Lunar rocks

Lunar rocks are among the materials brought to Earth from the surface of the Moon during the robotic Soviet Luna and the crewed American Apollo Moon missions of the 1960s and 1970s. These samples were analyzed by international teams of scientists in an effort to enhance understanding of the physical nature of the Moon and its origins. All samples were cataloged according to criteria relating to mineralogy, crystallography, geochronology, geochemistry, magnetism, radioactivity, and other characteristics.

Overview

When direct exploration of the surface of the Moon began during the 1960s, scientists around the world looked forward with great anticipation to the study and analysis of materials brought back to Earth from the lunar surface by the crewed space missions of the Apollo program and the uncrewed programs of the Soviet Union, such as the Luna missions. As new and different materials arrived at the completion of each successive mission, a wealth of information about the origin and physical character of the Moon began to accumulate. Many Apollo lunar samples continue to be studied decades after the original mission. A conscious effort was been made to save large portions of the Moon rock collection for emerging technologies that could be employed to study them in ways not possible back in 1969 through 1972, the era of the Apollo lunar landings.

Through scientific analysis of soil and rock gathered during Moon missions, researchers have identified the basic elements and minerals that constitute the Moon’s surface. Using that knowledge in combination with other information gathered by remote-sensing technology and sophisticated photography techniques, they have extrapolated information about the origin of the Moon. In addition, they have been able to determine a chronology of events in lunar evolution. While many mysteries remain about the Moon and its history, it is remarkable how much information was gathered from studying Moon rocks.

Moon rocks are defined as surface materials that exceed one centimeter in diameter. Anything smaller is considered lunar soil, or regolith, although lunar regolith is unlike Earth soil, which usually contains large amounts of decayed organic material and moisture. On the Moon, the lack of atmosphere and organic material gives the soil a composition similar to dry, clean sand. It is probably powdered rock, created when large objects, such as meteoroids, impacted violently with the surface of the Moon over many millennia.

Rocks on the Moon's surface are rich in calcium and aluminum. Below twenty kilometers, the surface is not broken as extensively as it is near the top. The upper mantle is 200 to 300 kilometers thick and contains high magnesium and iron silicate, pyroxene, and olivine concentrations. There is reason to believe that the core is iron-rich and produced a global magnetic field during the early days of the Moon’s history. Most of the radioactive materials on the Moon are located at or near the surface. Two major activities that have resulted in the character of the lunar surface are volcanism and impact events. Lowland regions exhibit evidence of great volcanic activity on the Moon, and across the lunar landscape, impacts have resulted in a broad redistribution of rock and soil. Occasionally, that activity has resulted in the breaking up of bedrock and basalt breccias that formed before later redistribution, recrystallization, or both.

Each lunar mission that returned to Earth with samples of Moon rock expanded the body of knowledge about the geological character of the Moon’s surface. Each mission landed at a different location on the lunar surface to ensure that as broad a collection of materials as possible would be acquired given the limited number of missions planned and the limited payload capacity available on returning spacecraft. For lunar exploration, two primary regions of the Moon were identified. Generally, the lunar highlands are regions that appear light on the surface of the Moon. The maria constitute that portion of the lunar surface that is dark. The highlands are generally composed of remnants of ancient volcanic cones and the ridges of giant impact craters. The maria are surface areas characterized by volcanic chamber depressions and impact crater basins. Analysis of rocks collected at various spots within the two regions demonstrates that each has unique physical characteristics.

Moon rocks are of two main types: crystallized (or igneous) and breccias. The igneous rock encountered on the lunar surface is most often volcanic in origin and appears to have been scattered over wide areas during violent eruptions. Some specimens are believed to have been thrown between 100 and 1,000 kilometers during eruptions or due to meteor impacts. Certain highland samples contain evidence of plutonic rock formed when volcanic material cools below the surface.

The Apollo 17 crew, working with the benefit of knowledge gained in previous crewed and uncrewed missions, further subcategorized lunar rock as follows: basalts; dark matrix breccias; glass-bonded agglutinates; vesicular green-gray breccias; blue-gray breccias; layered, foliated, light-gray breccias; and brecciated gabbroic rocks. The distribution of Moon-rock material is different for highland and mare regions. In the highlands, the surface consists of approximately 10 percent plutonic rock, 85 percent breccias, and 5 percent volcanic material, primarily basalts. Mare regions contain 90 percent volcanic basalts and 10 percent breccias.

It is important to note that the number of minerals identified on the Moon is dwarfed by the number found on Earth; this difference probably results from the Moon’s lack of atmosphere, particularly oxygen and moisture, which often interact with elements in the weathering process to form minerals. There are some two thousand minerals on Earth, whereas only two hundred are known to exist on the Moon. The number of primary minerals, however, is similar in both bodies. The principal minerals discovered on the Moon at the regolith level include clinopyroxene, plagioclase, olivine, ilmenite (by far the most abundant, 15-20 percent by volume), tridymite, cristobalite, and orthopyroxenes. Several other minerals have been identified, including armalcolite, pyroxferroite, and tranquillityite. It is interesting to note that armalcolite was named for the three astronauts on the Apollo mission that returned with it: Neil Armstrong, “Buzz” Aldrin, and Michael Collins. Armalcolite has since been found on Earth; it is formed when Crystallization occurs without moisture and oxygen.

