Moon

The Moon is Earth's sole permanent natural satellite and the fifth-largest satellite in the Solar System, orbiting at a mean geocentric semi-major axis of a = 384,400 km (60.27 R_⊕; perigee ~363,300 km, apogee ~405,500 km) with a sidereal orbital period of 27.321661 days and a synodic phase period (lunation) of 29.530589 days. With a mean volumetric radius of R = 1,737.4 km (0.2727 R_⊕) and a mass of M = 7.346 × 10²² kg (0.012300 M_⊕, or 1/81.3 of Earth's mass), the Moon has the largest satellite-to-planet mass ratio of any major planet. Its surface gravitational acceleration is g = 1.622 m s⁻² (~16.5% of Earth's gravity) and its escape velocity is 2.38 km s⁻¹, precluding a collisional atmosphere.

Locked in 1:1 synchronous tidal locking with Earth, the Moon rotates on its axis once per 27.322-day sidereal orbit, keeping its nearside hemisphere oriented toward Earth while optical librations in longitude (±7.9°, from e = 0.0549 eccentricity) and latitude (±6.7°) allow 59% of the surface to be seen from Earth. Visited by six crewed NASA Apollo landings (Apollo 11, 12, 14, 15, 16, and 17 between 1969 and 1972, returning 381.7 kg of rock and soil) plus Soviet Luna and Chinese Chang'e 5 and Chang'e 6 robotic sample returns, the Moon anchors radiometric impact crater chronologies across all rocky worlds from Mercury to Mars.

The Theia Giant Impact and Isotopic Equivalence

Geochemical analyses of Apollo samples alongside angular momentum constraints led William Hartmann, Donald Davis, Alastair Cameron, and William Ward in 1975–1976 to formulate the Giant-Impact Hypothesis. Approximately 50 to 100 million years after CAIs (~4.51–4.46 Ga, constrained by Lu–Hf and U–Pb zircon dating), the differentiated proto-Earth was struck obliquely by a Mars-sized protoplanet (~0.10–0.15 M_⊕) named Theia. While Theia's iron core merged into Earth's core, silicate mantle material from both bodies was vaporized and ejected into a circumterrestrial protolunar disk (or vaporized synestia) outside Earth's Roche limit (~2.9 R_⊕). Accretion from this high-temperature silicate disk explains three primary properties of the Moon:

1. Iron Depletion and Low Bulk Density: The Moon's mean density of 3.344 g cm⁻³ (versus Earth's 5.514 g cm⁻³) reflects a small metallic core comprising only 1% to 2% of the lunar mass.
2. Volatile Depletion: Lunar rocks are depleted in moderately volatile elements (K, Na, Rb, Zn, and Pb, yielding K/U ≈ 2,500 vs. ~12,000 on Earth) and lack hydrated phyllosilicates or ferric iron (Fe³⁺).
3. Isotopic Equivalence: Earth and the Moon share indistinguishable isotopic ratios across oxygen (Δ¹⁷O within ±5 ppm), titanium (⁵⁰Ti/⁴⁷Ti within ±4 ppm), chromium (⁵⁴Cr), and tungsten (¹⁸²W), recording turbulent equilibration inside the impact vapor disk or shared inner-disk provenance.

Lunar Magma Ocean Crystallization: Anorthosite Flotation and KREEP

Following accretion, the young Moon was enveloped in a global Lunar Magma Ocean (LMO) hundreds of kilometers deep. As the magma ocean cooled from the base upward, dense mafic minerals—first olivine ((Mg,Fe)₂SiO₄) and then pyroxene ((Mg,Fe)SiO₃)—crystallized and sank to form the mantle cumulates.

