The theoretical planet that impacted Earth to form the Moon is called Theia, a Mars-sized protoplanet from the early Solar System. This colossal event, which occurred approximately 4.5 billion years ago, is the foundation of the Giant Impact Hypothesis, the leading scientific explanation for the Moon’s origin. The theory suggests the collision was a massive, glancing blow that ejected enormous amounts of material into orbit around the proto-Earth. This catastrophic event explains many of the Moon’s unique physical and chemical characteristics.
The Giant Impact Hypothesis
The proposed collision took place during the Hadean Eon, only about 20 to 100 million years after the formation of the Solar System. Theia is estimated to have been roughly the size of Mars, which translates to a mass between 10% and 45% of Earth’s current mass. It is believed that Theia was a co-orbital body, likely situated in one of the Earth–Sun system’s stable Lagrange points before its orbit was destabilized.
The impact itself was a high-velocity, oblique strike, often modeled at an angle of around 45 degrees. This angle was crucial, as a head-on impact would likely have vaporized both bodies entirely. The immense energy released instantly vaporized much of Theia’s mantle and a significant portion of Earth’s outer layers.
Theia’s metallic core is thought to have merged with Earth’s core, while the lighter, silicate-rich material was blasted into space. This material, a mixture of vaporized rock and molten fragments from both Theia’s and Earth’s mantles, was ejected into orbit. The scale of the event ensured that a substantial amount of material was placed into a ring around Earth.
The Immediate Result The Birth of the Moon
The superheated, vaporized material that formed the debris ring rapidly expanded into a massive, gaseous cloud around the Earth. This cloud, consisting of molten rock droplets and silicate vapor, quickly began to cool and condense. The material needed to be outside of Earth’s Roche limit, where tidal forces would prevent accretion, to successfully clump together.
Gravity acted quickly on the orbiting debris, causing the particles to collide and stick together in a process called accretion. Within hundreds to thousands of years, the material outside the Roche limit coalesced to form a single, massive satellite. Early simulations suggest the Moon may have assembled in phases, starting with a “parent body” that then accreted smaller moonlets.
The Moon that resulted from this rapid formation process possessed unique characteristics explained by the high-energy impact. The intense heat of the collision caused a significant loss of volatile elements, such as water and certain gases, which evaporated and escaped into space. This volatile depletion is why the Moon is remarkably dry compared to Earth.
The Moon has a relatively small iron core, making up only a few percent of its total mass, compared to Earth’s core, which accounts for about 30%. This small core is a direct consequence of the formation mechanism. The Moon was assembled primarily from the lighter, iron-depleted silicate material of the merged mantles, leaving it with a lower overall density than Earth.
Supporting Scientific Evidence
The Giant Impact Hypothesis is the most widely accepted theory because it accounts for several specific observations about the Earth-Moon system. A crucial piece of evidence is the remarkably high angular momentum of the system. The oblique nature of the impact imparted a massive spin to the proto-Earth, which was then transferred to the Moon’s orbit through tidal interactions over billions of years.
Analysis of lunar samples returned by the Apollo missions provides compelling chemical support. Scientists found a striking similarity in the stable isotope ratios of oxygen, titanium, and chromium between lunar rocks and Earth’s mantle. Since most other Solar System bodies possess distinct isotopic “fingerprints,” this similarity strongly suggests that the Moon formed from material that was thoroughly mixed with Earth’s own silicates.
The low density and small iron core of the Moon align with the impact scenario. Since the Moon formed primarily from the ejected, iron-poor silicate mantles, its bulk composition lacks the heavy iron found in the cores of both the proto-Earth and Theia. The impact model explains why the Moon is essentially a composite of the outer layers of two planetary bodies.
This body of evidence—including the system’s rotation, the isotopic match, and the Moon’s iron-depleted structure—provides a cohesive narrative for the Moon’s origin. While variations on the hypothesis exist to address subtle discrepancies, the core concept of a Mars-sized impactor named Theia remains the leading explanation.