Ancient orange glass beads, thought to have been spewed out during volcanic eruptions, have given scientists a glimpse of what constitutes the core of the Moon’s interior. These beads contain high levels of lead, zinc, tellurium, and sulfur and are thought to have originated as deep as 300 kilometers below the surface of the Moon. Lunar rock samples have been individually but not uniformly rich in such elements as aluminum, magnesium, potassium, phosphorus, titanium, iron, chromium, and zirconium. Some lunar rocks are referred to as glassy agglutinates. These rocks are believed to have been formed when volcanic action or meteoric impact caused fine rock dust to weld together, a process known as impact melting. Their appearance ranges from dark opaque to transparent. Other rocks have different origins but are covered by glassy materials thought to have formed and distributed similarly. While the great majority of collected lunar rocks are igneous, there is some evidence that a metamorphosis has occurred due to the shock settling of soil caused by meteor impact and vibrations that occur during violent volcanic activity.

Breccias appear to contain material that impacts or volcanic eruptions have ejected. There are two types: soil breccia and ejecta blanket breccia. Soil breccia is composed of the same materials that exist as soil in a particular region and probably results from the shock of a meteor impact. Soil breccia is also characterized by weak cohesion. Ejecta blanket breccias, by contrast, are thought to have been formed when volcanically ejected materials fused to form a layer of rock that later underwent various forms of thermal metamorphism. The composition of such rock varies with the depth at which it has been found or at which it is thought to have originated.

The primary source of lunar erosion appears to have been impact cratering, although solar wind and cosmic-ray effects have also altered the character and composition of surface materials. Some lunar samples show cosmic-ray bombardment over long periods that resulted in the sample's surface alteration. The same activity has been helpful in helping scientists determine how long a sample fragment was in the exact location and at what level it resided within the regolith.

Scientists have also determined with some confidence the age of the Moon and the time of the earliest crystallization of Moon rock formations. Isotopic dating techniques indicate that the earliest crystallization occurred more than four billion years ago and that most volcanic activity appears to have spanned some 600 million years.

Methods of Study

Before the first direct contact with the surface of the Moon occurred, much was known about the composition and origin of that surface from sophisticated technology capable of analyzing materials remotely. Those techniques included indirect sensing processes involving the electromagnetic spectrum's visible, infrared, and microwave bands. The understanding derived from this technology was limited until samples of surface materials were brought to Earth. Scientists could then compare the data from the remote-sensing experiments with direct observations to determine what they were seeing at the time. Radar, which was used in determining the physical nature of the Moon’s surface, successfully established that it was loose, a mixture of sand and broken rock. Infrared studies confirmed that the surface was porous and exhibited low thermal conductivity. Photometric and polarimetric studies further refined the knowledge of the character of the surface by establishing that it contains high concentrations of iron-rich basalts (confirmed by the Apollo missions).

Once the first lunar samples arrived back on Earth, scanning electronic microscopes were used to photograph them, an essential step in categorizing individual sample fragments. Samples were also sieved to determine the relative abundance of materials in a particular location. In this way, researchers could determine where some materials were more abundant than others on the surface when collected using the same procedure. Some samples were cross-sectioned for better observation of stratification if it existed. Many were tested using standard chemical analysis techniques to determine the chemical nature of the materials. X-ray fluorescence analyses were also conducted to determine the relative concentrations of specific elements or components. Isotopic and carbon dating techniques were used to determine the relative ages of specific samples and to attempt a chronology of the events that have affected the evolution of the materials under study.

Context

There is enormous scientific interest in the Moon. Many scientists believe it may hold the key to understanding the physical nature of the solar system—indeed, of the universe itself. Perhaps more importantly, it may hold the key to a better understanding of the origin of Earth and its relationship with other objects in the universe. Lunar rocks have given scientists the first window into that unknown sphere, helping them understand planetary formation's nature. The Luna and Apollo programs demonstrated for the first time that many accepted ideas about the Moon and its composition were accurate, though many others were not. They also demonstrated that the Earth and its satellite have much in common. Except for the lack of a lunar atmosphere of composition similar to Earth, they would probably have evolved in much the same way.

Another significant outcome of the study of lunar rock samples has been the unequivocal knowledge that certain elements, metals, and other valuable materials exist in abundance on the Moon and may be important to future generations on Earth. Perhaps more important, however, is the knowledge gained of how specific elements interact under conditions that rarely exist on Earth, namely, with little oxygen and no water. The Moon has become a giant laboratory that is helping scientists to understand the nature of chemical and geological evolution and order.

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