Moon
The Moon is Earth's only natural satellite. It is a differentiated rocky body with a core, mantle, and crust, orbiting Earth at an average distance of about 384,400 km. Its synchronous rotation means it always shows the same hemisphere to our planet.
The Moon is Earth's only natural satellite. In NASA's NAIF ID code catalog, it is listed as “MOON” with code 301; Earth is listed as “EARTH” with code 399. The term “lunar” comes from the Latin name Luna. The Moon has no moons of its own and no rings.
According to NASA's account of its formation, the Moon probably formed several billion years ago, after a body about the size of Mars collided with Earth. Debris from both bodies accumulated and formed the satellite. At first, it was a molten world. Within about 100 million years, most of its global magma ocean had crystallized: less-dense materials rose and contributed to the formation of the crust.
The Moon is an average of about 384,400 km from Earth and is slowly moving away, by approximately one inch—about 2.5 cm—each year. It completes an orbit around Earth in 27 Earth days and rotates on its axis at the same rate. This synchronous rotation keeps the same hemisphere facing Earth. As a result, the far side was not seen from our planet until a Soviet spacecraft flew past it in 1959.
The far side does not remain in darkness all the time: it receives sunlight at different times as the Moon travels along its orbit. The changing illumination of the part we can see produces the lunar phases. From Earth's perspective, the apparent orbit cycle lasts about 29 days because Earth is also moving around the Sun.
The Moon is a differentiated body, organized into a core, mantle, and crust. Its core is proportionally small compared with those of other rocky bodies. It comprises a solid inner core, made mostly of iron, surrounded by molten material and a partially molten zone. NASA gives the radius of the solid inner core as about 240 km and the thickness of the liquid iron shell as about 90 km. The core also contains some nickel and sulfur; overall, it extends through about one-fifth of the distance from the lunar center to the surface.
The mantle is the thickest layer and is composed mainly of basalts rich in olivine and pyroxene, minerals that contain magnesium, iron, silicon, and oxygen. It is mostly solid, although the zone near the core is partially molten. The crust is asymmetrical: it is about 40 km thick in the hemisphere facing Earth and up to about 60 km thick on the far side. Its minerals contain mainly oxygen, silicon, magnesium, iron, calcium, and aluminum, as well as small amounts of titanium, uranium, thorium, potassium, and hydrogen.
The rocky surface is marked by impacts from asteroids, meteorites, and comets. Tycho Crater, for example, is more than 85 km wide. Impacts have ground up and fragmented rocks over billions of years; nearly the entire surface is covered by lunar regolith, a layer of gray dust and rock fragments. Beneath it lies the megaregolith, made up of fractured rock and impact ejecta.
The light-colored regions are the highlands; the dark regions, called maria (“seas”), are impact basins that filled with lava between 4.2 and 1.2 billion years ago. The crust preserves ancient craters and also has channels, volcanic domes, and ridges. The Moon had active volcanoes in the past; they are now dormant and have not erupted for millions of years. NASA cites surface temperatures of about 127 °C in full sunlight and as low as about −173 °C in darkness.
Exploration detected hydration distributed across the surface. In 2008, Chandrayaan-1 found hydroxyl molecules, concentrated at the poles. Missions such as Lunar Prospector, LCROSS, and the Lunar Reconnaissance Orbiter showed elevated concentrations of water ice in permanently shadowed polar regions. In 2020, SOFIA confirmed water molecules on the sunlit surface of Clavius Crater. Micrometeoroid impacts can also release water from the lunar soil; much of the released water escapes into space.
More than 105 robotic spacecraft have been launched to explore the Moon, and it is the only celestial body beyond Earth that humans have visited. Apollo astronauts brought back 382 kg of lunar rocks and soil. Seismometers installed during the Apollo program recorded data until September 1977; samples, seismic signals, spectroscopy, and orbital observations help researchers study the Moon's composition and interior.
The Moon has a very thin exosphere, which provides no protection from solar radiation or meteoroid impacts. Its surface gravity is one-sixth of Earth's. No evidence of life of its own has been found, and its weak exosphere and lack of liquid water cannot support life as we know it.
Values include their timeframe and uncertainty when available. This view does not simulate real-time positions.
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