The Moon, Earth’s nearest celestial neighbor, has captivated humanity for centuries. Its prominent presence in the night sky has inspired questions about its formation and what lies beneath its surface. Understanding the Moon’s composition has been a journey of scientific discovery, revealing insights into its past.
The Lunar Rocks
The Moon’s solid bedrock is composed of three main types of rocks: anorthosite, basalt, and breccia. Anorthosite dominates the ancient, brighter lunar highlands. This light-colored rock is largely made of plagioclase feldspar, a calcium-rich mineral, and represents the Moon’s primordial crust that formed early in its history.
The dark, smooth plains known as maria are predominantly made of basalt. Lunar basalts are volcanic rocks formed from solidified lava flows, similar to those found on Earth. They have higher concentrations of iron and magnesium than anorthosite, and their mineral composition typically includes pyroxene, olivine, plagioclase, and sometimes ilmenite, a titanium-rich oxide.
Breccia is a common rock type found across both the highlands and maria. These composite rocks form from fragments of older rocks, minerals, and glasses fused together. Meteorite impacts shatter existing rocks, and the heat and pressure from these collisions weld the fragments into new, coherent breccia formations.
The Lunar Surface Material
The Moon’s solid rock is covered by a layer of unconsolidated material called regolith. This material varies in thickness, from 4 to 5 meters in mare regions and 10 to 15 meters in the highlands. Lunar regolith differs from Earth’s soil because it lacks organic matter and forms through mechanical processes rather than biological or chemical ones.
Regolith is generated by the continuous bombardment of meteorites and micrometeorites impacting the lunar surface. These impacts pulverize the bedrock, breaking down rocks into a mixture of dust, soil, and fragmented rock pieces. High-velocity impacts can also melt and fuse material, creating glassy particles known as agglutinates, which are a common component of lunar regolith. The fine particles within the regolith are sharp, abrasive, and can be electrically charged, causing them to cling to surfaces.
How Scientists Study the Moon’s Composition
Understanding the Moon’s composition stems from direct analysis of lunar samples. The Apollo missions (11 through 17) brought back a collection of lunar material. Astronauts collected 2,196 samples, totaling 382 kilograms (about 842 pounds), for scientific study.
These returned samples underwent laboratory analysis on Earth. Scientists examined their chemical composition, mineralogy, and age using techniques like radiometric dating and zircon analysis. This direct examination revealed the minerals present and insights into the rocks’ formation conditions and their chronological history.
Robotic missions also map the Moon’s surface composition. Orbiters and landers, such as Lunar Prospector, Lunar Reconnaissance Orbiter (LRO), and the Chang’e missions, use instruments like spectrometers and radar to remotely analyze element and mineral distribution. This remote sensing complements direct sample analysis, providing a broader compositional context.
What Lunar Rocks Reveal About the Moon’s Origin
The composition of lunar rocks supports the Giant Impact Hypothesis for the Moon’s formation. This theory suggests the Moon formed from debris ejected after a Mars-sized body collided with the early Earth approximately 4.5 billion years ago. The impact generated superheated material that coalesced to form the Moon.
Evidence includes the stable isotope ratios of lunar and terrestrial rocks, particularly oxygen isotopes, which are similar. This similarity suggests a common origin for their material. Lunar rocks are depleted in volatile elements, which would have been vaporized and lost during the impact and subsequent formation.
The presence of anorthosite in the highlands and basalt in the maria supports the “lunar magma ocean” concept in the Moon’s early history. After the impact, the Moon was largely molten, forming a deep magma ocean. As this ocean cooled, lighter, calcium-rich plagioclase feldspar crystals floated to the surface, forming the anorthositic crust of the highlands. Denser minerals sank, and later volcanic activity brought basaltic magmas to the surface, creating the maria.