Last updated July 31, 2025

The Origin of the Moon: The Giant Impact Hypothesis

A titanic collision at the origin of our satellite

About 4.5 billion years ago, shortly after the formation of the Solar System, the primitive Earth collided with a Mars-sized body named Theia. This scenario, known as the Giant Impact Hypothesis, is today the most accepted explanation for the origin of the Moon. During this high-speed impact (≈ 10 km/s), an immense amount of material was ejected into Earth's orbit. Under the effect of tidal forces and gravitational aggregation, this material formed a debris disk that consolidated in less than 100 years to form our natural satellite.

Physical arguments in favor of the hypothesis

Particle hydrodynamic simulations link the oblique impact to the chemical composition of the Moon. The total angular momentum of the Earth-Moon system is also a direct consequence of this event. The oxygen isotopes present on Earth and the Moon are practically identical, suggesting a common origin or a deep mixing of materials. Furthermore, the low proportion of iron in the Moon compared to Earth indicates that it did not form from a differentiated body with its own metallic core.

Timeline of the giant impact

Key stages in the formation of the Moon
Stage	Estimated Duration	Dominant Physical Mechanism	Reference
Earth-Theia Impact	< 1 day	Oblique collision at ~10 km/s	Canup & Asphaug (2001)
Formation of the debris disk	A few hours	Evaporation + orbital ejection	Ward & Cameron (1976)
Accretion of the Moon	< 100 years	Gravitational regrouping	Ida et al. (1997)
Orbital stabilization	10⁵ – 10⁶ years	Earth-Moon tidal interactions	Touma & Wisdom (1994)

Sources: Canup R. & Asphaug E., *Nature*, 2001 – Ward W. & Cameron A.G.W., *Lunar Sci. Conf.*, 1976 – Ida S. et al., *Nature*, 1997 – Touma J. & Wisdom J., *AJ*, 1994.

A hypothesis still evolving

Although the giant impact hypothesis explains many orbital, chemical, and mechanical characteristics of the Earth-Moon system, it continues to be refined. Variants including multiple impacts or a different high-energy impactor are under study. The future of lunar missions, such as those of the Artemis program or the Japanese SELENE-2 mission, could provide additional data on the isotopic composition of the deep lunar crust, consolidating or revisiting this foundational cosmic scenario.