The Moon’s Crust: A Fragile Shell
The journey to the mantle begins at the surface. The lunar crust is a testament to a violent early history, a battered and fractured layer composed primarily of anorthosite, a light-colored rock rich in calcium and aluminum. This composition is a direct clue to the Moon’s formation; it represents the crystallized remnants of a global magma ocean that once enveloped the young Moon. As this ocean cooled, lighter minerals like plagioclase feldspar floated to the top, forming the primordial crust, while denser, iron- and magnesium-rich minerals like olivine and pyroxene sank, constituting the early mantle. The crust is asymmetrical, averaging about 50 kilometers thick on the near side but extending to over 100 kilometers on the far side, a fundamental asymmetry that continues to influence our understanding of the mantle’s dynamics and the Moon’s thermal evolution.
This crust is not a pristine shell. It is heavily cratered and fractured by billions of years of impacts, which have served as nature’s own drilling probes. Cataclysmic events, like the one that formed the South Pole-Aitken Basin, the largest known impact crater in the solar system, excavated material from depths of tens of kilometers, potentially scattering mantle material across the surface. These colossal impacts created vast fissures and faults that extend deep into the lunar interior, providing pathways for later magmatic activity and altering the underlying mantle’s structure. The regolith, a fine layer of dust and rocky debris covering the crust, acts as a historical archive, but one where the records of deep-seated materials are exceedingly rare and mixed with countless other components.
Seismic Clues: The Apollo Legacy
Our most direct knowledge of the lunar interior comes from the Apollo Passive Seismic Experiment (PSE). Deployed by astronauts from Apollo 11, 12, 14, 15, and 16, these sensitive instruments formed a network that recorded moonquakes and meteoroid impacts for nearly a decade. The data revealed a Moon with a complex, layered interior, fundamentally different from Earth’s. The analysis of seismic wave velocities—how P-waves and S-waves travel through the lunar body—allowed scientists to construct a preliminary model of the mantle’s structure.
The lunar mantle appears to be divided into distinct layers. Directly beneath the crust lies the upper mantle, extending to a depth of roughly 500 kilometers. This region is characterized by a high seismic velocity, indicating it is cold, rigid, and likely composed of minerals like olivine and pyroxene. However, a critical discovery was the presence of a deep zone of attenuation around 500-1000 kilometers depth, where seismic waves are significantly dampened. This suggests a region that is potentially partially molten or at least thermally and mechanically weak, acting as a boundary layer. Below this, the lower mantle extends down to the core-mantle boundary. The entire mantle is far more rigid and colder than Earth’s, with a much lower level of internal heat production due to the Moon’s small size and lack of large-scale plate tectonics.
The moonquakes themselves provided further insight. Deep moonquakes, originating at depths of 700-1000 kilometers, are the most numerous and occur periodically, triggered by tidal stresses from Earth’s gravity. Their existence confirms that the deep mantle is brittle and capable of storing and releasing stress, despite the potentially warmer temperatures at those depths. In contrast, shallow moonquakes, though rare, are far more powerful, and their origins remain enigmatic, possibly linked to tectonic activity driven by global contraction as the Moon’s interior slowly cools.
Geochemical Fingerprints from the Mantle
While seismology provides a structural image, geochemistry offers a compositional one. The primary sources of mantle material are the lunar samples returned by the Apollo and Luna missions, particularly the basaltic maria. These dark plains are floods of lava that erupted between about 4.2 and 1.2 billion years ago. The magma that formed these plains originated from partial melting deep within the mantle. By analyzing the chemistry of these basalts—their ratios of elements like iron, magnesium, titanium, and rare-earth elements—scientists can work backward to infer the composition and mineralogy of the source regions in the mantle.
These studies reveal that the lunar mantle is not uniform. It is a heterogeneous mixture, a relic of the incomplete crystallization of the magma ocean. Some mare basalts are high in titanium, suggesting they came from mantle reservoirs rich in ilmenite, a dense titanium-iron oxide. Others are low in titanium, pointing to different source regions. The presence of volatiles, or the surprising lack thereof, is another key signature. The Moon is exceptionally depleted in volatile elements compared to Earth, a characteristic attributed to the high-energy conditions of its formation. However, recent remote sensing data has hinted at the presence of water ice in permanently shadowed craters, and some lunar samples contain minute amounts of water within apatite crystals, raising profound questions about the distribution and origin of water in the lunar mantle.
A major goal of modern lunar exploration is to find a direct sample of the mantle—a piece of rock that was excavated by an impact but not melted into magma. Candidates for such material are often associated with large impact basins. For instance, some rocks collected from the Apollo 16 site, believed to be ejecta from the Nectaris basin, have compositions suggestive of mantle origins, such as dunite (rich in olivine). However, definitive proof remains elusive, driving missions to target specific geologic features like the South Pole-Aitken Basin, where the impact may have penetrated the crust and scattered mantle material across the surface.
