The Moon is a planetary body whose composition differs significantly from Earth’s, representing a geological environment shaped by ancient events and a lack of atmosphere and water. Its surface rocks provide a record of its formation and evolution over billions of years, largely free from erosion and plate tectonics. Most of what is known about the Moon’s composition comes from direct physical samples and orbiting spacecraft data. The resulting chemical environment on the surface is characterized by a scarcity of free oxygen and water, with its rock types reflecting a world born from a massive, high-energy impact.
Retrieving Lunar Samples
Direct physical samples are the foundation of understanding the Moon’s geology, providing a tangible record of its history. The six crewed Apollo missions conducted between 1969 and 1972 were the primary source, returning a total of 382 kilograms of lunar rock and soil to Earth. The materials collected included rocks, fragments of minerals, and fine-grained lunar soil, often referred to as regolith. Later missions, like Apollo 15 and 17, used core tubes to retrieve material from up to 40 centimeters below the surface and employed rakes to collect statistical samples. This extensive collection is supplemented by approximately 300 grams of material returned by three robotic Soviet Luna spacecraft, as well as naturally occurring lunar meteorites found on Earth.
The Primary Rock Types
Lunar rocks are broadly categorized into two main igneous rock types—anorthosite and basalt—along with a third category of impact-formed materials. All lunar rocks are largely made of common rock-forming minerals like olivine, pyroxene, and plagioclase feldspar. They lack the oxidized iron and silica minerals common on Earth. This composition reflects the Moon’s unique formation and thermal history.
Anorthosite
Anorthosite is a light-colored, low-density igneous rock that is rich in aluminum and calcium. It is composed predominantly of the mineral plagioclase feldspar, specifically the calcium-rich variety called anorthite. This rock type represents the Moon’s earliest crust, formed when lighter plagioclase crystals floated to the surface of a global magma ocean during the Moon’s initial cooling phase. The most pristine examples of anorthosite average about 96% plagioclase by volume.
Mare Basalt
Mare basalts are dark, dense volcanic rocks formed from the rapid cooling of ancient lava flows. Compared to the highland rocks, basalts have higher concentrations of iron and magnesium, and they contain greater amounts of minerals like pyroxene and olivine, with less plagioclase. Lunar basalts are also richer in iron than terrestrial basalts. Some varieties contain high abundances of the titanium-rich oxide mineral ilmenite. These flood lavas erupted onto the surface from the partial melting of the Moon’s mantle, reflecting variations in the source regions beneath the surface.
Breccia and Regolith
The majority of samples returned from the Moon are breccias, which are composite rocks formed by repeated impact events, rather than pristine bedrock. A breccia is a rock composed of angular fragments of older rocks and minerals that have been compacted and welded together by the heat and pressure of meteoroid impacts. These mixtures can range from strong, coherent masses to crumbly aggregates.
Lunar regolith is the blanket of fine, loose, fragmental material covering the entire surface of the Moon. It is often called lunar soil, though it lacks the organic content of Earth’s soil, and it consists of pulverized rock fragments, mineral grains, and tiny glassy particles called agglutinates.
Geological Distinction: Highlands and Maria
The Moon’s surface is visibly divided into two distinct geological regions: the bright, heavily cratered highlands and the darker, smoother maria. The highlands, or terrae, are the elevated areas that cover most of the Moon’s surface, dominated by the older, lighter anorthosite. Their anorthositic composition gives them their bright, high-reflectivity appearance, characterized by mountainous topography and an extremely high density of overlapping impact craters.
In contrast, the dark, flat plains known as the maria (Latin for “seas”) are vast, low-lying regions that were misidentified as bodies of water by early astronomers. The maria are composed predominantly of the dark, iron-rich mare basalt, which formed from massive eruptions of lava that flooded large impact basins. The maria are much less cratered than the highlands, indicating they are geologically younger surfaces.
The Moon’s Internal Structure
The Moon is a differentiated body, meaning it has separated into layers of distinct chemical composition, much like Earth. This layering resulted from the fractional crystallization of a deep magma ocean early in its history.
The Crust
The outermost layer is the crust, which is largely anorthositic in composition, consistent with the floating of lighter plagioclase minerals in the ancient magma ocean. The crust is not uniformly thick, averaging about 50 kilometers, but it is notably thinner on the near side facing Earth compared to the far side.
The Mantle
Beneath the crust lies the mantle, composed primarily of the iron-rich minerals olivine and pyroxene. This layer is the source of the magma that erupted to form the mare basalts on the surface. Seismic data from Apollo missions revealed that moonquakes occur deep within the mantle, sometimes as far down as 1,000 kilometers below the surface, often linked to tidal stresses from Earth’s gravity.
The Core
At the very center is a small core, thought to be composed of a metallic iron alloy with small amounts of sulfur and nickel. Recent analysis of seismic data suggests the core has a radius of about 330 kilometers or less, making it only about 20% of the Moon’s total diameter. The core is believed to have two components: a solid iron-rich inner core with a radius of around 240 kilometers, surrounded by a partially molten outer core.