The Moon, Earth’s natural satellite, possesses a complex internal structure. Understanding what lies beneath its surface is crucial for unraveling its formation and evolution.
The Moon’s Layered Interior
Similar to Earth, the Moon is a differentiated body with distinct layers: a crust, mantle, and core. This layered structure developed as the Moon solidified from a molten state, with denser materials sinking towards the center and lighter materials rising.
The outermost layer is the lunar crust, which varies in thickness, averaging about 50 kilometers (31 miles). It is thinner on the near side, facing Earth, at approximately 40 kilometers (25 miles), and thicker on the far side, extending up to 60 kilometers (37 miles). Beneath the crust lies the Moon’s mantle, which makes up most of its volume. The innermost region is the lunar core, composed of a solid inner core surrounded by a fluid outer core.
Properties of the Lunar Mantle
The Moon’s mantle is its thickest layer, extending from the base of the crust to the core, with an estimated thickness of roughly 1,338 kilometers (831 miles). This substantial layer is primarily solid, though some scientific models suggest the deepest part, closest to the core, might contain a partially molten boundary layer.
The lunar mantle is characterized by iron- and magnesium-rich silicates, specifically minerals like olivine and pyroxene. These minerals are denser than those found in the crust. Analyses of lunar basalts, which are volcanic rocks that erupted from the Moon’s interior, confirm this mafic (magnesium and iron-rich) mantle composition. Some studies also indicate the lunar mantle is more iron-rich than Earth’s mantle.
The physical state and composition of the lunar mantle are important for understanding the Moon’s geological history. Partial melting within the mantle historically led to the eruption of mare basalts, forming the dark plains visible on the lunar surface. While the Moon is considered geologically less active than Earth, processes within its mantle, such as potential mantle overturn, might have influenced past volcanic activity and even its ancient magnetic field.
How Scientists Study the Moon’s Depths
Scientists study the Moon’s interior primarily through seismic experiments, similar to how Earth’s internal structure is studied. Apollo missions, specifically Apollo 11, 12, 14, 15, and 16, deployed seismometers on the lunar surface between 1969 and 1977.
These instruments detected vibrations and tilting of the lunar surface caused by moonquakes or impacts from meteoroids. The data collected from these seismometers, which operated for several years, provided the first detailed insights into the Moon’s internal structure.
By analyzing how seismic waves propagated through the Moon, scientists inferred the presence and characteristics of its layers, including the mantle. Different materials transmit seismic waves at varying speeds, allowing researchers to create a “geological X-ray” of the Moon’s interior. For instance, P-wave velocities of about 6.8 km/s and S-wave velocities of about 4.5 km/s were observed in the mantle down to 1200 km depth.
Moonquakes originate from various sources, including tidal forces from Earth, meteoroid impacts, and thermal stresses. Deep moonquakes occur within the mantle, often around 1,000 kilometers below the surface, and are linked to tidal stresses. The long duration and scattering of seismic waves from moonquakes suggest a highly fractured and heterogeneous upper lunar crust. Beyond seismic data, missions like GRAIL (Gravity Recovery and Interior Laboratory) also used precise gravity measurements to map density variations and crustal thickness, further refining our understanding of the Moon’s internal layers.
Lunar Mantle Versus Earth’s Mantle
While both the Moon and Earth possess mantles, significant differences exist in their characteristics and behavior. Both are primarily solid, silicate-rich layers, composed largely of minerals like olivine and pyroxene. The Moon’s mantle is proportionally much thicker relative to its overall size compared to Earth’s mantle. Earth’s mantle is approximately 2,900 kilometers (1,800 miles) thick, making up about 84% of Earth’s volume, whereas the Moon’s mantle is about 1,338 kilometers thick.
A primary distinction lies in their physical states and thermal activity. Earth’s mantle is characterized by convection, a slow churning of solid silicate rock that drives plate tectonics on the surface. This convective motion is fueled by internal heat, leading to significant geological activity. In contrast, the Moon’s mantle shows no evidence of large-scale convection, and its overall interior is considered relatively isothermal, or having a uniform temperature. The Moon’s smaller size allowed it to cool more quickly and completely, resulting in a much thicker, rigid lithosphere compared to Earth’s.
Furthermore, Earth’s mantle plays a direct role in generating its strong magnetic field through the convection of molten metals in its core. The Moon, however, lacks a strong global magnetic field today. While some evidence suggests the Moon may have had an ancient magnetic field, its small core and lack of significant mantle convection make it unlikely to have sustained a strong, long-lived magnetic dynamo.