The Role of the Magma Ocean
The concept of the Lunar Magma Ocean (LMO) is central to understanding the mantle’s formation and initial structure. Shortly after the Moon’s accretion, the immense energy from the Giant Impact, combined with heat from radioactive decay, is thought to have melted a significant portion, if not the entirety, of the Moon. As this global ocean of molten rock began to cool, it underwent fractional crystallization. This process was not a simple, orderly sequence; it was a complex geochemical evolution that set the stage for the Moon’s entire geological history.
The first minerals to crystallize were the dense, mafic minerals olivine and pyroxene, which sank through the melt to begin forming the mantle. Later, the buoyant plagioclase feldspar crystallized and floated upward to form the anorthositic crust. This simple model is complicated by the fact that the accumulating mantle crystals would have formed a thick, mushy pile at the bottom of the magma ocean. Furthermore, the final dregs of the LMO would have become increasingly enriched in incompatible elements and radioactive heat-producing elements like uranium, thorium, and potassium. This geochemically distinct layer, known as KREEP (for Potassium (K), Rare-Earth Elements, and Phosphorus), became sandwiched between the crust and mantle, primarily on the lunar near side. The uneven distribution of this hot, radioactive layer explains the asymmetry in volcanic activity, with the near side featuring extensive mare plains while the far side is largely devoid of them.
Modern Investigations: Gravity and Topography
Post-Apollo missions have revolutionized our understanding of the lunar mantle through global remote sensing. NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, which flew twin spacecraft in precise formation around the Moon, mapped its gravitational field with unprecedented detail. By measuring subtle changes in the distance between the two craft caused by variations in the Moon’s gravity, GRAIL revealed the distribution of mass beneath the surface.
The data confirmed the crustal thickness variations and, more importantly, revealed numerous mass concentrations, or mascons, beneath the large mare-filled impact basins. These mascons are dense regions of uplifted mantle material that cause local increases in gravity. GRAIL’s findings showed that the mantle’s rigidity supported these mascons for billions of years, further evidence of a lithosphere that is much thicker and stronger than Earth’s. The mission also helped refine estimates of the depth to the core-mantle boundary and provided evidence for ancient magmatic intrusions, or dikes, extending deep into the mantle, suggesting a more dynamic early thermal history than previously thought.
Complementary data from missions like NASA’s Lunar Reconnaissance Orbiter (LRO), which provides high-resolution topographic maps, allows scientists to correlate surface features with subsurface structures. The combination of gravity and topography data enables the modeling of the lunar lithosphere—the rigid outer shell comprising the crust and the uppermost mantle. These models suggest that the lithosphere thickened rapidly as the Moon cooled, effectively shutting down large-scale mantle convection and volcanic activity by about one billion years ago.
The Enigmatic Core-Mantle Boundary
The boundary between the mantle and the core is one of the most enigmatic regions of the Moon. Seismic data from Apollo is ambiguous regarding the core’s exact size and state, but recent re-analyses and geophysical modeling suggest a small, partially liquid iron core with a radius of about 330 kilometers. The core-mantle boundary is thus a critical interface where heat from the core is transferred to the overlying mantle.
This region may hold a key to the Moon’s past magnetic field. Apollo samples indicate that the Moon once possessed a surprisingly strong magnetic field, generated by a dynamo effect in its core. The energy source for this dynamo is a subject of debate, but one possibility involves a process called core crystallization. As the liquid core cools and solidifies from the center outward, the release of latent heat and light elements could have driven vigorous convection, powering the dynamo. The style of heat transfer across the core-mantle boundary—whether by conduction or small-scale convection—would have profoundly influenced the thermal evolution of the deep mantle above it. The presence of a partially molten layer at the base of the mantle, as suggested by seismic attenuation, could have acted as a thermal insulator or a chemical reactor, further complicating the picture.
Future Missions: Probing Deeper
The next leap in understanding the lunar mantle will come from a new generation of missions specifically designed to probe the deep interior. International efforts are focusing on the lunar south pole and the far side. NASA’s Artemis program aims to land astronauts in this region, where they could potentially sample ejecta from the South Pole-Aitken Basin, offering a direct window into the mantle. The deployment of a new, more sophisticated seismic network is a high priority. Modern seismometers, far more sensitive than their Apollo-era predecessors, could detect subtle signals that would clarify the state of the deep mantle and core.
Planned missions like the Lunar Geophysical Rover concept envision a rover that would deploy a suite of instruments, including a seismometer and a heat flow probe, to make long-term measurements. Furthermore, orbital radar sounders, like those used on Mars, could be employed to probe the upper layers of the crust and mantle for subsurface structures. Missions targeting the largest basins will use spectrometers to map the mineralogical composition of the surface with high precision, searching for the unique spectral signatures of mantle minerals like olivine. Each of these endeavors represents a step toward answering fundamental questions about the Moon’s formation, its relationship to Earth, and the processes that govern the evolution of terrestrial planets. The lunar mantle, long hidden from view, is gradually revealing its secrets, reshaping our understanding of our closest celestial neighbor.