The Giant Impact: How Our Moon Came to Be

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A titanic shock at the origin of our satellite

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

Physical arguments supporting the hypothesis

Hydrodynamic particle 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

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 by new simulations and isotopic analyses.

A formation in a few hours rather than a century?

In 2022, very high-resolution simulations conducted by Jacob Kegerreis's team (NASA Ames / Durham University) proposed an alternative scenario: the Moon would have formed in just a few hours, projected directly into orbit during the impact, rather than by progressive accretion of a debris disk over about 100 years. This "immediate" model would more naturally explain the nearly perfect isotopic resemblance between Earth and the Moon.

Theia, an origin now better constrained

In November 2025, a study published in Science by Timo Hopp and his team (Max Planck Institute for Solar System Research) measured iron isotopes in lunar rocks, terrestrial rocks, and meteorites to reconstruct Theia's origin: the impactor would have formed in the inner Solar System, probably in close proximity to Earth.

Towards new data with Artemis

Variants including multiple successive impacts are also being studied. Future lunar missions, such as the Artemis program, will provide additional data on the isotopic composition of the deep lunar crust, allowing to decide between these competing scenarios.

Why is the Moon essential to Earth's habitability?

The giant impact did not just create a satellite: it lastingly shaped the conditions that make Earth habitable. Three major consequences directly derive from this foundational event.

A stabilized axial tilt

Without the Moon, Earth's rotation axis would be unstable. The work of Jacques Laskar and his colleagues (Nature, 1993) showed that lunar attraction limits Earth's obliquity to a narrow range, between 22.1° and 24.5°. In the absence of the Moon, this obliquity could vary chaotically between 0° and 85°, causing radical climatic upheavals and making the development of a stable climate, conducive to complex life, much more uncertain.

A progressive slowing of Earth's rotation

Just after the impact, Earth rotated much faster than today: a day then lasted only a few hours. The tidal forces exerted by the Moon have continuously slowed Earth's rotation while transferring angular momentum to the lunar orbit, which has gradually widened. This process continues today: the Moon moves away from Earth by about 3.8 cm per year, precisely measured by laser telemetry on reflectors placed by the Apollo missions.

A driver for tidal cycles and the evolution of life

The tides generated by the Moon created intertidal zones marked by regular cycles, an environment that several scientific hypotheses associate with key steps in the evolution of life, notably the transition between marine and terrestrial environments. While this role remains debated, the climatic stability offered by the Moon is widely recognized as a determining factor in the long-term habitability of our planet.

The rival hypotheses, discarded before the giant impact

Before the giant impact hypothesis prevailed in the 1980s, three other scenarios long competed to explain the origin of the Moon. Each eventually fell short because it failed to resist the observed physical and chemical constraints.

The fission hypothesis

Proposed as early as 1879 by George Darwin (son of Charles Darwin), this hypothesis suggested that the Moon had detached from an early Earth spinning so rapidly that centrifugal force ejected part of its matter, leaving behind the Pacific Ocean basin. This scenario was abandoned because it requires an initial angular momentum much higher than that observed today in the Earth-Moon system, a physically untenable condition.

The capture hypothesis

According to this scenario, the Moon would have formed elsewhere in the Solar System before being captured by Earth's gravity during a close pass. However, stable gravitational capture is an extremely improbable event, requiring a very precise adjustment of speed and trajectory. This hypothesis also does not explain the isotopic identity almost perfect between Earth and the Moon, which presupposes a common origin.

The co-accretion hypothesis

This model proposed that Earth and the Moon formed simultaneously, side by side, from the same protoplanetary gas and dust cloud. However, it encounters a major problem: it cannot explain why the Moon has a proportionally much smaller metallic core than Earth, nor the high angular momentum of the current system.

Why the giant impact prevailed

Unlike these three scenarios, the giant impact hypothesis explains simultaneously the low iron content of the Moon, the isotopic identity of oxygen, and the angular momentum of the Earth-Moon system: it is this overall coherence, confirmed by hydrodynamic simulations and lunar samples brought back by the Apollo missions, that made it the current standard model.

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