The term “moonrock” refers to geological samples collected from the Moon’s surface, primarily by the Apollo missions. These extraterrestrial materials offer a direct record of the Moon’s formation and evolution. By analyzing their elemental and mineral composition, scientists reconstruct the ancient processes of planetary differentiation and crust formation that occurred billions of years ago. The study of these rocks helps establish a timeline for bombardment events and volcanic activity that shaped the lunar landscape.
Fundamental Mineral Components
The primary composition of lunar rocks is dominated by silicates and oxides. Oxygen is the most abundant element, chemically bound within these mineral structures. The most common silicates are plagioclase feldspar (calcium-rich anorthite), pyroxene, and olivine. Unlike Earth, nearly all silicon is contained within these three main groups.
Plagioclase feldspar is high in calcium and aluminum, contributing to the lighter color of the lunar highlands. Pyroxene and olivine carry iron and magnesium, making them abundant in the darker volcanic plains known as maria. Lunar rocks have a high concentration of iron, present almost entirely in its ferrous state (Fe\(^{2+}\)), indicating extremely low oxygen availability. The iron-titanium oxide mineral ilmenite is also common in mare basalts.
The lack of volatile elements, such as water or potassium, distinguishes these samples from terrestrial rocks. This results from the extreme heat and volatile loss during the Moon’s formation. The presence of metallic iron in its pure, unoxidized state confirms the highly reducing conditions under which the lunar crust solidified.
Structural Classification of Lunar Samples
The fundamental mineral ingredients form three main structural categories defining the lunar surface regions.
Mare Basalts
Mare basalts are dense, fine-grained volcanic rocks that cooled rapidly from lava flows filling large impact basins. These rocks are rich in iron and titanium, and their dark color results from abundant iron-bearing pyroxene and ilmenite. They represent younger, extrusive volcanism that resurfaced about 17 percent of the Moon’s near side.
Highlands Rocks
Highlands rocks constitute the brighter, heavily cratered crust and are generally older. The most common type is anorthosite, a coarse-grained rock composed of over 90 percent calcium-rich plagioclase feldspar. This light composition reflects the early differentiation of the Moon, where lighter minerals floated to the surface of a global magma ocean to form the initial crust. Other types include norites and troctolites, which contain varying amounts of pyroxene and olivine alongside the plagioclase.
Breccias and Regolith
The third classification is breccias and regolith, mixtures created by constant meteorite bombardment. Breccias are rocks composed of fragmented rock cemented together by heat and pressure from impacts. Lunar regolith is the unconsolidated layer of dust, soil, and small rock fragments covering the bedrock. These fragmented materials record the Moon’s intense impact history.
Unique Geochemical Signatures
Lunar rocks possess unique geochemical signatures that reflect their extreme formation conditions. A primary difference is the near-total absence of water-derived minerals (clays or micas), which are common products of hydrothermal alteration on Earth. This lack of hydrated minerals is consistent with the Moon’s formation in a hot, dry, atmosphere-free environment.
The presence of highly siderophile elements (HSEs), which prefer to bond with iron, provides another distinctive signature. These elements (iridium, osmium, and platinum) are extremely rare in the indigenous lunar crust because they sank to the core during early differentiation. Observed concentrations are attributed to the constant infall of meteorites. Scientists use the abundance and ratios of HSEs in impact melt rocks to identify the composition of the meteorites that bombarded the surface.
Impact melt glasses are a unique feature, formed when the heat of a meteorite impact instantly melts the surrounding rock, which then cools rapidly into glassy beads or fragments. The abundance of these impact products demonstrates the pervasive role of cosmic bombardment. The lunar mantle’s depletion in HSEs is at least 20 times greater than Earth’s mantle, constraining theories about the giant impact that formed the Earth-Moon system.
Curation and Access to Lunar Samples
The vast majority of lunar samples (382 kilograms) were collected during the six crewed Apollo missions (1969–1972). These geological specimens are meticulously preserved to maintain their scientific integrity. The primary repository is the Lunar Sample Laboratory Facility (LSLF) at NASA’s Johnson Space Center in Houston, Texas.
To prevent contamination, the rocks are stored and handled in a pristine environment. They reside in cabinets flushed with high-purity nitrogen gas, an inert atmosphere that prevents reaction with oxygen or water vapor. The facility is a certified clean room where oxygen and water levels are strictly maintained below 10 parts per million. Scientists access the samples through gloveboxes using specialized tools.
Researchers worldwide submit proposals to NASA to study the lunar material. This ensures that advanced analytical techniques can continue to extract information from the original samples. The curation facility maintains an archive of data and records, cataloging over 128,000 individual subsamples available for research and education.