base and to develop space manufacturing facilities have focused attention on the lunar soils as raw materials for the manufacturing process. Procedures for the extraction of aluminum from the plagioclase, titanium from the ilmenite, and magnesium from the olivine grains in the soils have been described. As a by-product of these extractions, silicon and oxygen can also be produced. Although lunar soils contain essentially no water, hydrogen implanted by the solar wind might be obtained from the ilmenite and then be used to react with oxygen to produce water. The use of implanted hydrogen as a rocket fuel has also been proposed. Thus, it appears that the high cost of launching materials from Earth’s surface for space manufacturing can be circumvented by acquiring the bulk of the raw materials in the low-gravity environment of the Moon.

Knowledge Gained

Lunar regolith was found to cover almost all of the Moon to an estimated depth of a few meters. The fine component of the regolith, generally referred to as soil, was shown to be very different from terrestrial soil. On Earth, the soil is formed by the complex action of atmospheric weathering and biological activity. Lunar soil is produced by the bombardment of the surface rocks by meteorites and micrometeorites, producing craters and microcraters; the impact debris is then scattered over the lunar surface.

The chemical composition of the soil was discovered to differ slightly from that of the lunar rocks. By subtracting the rock composition from that of the soil, the chemical composition of the added component was found to be similar to the composition of primitive stone meteorites that fall on Earth. Thus, meteoritic bombardment was proved to be an important alteration process on the Moon.

Analyses of the layered structure of the tube samples suggested that large meteorite impacts have thrown layers of soil across the lunar landscape. The top surface of this layer has been stirred by impacts of more numerous micrometeorites. The layer structure observed in long core tubes collected on later Apollo missions agrees well with computer simulations of the rate of soil mixing, calculated assuming the rate at which small particles strike the Moon has been approximately equal to the measured current rate for millions of years. Radiation damage in grains of the lunar soil, as well as in lunar rocks, suggests that the rate of emission of heavy ions by the Sun has also been relatively constant over the past few million years.

Layers in the lunar soil record the history of bombardment of the Moon, with a single core providing samples of material ejected from craters far apart on the lunar surface. One core (collected during the Apollo 12 mission) contains a layer, light gray in color and rich in silica, that has been identified as ejecta (material thrown out by volcanic eruptions) from the crater Copernicus, seventy-five kilometers from the core site. Thus, Apollo core samples have provided information on the composition and mineralogy of locations far from the sampling site.

Context

Both the Apollo and Luna programs returned samples of the lunar surface material to Earth. Though these lunar rocks could be compared to similar terrestrial rocks, lunar soil was clearly different from its terrestrial counterpart. Analysis of soils and the stratigraphy of core samples provided information necessary to confirm the importance of meteoritic bombardment in producing rock fragments that make up lunar soil and in stirring the upper layer of the soil once placed on the lunar surface. Measurement of the effects of cosmic rays in an Apollo 15core sample indicated that layered structures within the lunar soil can be preserved undisturbed on the lunar surface for periods of 500 million years. Cores established that the lunar erosion process proceeds at a rate of between one and two millimeters per million years, about one thousand times slower than on Earth.

The depth of the lunar soil layer confirmed that the rate at which meteorites and micrometeorites are hitting the Moon has remained essentially unchanged over the past four billion years. The presence of ion damage caused by solar flares in soils buried for millions of years also confirmed that the flux of charged particles from the Sun has been relatively constant.

The chemical composition of the bombarding material was also deduced from the difference between soil and bulk rock compositions. Several distinct compositions were observed, but all were generally similar to the known meteorites collected on Earth.

Studies of returned lunar soils also demonstrated their potential as a raw material for lunar base construction and for space manufacturing. Proposals for lunar mining have generally assumed the soils as the starting material. Since the delivery of large masses of material from the lunar surface to near-Earth orbit requires less energy than the delivery of the same mass of material from the surface of Earth, lunar soils are expected to play a significant role in the industrialization of space.

