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Last updated August 12, 2025

The Origins of the Moon: From Chaos to Formation

Giant impact forming the Moon

The Enigma of Lunar Origins

For centuries, the origin of the Moon has remained a profound mystery. Early theories included gravitational capture, co-formation with Earth, and even terrestrial fission. However, the analysis of lunar samples brought back by the Apollo missions revolutionized our understanding, leading to today's dominant hypothesis: the giant impact with Theia.

Hypotheses on the origin of the Moon before the Apollo missions
HypothesisPeriodMain ArgumentsMajor ProblemsMain Proponents
Terrestrial Fission1878-1960
  • Assumed compositional similarity
  • Explanation of the Pacific Basin
  • Compatibility with angular momentum
  • Too rapid initial Earth rotation required
  • Unlikely fission mechanism
  • Pacific Basin too young geologically
George Darwin (son of Charles Darwin), Harold Jeffreys
Gravitational Capture1909-1969
  • Allows for different composition
  • Explains orbital inclination
  • Extremely low dynamic probability
  • Requires unlikely atmospheric braking
  • Incompatible with later revealed isotopic similarity
Thomas Jefferson Jackson See, Gordon MacDonald
Co-formation (Binary Accretion)1940-1969
  • Natural process in the nebular model
  • Observed analogues in other systems
  • Does not explain the low lunar density
  • Difficult to justify the current angular momentum
  • Composition too different from expected
Carl Friedrich von Weizsäcker, Gerard Kuiper
Precipitation (Condensation)1960-1969
  • Simple model of rapid formation
  • Compatibility with certain thermal models
  • Unable to explain compositional differences
  • Problems with the distribution of volatile elements
Harold Urey

Source: NASA Historical Archives, Journal for the History of Astronomy (2004), Annual Review of Earth and Planetary Sciences (1975)

The Giant Impact Hypothesis

According to this theory, about 4.5 billion years ago, a celestial body the size of Mars, named Theia, collided with the proto-Earth. This cataclysmic impact vaporized part of the Earth's mantle and ejected debris into orbit. Within a few centuries, these debris aggregated to form our Moon.

Geochemical Evidence of the Giant Impact

Isotopic analyses reveal a striking similarity between the composition of the Moon and that of the Earth's mantle, particularly for oxygen isotopes (\(^{16}O\), \(^{17}O\), \(^{18}O\)). This similarity strongly suggests a common origin, supporting the impact hypothesis.

Table of Evidence for the Giant Impact Hypothesis
Type of EvidenceObservationInterpretationImplications for the ImpactKey References
Oxygen IsotopesΔ17O identical to ±0.016‰ between Earth and MoonSame source reservoir for materialsTheia and proto-Earth must have had similar isotopic compositionsWiechert et al. (2001), Science
Siderophile ElementsDeficit in siderophile elements (Ni, Co, W) in the lunar mantleDepletion due to Theia's metallic coreThe Moon formed mainly from the mantle of the impactor and EarthRingwood (1979), EPSL
VolatilesMarked depletion in K, Na, Pb compared to EarthEvaporation during high-energy impactTemperatures >2000K necessary during accretionDay & Moynier (2014), Phil. Trans.
Fe/Mn RatiosFe/Mn ∼70 identical in lunar and terrestrial basaltsSame mantle formation processCommon source for mantle materialsDrake et al. (1989), GCA
Titanium Isotopesε50Ti identical to ±0.05 ε-unitsComplete mixing of reservoirs after impactEfficient homogenization of the accretion diskZhang et al. (2012), Nature
Tungsten IsotopesExcess 182W in lunar rocks (∼27 ppm)Early differentiation in the first 60 million yearsRapid formation after the giant impactTouboul et al. (2015), Nature
Mg/Si Ratio∼1.2 higher than in chondritesEnrichment in forsterite (Mg2SiO4)Selective partial melting during impactTaylor & Jakes (1974), Proc. LSC

Sources: Wiechert et al. (2001), Ringwood (1979), Day & Moynier (2014), Zhang et al. (2012), Touboul et al. (2015)

The Spherical Formation of the Moon

Accretion Process

After the impact, the ejected debris went into orbit around the Earth. Due to gravity, these debris began to clump together to form increasingly larger bodies. This process, called accretion, eventually led to the formation of the Moon.

N.B.: The Moon likely reached 90% of its current mass in less than 100 years, but it took up to 10 million years to cool completely and acquire its definitive internal structure.

Cooling and Differentiation

Once formed, the Moon was initially in a molten state due to the energy released during the impact and accretion. Over time, it cooled and underwent a differentiation process, where the densest materials sank to the center to form the core, while the less dense materials formed the crust.

Formation of the Lunar Surface

The surface of the Moon that we observe today is the result of billions of years of evolution. Craters, lunar seas (or "maria"), and mountains are all features that testify to its complex geological history. Meteorite impacts, volcanic activity, and tidal forces have all played a role in shaping the lunar surface.

Timeline of Formation

Table of the timeline of the Moon's formation
PhaseDuration after impactKey Event
Debris Disk0-10 yearsMaterials in chaotic orbit
Condensation10-100 yearsSolidification of vapors
Accretion100-1000 yearsFormation of the first planetesimals
Spherization1000-10,000 yearsGravitational balancing
Differentiation10,000-1M yearsFormation of the mantle and crust

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