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.
Hypothesis | Period | Main Arguments | Major Problems | Main Proponents |
---|---|---|---|---|
Terrestrial Fission | 1878-1960 |
|
| George Darwin (son of Charles Darwin), Harold Jeffreys |
Gravitational Capture | 1909-1969 |
|
| Thomas Jefferson Jackson See, Gordon MacDonald |
Co-formation (Binary Accretion) | 1940-1969 |
|
| Carl Friedrich von Weizsäcker, Gerard Kuiper |
Precipitation (Condensation) | 1960-1969 |
|
| Harold Urey |
Source: NASA Historical Archives, Journal for the History of Astronomy (2004), Annual Review of Earth and Planetary Sciences (1975)
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.
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.
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)
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.
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.
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.
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 |
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