No soil samples were returned to Earth after the Soviet Luna 24 mission in 1976, but in the 1990s, a pair of spacecraft, Clementine and Lunar Prospector, mapped the surface for mineral content. A renewed interest in the Moon led the European Space Agency (ESA), Japanese Aerospace Exploration Agency (JAXA), the Chinese National Space Administration (CNSA), and the Indian Space Research Organization (ISRO) to send probes to the Moon in the first decade of the twenty-first century. These spacecraft were named SMART 1 (ESA), Kaguya (JAXA), Chang’e 1 (CNSA), and Chandrayaan 1 (ISRO).

In 2009, NASA launched the Lunar Reconnaissance Orbiter to begin a comprehensive mapping of lunar resources and assist in the determination of the best location near the Moon’s south pole for the construction of a permanently occupied lunar base. That goal was the first objective of the Bush administration’s Vision for Space Exploration set up in the aftermath of the 2003 Columbia accident. NASA was charged with a return of astronauts to the Moon by 2019, the fiftieth anniversary of the Apollo 11 lunar landing. However, that mission was canceled in 2010 due to budget cuts. Though the United States has not returned to the Moon, in 2022, scientists opened a lunar sample from the Apollo 17 mission that had been preserved in a vacuum for over fifty years, hopefully bringing new insights to the Moon’s composition. The Chinese have also stated a firm commitment to evolve their early Shenzhou-piloted missions into flights to the Moon. Several Chang'e spacecraft have orbited the Moon since 2007, and the Chinese space program launched the Chang'e 5 probe in 2020 which collected samples of lunar material and returned it to Earth for analysis. Regardless of which nations return to the Moon, the renewed interest in lunar exploration promises advances in understanding the history of our solar system and a first step in lunar colonization.

Bibliography

Baldwin, R. A. The Measure of the Moon. Chicago: U of Chicago P, 1963.

Beattie, Donald A. Taking Science to the Moon: Lunar Experiments and the Apollo Program. Baltimore: Johns Hopkins UP, 2003.

French, Bevan M. The Moon Book. New York: Penguin, 1977.

Hartmann, William K. Moons and Planets. 5th ed. Belmont: Thomson Brooks/Cole, 2005.

Iai, Masafumi, and Leslie Gertsch. "Excavation of Lunar Regolith with Large Grains by Rippers for Improved Excavation Efficiency." Journal of Aerospace Engineering 26.1 (2013): 97–104.

King, Elbert A., Jr. Space Geology: An Introduction. New York: Wiley, 1976.

Levinson, Alfred Abraham, ed. Apollo 11 Lunar Science Conference: Proceedings. 3 vols. Elmsford: Pergamon, 1970.

Mason, Brian, and William G. Melson. The Lunar Rocks. New York: Wiley-Interscience, 1970.

Schmitt, Harrison J. Return to the Moon: Exploration, Enterprise, and Energy in the Human Settlement of Space. New York: Copernicus, 2006.

Sinitsyn, M. "Some Astronomical Aspects of the Study of Lunar Regoliths." Moon and Planets 110.1/2 (2013): 29–39.

Strickland, Ashley. “Apollo 17 Lunar Sample Newly Opened and Unveiled by NASA.” CNN, 25 Mar. 2022, www.cnn.com/2022/03/25/world/apollo-lunar-samples-scn/index.html. Accessed 16 Sept. 2023.

Taylor, S. R. Planetary Science: A Lunar Perspective. Houston: Lunar and Planetary Inst., 1982.

Wilhelms, Don E. To a Rocky Moon: A Geologist’s History of Lunar Exploration. Phoenix: U of Arizona P, 1994.

Wood, John A. “The Lunar Soil.” Scientific American 223 (1970): 14–23.

Zbik, Marek S., et al. "Discovery of Discrete Structured Bubbles within Lunar Regolith Impact Glasses." Astronomy and Astrophysics (2012): 1–3.