Once ~75% to 80% of the magma ocean had solidified, calcium-rich plagioclase feldspar (anorthite, CaAl₂Si₂O₈) crystallized. Because plagioclase (ρ ≈ 2.73 g cm⁻³) was less dense than the iron-enriched residual silicate melt (ρ ≈ 3.1 g cm⁻³), buoyant plagioclase crystals floated upward to form a primary flotation crust of light-colored ferroan anorthosite (FAN)—identified in 1970 by John Wood and colleagues in Apollo 11 soil fragments. The final 1%–2% residual liquid trapped beneath the crust became enriched in incompatible elements, crystallizing as urKREEP—rich in K (potassium), REE (rare-earth elements), and P (phosphorus), plus heat-producing radioactive uranium (U) and thorium (Th)—alongside dense titanium-iron ilmenite (FeTiO₃) cumulates.

Nearside–Farside Dichotomy, Lunar Maria, and South Pole–Aitken Basin

Photographed for the first time by the Soviet Luna 3 probe in October 1959, the lunar farside differs markedly from the nearside in a hemispherical crustal dichotomy. Gravity gradiometry from NASA's GRAIL mission (2012) established that the lunar crust has ~12% bulk porosity and averages 34 to 43 km in thickness, but is ~15–20 km thinner on the nearside (~30 km) than on the farside highlands (50–60 km).

Furthermore, heat-producing KREEP and thorium (up to 10–12 ppm Th) are concentrated on the nearside within the Procellarum KREEP Terrane (PKT). Radiogenic heating inside the PKT sustained mantle partial melting from ~3.9 to 3.1 Ga (continuing locally to ~2.0 Ga in Oceanus Procellarum as dated in Chang'e 5 samples), extruding low-viscosity iron- and titanium-rich flood basalts into nearside impact basins (Imbrium, Serenitatis, Crisium, and Tranquillitatis) to form the dark lunar maria (~31% of the nearside vs. <2% of the farside) and positive gravity mascons. Dominating the southern farside is the South Pole–Aitken basin (SPA), a 2,500-km-diameter, 8–13-km-deep impact structure (~4.2–4.3 Ga) from which China's Chang'e 6 returned ~2.8 Ga farside basalts in June 2024.

Apollo Seismology, Space Weathering, and Tidal Recession

Four passive seismometers deployed by Apollo 12, 14, 15, and 16 recorded over 12,000 seismic events, including monthly deep moonquakes (700–1,100 km depth) triggered by Earth's tidal forces and shallow tectonic moonquakes. Array polarization analysis confirmed a ~330-km metallic core (~240-km solid inner core inside a ~90-km fluid outer core) that powered an ancient lunar magnetic dynamo (20–100 μT) between 4.2 and 3.5 Ga.

Exposed directly to micrometeoroid bombardment and the solar wind, the surface is blanketed by 4–15 m of lunar regolith in which ferrous iron (Fe²⁺) is reduced to nanophase metallic iron (npFe⁰) via space weathering, while cryogenic Permanently Shadowed Regions (PSRs) near the poles (25–50 K) trap up to 5.6 wt% water ice. Finally, gravitational coupling with Earth's rotationally advanced tidal bulges drives a lunar tidal recession rate of 38.2 ± 0.1 mm yr⁻¹ (3.82 cm yr⁻¹) measured by Lunar Laser Ranging, lengthening Earth's solar day by ~2.3 ms century⁻¹ while stabilizing Earth's obliquity unlike chaotic Mars.

Does the Moon have a permanent "dark side" that never receives sunlight?

No. "Dark side" is a misnomer for the farside—the hemisphere that faces away from Earth due to synchronous tidal locking. Both the nearside and farside receive ~14.77 Earth days of sunlight followed by 14.77 days of night during each 29.53-day synodic cycle; at new moon, the farside is in full midday sunlight.

Why are the dark volcanic plains (lunar maria) concentrated on the nearside?

The nearside crust is 15 to 20 km thinner (~30 km) than the farside crust (~50–60 km), and radioactive heat-producing elements (U, Th, and K) are concentrated in the nearside Procellarum KREEP Terrane, allowing basaltic magmas to melt and breach the thin nearside basin floors.

Why do astronauts' footprints on the Moon last for millions of years?

Without wind, rain, or plate tectonics, lunar surface modification occurs only via slow micrometeoroid gardening and space weathering, which overturns the top centimeter of regolith only once every 10 million to 20 million years.

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