The Moon's Composition, Structure, and Physical Properties
The Moon is the most studied body beyond Earth, thanks largely to the Apollo missions and decades of remote sensing from orbit. Understanding its composition, structure, and surface features gives us a window into the early history of the solar system, since the Moon's surface has barely changed in billions of years.
Lunar Exploration's Scientific Impact
The Apollo missions (1969–1972) brought back about 382 kg of lunar rock and soil, giving scientists direct samples to analyze. Those samples revealed that the Moon's crust is primarily composed of anorthosite, an igneous rock rich in calcium and aluminum.
Astronauts also placed seismometers on the surface during Apollo missions. The seismic data showed that the Moon has a layered interior:
- A thick crust (roughly 60–100 km, thicker on the far side than the near side)
- A rocky mantle beneath the crust
- A small, partially molten core at the center
Beyond Apollo, remote sensing instruments on lunar orbiters have mapped the Moon's surface composition in detail. Spectral data identify minerals like pyroxene, olivine, and ilmenite alongside the dominant anorthosite.
Key Properties of the Moon
- Mass: kg, roughly 1.2% of Earth's mass
- Surface gravity: About 1/6th of Earth's, at . This lower gravity is a direct result of the Moon's smaller mass. It affects everything from how astronauts move to the shape of impact craters.
- Atmosphere: The Moon has essentially no atmosphere. What exists is technically an exosphere, with density and pressure so low they're nearly zero. The Moon's weak gravity simply can't hold onto atmospheric gases. With no real atmosphere, there's no wind, no rain, and no weather of any kind.
- Synchronous rotation: The Moon always shows the same face toward Earth. Its rotation period matches its orbital period, so we never see the far side from Earth's surface.
Moon vs. Earth Surface Features
Earth's surface is constantly reshaped by plate tectonics, erosion, and weather. The Moon has none of these processes, so its surface is far more ancient and static. That's what makes it such a valuable record of early solar system history.
Impact craters are the Moon's most prominent features. They're far more numerous and better preserved than on Earth because there's no atmosphere to burn up incoming objects and no erosion to wear craters away. Lunar craters range from microscopic pits to basins hundreds of kilometers across.
Volcanic features tell us the Moon was once geologically active:
- Mare basalts are the vast, dark plains visible from Earth. These are solidified lava flows that filled large impact basins billions of years ago, forming the lunar maria.
- Sinuous rilles are winding, channel-like features carved by ancient lava flows.
- Domes are small, rounded hills that likely formed as volcanic constructs.
Tectonic structures on the Moon formed as it cooled and contracted over time, not from plate tectonics:
- Wrinkle ridges are raised, linear features caused by compressional stress in the crust.
- Graben are linear depressions bounded by parallel faults, formed by extensional (stretching) stress.
The primary weathering process on the Moon is space weathering, caused by constant micrometeorite impacts and solar wind bombardment. Over time, space weathering darkens and reddens the lunar regolith (the layer of loose, broken rock covering the surface) and alters its chemical composition.
Lunar Surface Features
- Highlands: The light-colored, heavily cratered regions that make up most of the Moon's surface. They're composed mainly of anorthosite and represent the Moon's oldest surviving crust.
- Mascons: Short for "mass concentrations," these are regions of unusually dense material beneath the surface, typically found under large impact basins. They create measurable variations in the Moon's gravitational field, which can affect the orbits of spacecraft passing overhead.