The formation of our Solar System was a violent and chaotic sequence of collisions and mergers. While the planets were still forming from the primordial disk of gas and dust, massive impacts were commonplace. The origin of Earth’s large, singular Moon has long been a mystery, but the prevailing scientific explanation links its creation to a single, cataclysmic event in the planet’s infancy. This hypothesis suggests that a massive, ancient protoplanet slammed into the young Earth, instantly ejecting a cloud of superheated material that eventually coalesced to become our satellite.
The Giant Impact Hypothesis and the Impactor Theia
The leading scientific model explaining the Moon’s origin is known as the Giant Impact Hypothesis. This theory identifies the colliding body as a hypothetical protoplanet named Theia, which was roughly the size of Mars. The name Theia comes from Greek mythology, where she was the mother of Selene, the goddess of the Moon. The collision involved an early version of Earth, often called proto-Earth, and Theia, which was likely following a similar orbit around the Sun.
Computer simulations suggest the impact was not a head-on collision, but rather a high-speed, oblique, or glancing blow. This angle was necessary to eject substantial material into orbit without completely destroying Earth. The immense energy of the collision vaporized much of the outer layers of both Theia and the proto-Earth’s mantle. This superheated material, composed primarily of silicate rock, was flung into a vast orbit. Theia’s dense iron core is thought to have merged almost entirely with Earth’s core upon impact, explaining why the resulting debris cloud, and thus the Moon, is largely iron-deficient.
The Timing and Mechanics of the Collision
The violent impact that created the Moon occurred approximately 4.5 billion years ago, placing the event squarely in the Solar System’s earliest epoch, the Hadean Eon. This timeframe is only about 20 to 100 million years after the initial formation of the Sun and the rest of the planets. The impacter, Theia, struck the proto-Earth at a relatively high velocity, estimated to be around 10 kilometers per second.
The oblique angle of the impact was mechanically necessary because a direct, head-on collision would have likely resulted in the complete fragmentation of both bodies. The grazing blow caused the outer layers of both planets to be stripped away and vaporized, leaving the proto-Earth intact. The sheer kinetic energy released would have instantaneously melted Earth’s surface, turning the entire planet into a swirling globe covered by a deep magma ocean. This intense heat ensured that the ejected material was highly processed and depleted of lighter, more volatile compounds before it began to coalesce.
Physical Evidence Supporting the Theory
The most persuasive scientific support for the Giant Impact Hypothesis comes from the analysis of lunar rock samples returned by the Apollo missions. Scientists focused on the isotopic composition of these rocks, which provides a unique chemical fingerprint of their origin. The ratio of oxygen isotopes (oxygen-16 and oxygen-17) is a distinctive marker that varies across different bodies in the Solar System.
Lunar rocks and Earth rocks exhibit oxygen isotope ratios that are nearly indistinguishable. This similarity is difficult to explain unless the two bodies share a common source material. This finding suggests a vigorous mixing and homogenization of the material from both the proto-Earth and Theia during the impact, leading to a Moon made of a blend of both planets.
The Moon is also depleted in volatile elements, such as hydrogen and nitrogen compounds, compared to Earth. This lack of volatile material is consistent with a formation scenario involving extreme heat and vaporization, where lighter elements were boiled away and lost to space. The Moon’s overall low density further supports the idea that it formed primarily from the iron-poor, silicate-rich mantle material ejected during the collision. Its small metallic core makes up only about 1 to 3 percent of its total mass.
The Resulting Structure of the Earth-Moon System
Gravity caused the ejected material to quickly accrete, forming the Moon within a relatively short period, perhaps less than a century. The orbital arrangement of the final Earth-Moon system provides further constraints that align with the Giant Impact Hypothesis. One of the system’s most defining features is its anomalously high angular momentum, which represents the total rotational energy of the two bodies.
A large, off-center impact is an effective mechanism for imparting this massive amount of spin and orbital energy, which cannot be easily explained by other formation theories like capture or co-accretion. The Moon’s orbital plane is also tilted significantly relative to Earth’s equator, a natural consequence of the off-axis angle of the initial collision. Over the eons, intense tidal interactions have caused the Moon to slowly spiral outward. The Moon continues to recede from Earth today at a rate of approximately 3.8 centimeters per year.