The Moon is a differentiated world. This means that it is composed of different layers with different compositions. The heaviest materials have sunken down into the Moon’s center, and the lightest materials have risen to the outermost layer. Seismic, rotational, and gravity measurement studies have allowed us to gain insights into the different layers within the Moon.
At the center is the Moon’s dense, metallic core. The core is largely composed of iron and some nickel. The inner core is a solid mass about 480 km in diameter. Surrounding the solid inner core is a fluid outer core, that brings the total diameter of the core to about 660 km. The Moon’s core is small (about 20% of the Moons diameter) as opposed to other terrestrial worlds (like the Earth) with cores measuring closer to 50% of their diameters.
Above the core are the mantle and crust. Differences in compositions between these layers tell a story of the Moon being largely, or even completely, composed of a great ocean of magma in its very early history. As the magma ocean began to cool, crystals began to form within the magma. Crystals of denser mantle minerals, such as olivine and pyroxene sank down to the bottom of the ocean. Lighter minerals, notably anorthositic plagioclase feldspar, crystalized and floated to the surface to form the Moon’s crust. The mantle, with a thickness of roughly 1350 km is far more extensive than the crust, which has an average thickness of about 50 km. Interestingly, the crust of the Moon seems to be thinner on the side of the Moon facing the Earth, and thicker on the side facing away. Researchers are still working to determine why this might be.
Seismometers left on the surface of the Moon by the Apollo astronauts have revealed that the Moon does experience moonquakes. Deep moonquakes, occurring broadly around 700 km beneath the lunar surface are tidal events, caused by the pull of Earth’s gravity tugging and stretching the internal structures of the Moon. Moonquakes originating on or near the surface can be caused by meteoroid impacts with the Moon. Another type of extremely shallow moonquake can come from thermal expansion and contraction of rock on or near the surface as it goes from the extremely frigid lunar night to the very hot lunar daytime. A fourth type of moonquake originates at the moderately shallow depths of 20-30 km, can register up to a startling 5.5 on the Richter scale, and can last for over 10 minutes! The causes of this fourth type of moonquake are still being investigated.
The regular daily and monthly rhythms of Earth’s only natural satellite, the Moon, have guided timekeepers for thousands of years. Its influence on Earth’s cycles, notably tides, has been charted by many cultures in many ages.
The Moon moderates Earth’s wobble on its axis, leading to a relatively stable climate over billions of years. From Earth, we always see the same face of the Moon because the Moon is spinning on its axis at the same speed that it is going around Earth (that is, it is in synchronous rotation with Earth).
The light areas of the Moon are known as the highlands. The dark features, called maria (Latin for seas), are impact basins that were filled with lava between 4.2 and 1.2 billion years ago. These light and dark areas represent rocks of different composition and ages, which provide evidence for how the early crust may have crystallized from a lunar magma ocean. The craters themselves, which have been preserved for billions of years, provide an impact history for the Moon and other bodies in the inner solar system.
The leading theory of the Moon’s origin is that a Mars-sized body collided with Earth approximately 4.5 billion years ago, and the resulting debris from both Earth and the impactor accumulated to form our natural satellite. The newly formed Moon was in a molten state. Within about 100 million years, most of the global “magma ocean” had crystallized, with less-dense rocks floating upward and eventually forming the lunar crust.
The early Moon may have developed an internal dynamo, the mechanism for global magnetic fields for terrestrial planets. Since the ancient time of volcanism, the arid, lifeless Moon has remained nearly unchanged. With too sparse an atmosphere to impede impacts, a steady rain of asteroids, meteoroids, and comets strikes the surface. Over billions of years, the surface has been ground up into fragments ranging from huge boulders to powder.
Nearly the entire Moon is covered by a rubble pile of charcoal-gray, powdery dust and rocky debris called the lunar regolith. Beneath is a region of fractured bedrock referred to as the megaregolith.
The Moon was first visited by the Soviet Union’s uncrewed Luna 1 and 2 in 1959, and, as of April 2019, seven nations have followed. The U.S. sent three classes of robotic missions to prepare the way for human exploration: the Rangers (1961–1965) were impact probes, the Lunar Orbiters (1966–1967) mapped the surface to find landing sites, and the Surveyors (1966–1968) were soft landers.
The first human landing on the Moon was on July 20, 1969. During the Apollo missions of 1969–1972, 12 American astronauts walked on the Moon and used a Lunar Roving Vehicle to travel on the surface and extend their studies of soil mechanics, meteoroids, lunar ranging, magnetic fields, and solar wind. The Apollo astronauts brought back 382 kilograms (842 pounds) of rock and soil to Earth for study.