The Moon formed through a cataclysmic collision in the early solar system, an event now widely accepted by the scientific community as the Giant Impact Hypothesis.wikipedia+1
The Giant Impact Event
Approximately 4.5 billion years ago, during the early Hadean eon of Earth's history, a Mars-sized protoplanet called Theia collided with the proto-Earth. This collision occurred roughly 20 to 100 million years after the solar system itself began forming. The impact was so tremendous that it vaporized significant amounts of rock and metal from both the proto-Earth and the impacting body. This tremendous energy re-melted the proto-Earth after it had already undergone initial differentiation into a rocky mantle and metallic core.news.uchicago+1
The collision ejected vast quantities of debris into orbit around Earth. Over time, this orbiting material cooled and gradually condensed, with particles clumping together through gravitational attraction. Eventually, this debris coalesced into the Moon as we know it today.universetoday
Evidence Supporting the Theory
Several compelling lines of evidence support the giant impact hypothesis:
Isotopic Composition: The Moon's oxygen isotopic ratios are nearly identical to Earth's, which is remarkably distinctive throughout the solar system. Research published in 2025 examining 14 lunar rock samples found similar levels of oxygen-17 isotopes between lunar and terrestrial materials, suggesting they shared a common source. The Moon's titanium isotope ratio also closely matches Earth's to within 4 parts per million, which would be extremely unlikely if the Moon had formed from a separate protoplanet with a distinct composition.wikipedia+2
Iron Deficiency: The Moon has significantly less iron than Earth, with a mean density of 3.3 times that of water compared to Earth's 5.5 times. The Moon's iron core must be small, with its radius comprising less than about 25% of the Moon's total radius, in contrast to about 50% for most other terrestrial bodies. This composition resembles Earth's rocky mantle rather than Earth's overall composition, suggesting the Moon formed primarily from mantle material ejected during the impact.astronomy+1
Zinc Isotopes: Moon rocks contain more heavy isotopes of zinc and overall less zinc than corresponding igneous rocks from Earth or Mars. This depletion is consistent with zinc being vaporized and lost during the giant impact event, as zinc becomes strongly fractionated when volatilized in planetary rocks but not during normal igneous processes.wikipedia
Energy and Melting: The anorthositic composition of the lunar crust and KREEP-rich samples indicate that a large portion of the Moon was once molten. The energy required to create such a magma ocean would have been readily supplied by a giant impact scenario.wikipedia
Refinements to the Theory
Recent research has refined understanding of how the collision produced such isotopically similar bodies. A 2025 study proposed that Theia may have lost its rocky mantle in earlier collisions before striking the proto-Earth. Under this scenario, Theia would have been essentially a metallic core by the time of impact, colliding with Earth "like a metallic cannonball". If this occurred, Theia's core would have accreted to Earth's core (explaining Earth's exceptional density), while the Moon formed almost entirely from ejected material from Earth's rocky mantle.skyatnightmagazine
Timing Uncertainties
While the general timeframe of Moon formation is well established, researchers continue refining the exact timing. Evidence suggests formation occurred around 60 to 175 million years after the solar system's birth, though estimates range from approximately 4.52 billion to 4.35 billion years ago. Dating methods using hafnium and tungsten isotopes prove particularly valuable, as hafnium decays to tungsten over about 10 million years, allowing scientists to determine when Earth's layers re-differentiated following the impact.news.uchicago+1
Legacy of Apollo Samples
The Apollo missions' return of nearly 900 pounds of lunar samples to Earth proved crucial to developing and validating the giant impact hypothesis. These samples, combined with lunar meteorites and increasingly sophisticated analytical techniques, provided the evidence base that led to the theory's broad acceptance at the 1984 "Origin of the Moon" international conference in Hawaii. Despite ongoing research into refinements and challenges like the "isotope crisis," the giant impact hypothesis remains the most widely accepted explanation for the Moon's formation.spj.science+2
- https://en.wikipedia.org/wiki/Giant-impact_hypothesis
- https://news.uchicago.edu/explainer/formation-earth-and-moon-explained
- https://www.universetoday.com/articles/the-moon-might-have-formed-earlier-than-we-thought
- https://en.wikipedia.org/wiki/Origin_of_the_Moon
- https://www.skyatnightmagazine.com/news/theia-moon-formation-theory
- https://www.astronomy.com/science/what-evidence-supports-the-collision-theory-for-the-formation-of-the-moon-and-what-alternate-theories-exist/
- https://spj.science.org/doi/10.34133/space.0153
- https://www.astronomy.com/science/giant-impact-hypothesis-an-evolving-legacy-of-apollo/
- https://www.esa.int/Education/Teach_with_the_Moon/What_is_the_origin_of_the_Moon
- https://www.lpi.usra.edu/education/explore/marvelMoon/background/moon-formation/
