Moon - Interior Structure and Geophysics

The Moon's Physical Characteristics and Internal Structure Introduction The Moon is a substantial celestial body that has profoundly influenced Earth throughout its history. Understanding its physical properties—from its size and gravity to its internal layering—is essential for comprehending lunar geology, its past, and its interaction with Earth. The Moon's characteristics reveal a world that was once geologically active but has since cooled and quieted over billions of years. Size and Mass The Moon is impressively large compared to other natural satellites. Its diameter is approximately 3,474 km, which is roughly 27 percent of Earth's diameter. This makes the Moon substantially bigger than planets like Mercury, but much smaller than Earth. If you could see Earth and the Moon side by side, the Moon would look like a modest companion rather than a tiny orbiting speck. The Moon's mass is $7.35 \times 10^{22}$ kg, which equals about 1.23 percent of Earth's mass (or 1/81 of Earth's mass, depending on which comparison you prefer). Despite being smaller than Earth in both diameter and mass, the Moon is the fifth-largest natural satellite in the Solar System and notably, it's the largest natural satellite relative to its primary planet—meaning the Moon is more significant compared to Earth than any other major moon is to its host planet. Density and Gravity The Moon's average density is about 60 percent of Earth's average density ($3.34 \text{ g/cm}^3$ for the Moon versus $5.52 \text{ g/cm}^3$ for Earth). This difference is significant: it tells us that the Moon has less dense material overall and a smaller iron core relative to its size than Earth does. This lower density directly affects the gravitational environment on the lunar surface. The Moon's surface gravity is approximately 1.62 m/s², which is about one-sixth of Earth's surface gravity (9.8 m/s²). This means an astronaut weighing 180 pounds on Earth would weigh only 30 pounds on the Moon. This weaker gravity has important consequences: it means the Moon cannot retain a thick atmosphere, and it also affects how objects move across the lunar surface. Related to surface gravity is escape velocity—the speed needed to leave the Moon's gravity entirely. On the Moon, the escape velocity is 2.38 km/s (about 8,600 km/h), compared to Earth's 11.2 km/s. The Moon's lower escape velocity made it possible for astronauts to launch from the lunar surface with relatively small rockets. Shape and Geological History The Moon's shape provides a window into its past. The Moon exhibits what geologists call a fossil bulge—a subtle deformation in its shape that was "frozen in" as the Moon solidified. This bulge is important because it tells us that the Moon once orbited Earth at about half its current distance. As the Moon has gradually moved away from Earth over billions of years, this equatorial bulge remained, a fossil record of an earlier orbital configuration. <extrainfo> The Moon also shows significant variations in crust thickness between its near side (facing Earth) and far side, another feature influenced by its orbital history and the gravitational interactions with Earth early in the Moon's existence. </extrainfo> Internal Structure: The Layered Moon The Moon, like Earth, has a layered internal structure consisting of a crust, mantle, and core. Understanding these layers is crucial for comprehending the Moon's geology and history. The Lunar Crust The Moon's crust is the outermost layer of solid rock. Its average thickness is about 50 km on the near side (the side facing Earth), but it thickens significantly to up to 100 km on the far side. This variation is one of the Moon's most interesting asymmetries and reflects different cooling histories of the two hemispheres. The crust is primarily composed of anorthosite, a feldspar-rich rock. Anorthosite is light-colored and aluminum-rich, which is why the lunar highlands (the ancient highlands visible as the bright regions on the Moon) have such a distinctive appearance. The Lunar Mantle Beneath the crust lies the mantle, which extends from the base of the crust down to the core. The lunar mantle is mafic in composition, meaning it is richer in iron and magnesium compared to Earth's mantle. Importantly, the lunar mantle contains more iron than Earth's mantle, which affects how the Moon has cooled and evolved over time. The Lunar Core The Moon has both an inner solid core and an outer liquid core, as revealed by seismic studies. These cores are primarily composed of iron with lighter elements mixed in. The core is much smaller than Earth's core relative to the Moon's size—it has a radius of only a few hundred kilometers, whereas Earth's core is proportionally larger. How We Know the Moon's Interior Scientists have learned about the Moon's internal structure through several crucial techniques: Seismic Studies: Instruments left on the lunar surface by Apollo astronauts recorded moonquakes and vibrations from meteorite impacts. By analyzing how seismic waves traveled through the Moon—waves that move differently through different rock types—scientists could map the internal layers, much as we map Earth's interior. Laser Ranging: Lunar laser ranging experiments work by bouncing laser beams off reflectors left on the Moon and measuring the precise time for the light to return. This provides extraordinarily accurate measurements of the Moon's distance and reveals its physical librations (slight wobbles in its rotation). These measurements also help reveal information about the Moon's interior density distribution. Thermal Evolution and the Lost Magnetic Field The Moon's internal history is a story of cooling. Early in its history, the Moon was warmer and more geologically active. Its iron core generated a magnetic field through a core dynamo—a process similar to what creates Earth's magnetic field today. However, billions of years ago, as the core cooled, this dynamo shut down and the magnetic field ceased. The evidence for this ancient magnetic field is preserved in lunar rocks brought back by Apollo astronauts, which show magnetic properties from a time when the Moon had a planetary magnetic field. This cooling has had cascading effects: as the core cooled, volcanic activity reduced, and eventually the Moon became the quiet, geologically inactive (though not tectonically entirely dead) world we observe today. <extrainfo> This thermal cooling also affected the Moon's internal stress state and may have contributed to the formation of the wrinkle ridges and other surface features we observe today. The Moon continues to cool very slowly even now, billions of years after its formation. </extrainfo>