Last updated August 12, 2025

The Origins of the Moon: From Chaos to Formation

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
Hypothesis	Period	Main Arguments	Major Problems	Main Proponents
Terrestrial Fission	1878-1960	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 Capture	1909-1969	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 Evidence	Observation	Interpretation	Implications for the Impact	Key References
Oxygen Isotopes	Δ¹⁷O identical to ±0.016‰ between Earth and Moon	Same source reservoir for materials	Theia and proto-Earth must have had similar isotopic compositions	Wiechert et al. (2001), Science
Siderophile Elements	Deficit in siderophile elements (Ni, Co, W) in the lunar mantle	Depletion due to Theia's metallic core	The Moon formed mainly from the mantle of the impactor and Earth	Ringwood (1979), EPSL
Volatiles	Marked depletion in K, Na, Pb compared to Earth	Evaporation during high-energy impact	Temperatures >2000K necessary during accretion	Day & Moynier (2014), Phil. Trans.
Fe/Mn Ratios	Fe/Mn ∼70 identical in lunar and terrestrial basalts	Same mantle formation process	Common source for mantle materials	Drake et al. (1989), GCA
Titanium Isotopes	ε⁵⁰Ti identical to ±0.05 ε-units	Complete mixing of reservoirs after impact	Efficient homogenization of the accretion disk	Zhang et al. (2012), Nature
Tungsten Isotopes	Excess ¹⁸²W in lunar rocks (∼27 ppm)	Early differentiation in the first 60 million years	Rapid formation after the giant impact	Touboul et al. (2015), Nature
Mg/Si Ratio	∼1.2 higher than in chondrites	Enrichment in forsterite (Mg₂SiO₄)	Selective partial melting during impact	Taylor & 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
Phase	Duration after impact	Key Event
Debris Disk	0-10 years	Materials in chaotic orbit
Condensation	10-100 years	Solidification of vapors
Accretion	100-1000 years	Formation of the first planetesimals
Spherization	1000-10,000 years	Gravitational balancing
Differentiation	10,000-1M years	Formation of the mantle and crust