Moon

The Moon is a differentiated rocky body with a crust, mantle, and core. Its mean radius is about 1,737 km, roughly one-quarter of Earth's, and its mass is 7.35 × 10²² kg. The surface is covered with regolith, a fine dust produced by meteorite impacts, and features maria (dark basaltic plains) and highlands (bright, heavily cratered terrains). The Moon has no substantial atmosphere, leading to extreme temperature swings from about 127°C in sunlight to -173°C in shadow.¹

The Moon's interior is thought to have a small iron-rich core, about 240 km in radius, surrounded by a partially molten layer. Seismic data from the Apollo missions revealed moonquakes, some originating deep within the mantle. The Moon's crust is thicker on the far side, which may explain the asymmetry in volcanic activity.²

The leading explanation for the Moon's origin is the giant-impact hypothesis: a Mars-sized body, named Theia, collided with the early Earth about 4.5 billion years ago. The debris from this impact coalesced to form the Moon. This theory is supported by the Moon's isotopic composition, which is nearly identical to Earth's mantle, and by computer simulations.³

After formation, the Moon underwent a magma ocean phase, cooling to form the crust. Subsequent impacts created the large basins that later filled with basaltic lava, forming the maria. The Moon's surface has remained largely unchanged for the last 3 billion years, providing a record of Solar System history.⁴

The Moon orbits Earth at an average distance of 384,400 km, with an eccentricity of 0.0549. Its orbital period is 27.3 days, but due to Earth's motion, the synodic month (from new moon to new moon) is 29.5 days. The Moon is tidally locked, showing the same face to Earth, a result of gravitational torque over billions of years.

The Moon's gravitational pull creates ocean tides, with two high tides and two low tides each day. It also causes tidal braking, which slows Earth's rotation and causes the Moon to recede at about 3.8 cm per year. This process has been measured by lunar laser ranging experiments.⁵

The Moon was first reached by a human-made object in 1959 when the Soviet Luna 2 impacted its surface. The first crewed landing was Apollo 11 on July 20, 1969, with Neil Armstrong and Buzz Aldrin walking on the Moon. A total of six Apollo missions landed 12 astronauts, who collected 382 kg of rock and soil samples.⁶

After Apollo, robotic missions from the US, USSR, Japan, China, India, and Europe have mapped the Moon and studied its composition. China's Chang'e missions have landed on the far side and returned samples. The Artemis program aims to return humans to the Moon, including the first woman and person of color, with a goal of establishing a sustainable presence.⁷

Beyond the well-known facts, the Moon has several surprising features. It has a very thin exosphere of sodium and potassium, detected by spacecraft. There are also transient lunar phenomena, brief flashes of light on the surface, possibly caused by outgassing or meteorite impacts.¹

The Moon's far side, often called the dark side, is not always dark; it receives sunlight half the time. The far side lacks the large maria seen on the near side, and its crust is thicker. The Moon also has a slight wobble called libration, which allows us to see about 59% of its surface over time.

Water ice exists in permanently shadowed craters at the poles, a resource for future exploration. The Moon also has a very weak magnetic field, and its surface is covered with a layer of dust that can be electrostatically charged.²

Maria

Large, dark basaltic plains on the Moon formed by ancient volcanic eruptions.

Regolith

Layer of loose, fragmented material covering solid rock, produced by impacts.

Tidal locking

Synchronous rotation where a body always shows the same face to its partner.

Libration

Apparent wobble of the Moon that reveals slightly more than half its surface.