Giant-impact hypothesis

• 1. Introduction

  ›

  The giant-impact hypothesis proposes that the Moon was born from catastrophe. About 4.5 billion years ago, a Mars-sized body slammed into a proto-Earth still taking shape in the early Hadean eon. The collision was so violent that enormous quantities of rock were flung into orbit, and that debris gradually assembled itself into the Moon now visible from every part of our planet.

  The impactor has a name: Theia, drawn from Greek mythology. Theia was the Titan mother of Selene, the goddess of the Moon, and the name fits the story almost too well. Without Theia's existence and its fateful trajectory, the night sky might look entirely different.

  This hypothesis is not ancient wisdom. It was first proposed in 1946 by Canadian geologist Reginald Daly, largely ignored for nearly three decades, then revived at a pivotal 1984 conference in Kona, Hawaii, where it won over most of the scientific community. Today it stands as the favored explanation for lunar formation among astronomers. What makes it compelling are the clues left in Moon rocks, the physics of planetary collisions, and a set of coincidences in the Earth-Moon system too striking to dismiss as chance.

• 2. A Long Road to Recognition

  ›

  In 1898, George Darwin put forward the idea that Earth and the Moon were once a single fused body. His proposal held that a molten Moon had been flung outward by centrifugal force, and it became the dominant academic explanation for lunar origin for decades. Darwin also calculated, using Newtonian mechanics, that the Moon had once orbited much closer to Earth and was slowly drifting away. American and Soviet experiments later confirmed that drift, using laser ranging targets left on the lunar surface.

  Darwin's mechanics could not, however, trace the Moon's path all the way back to Earth's surface. That gap opened space for Reginald Aldworth Daly of Harvard University, who in 1946 challenged Darwin's centrifugal model and proposed instead that an impact was responsible. For roughly three decades, Daly's idea attracted little serious attention.

  The turning point came from a pair of researchers working in the journal Icarus in 1975. William K. Hartmann and Donald R. Davis published models suggesting that at the end of the planet-formation period, several satellite-sized bodies had accumulated across the Solar System. One of those bodies, they proposed, may have struck Earth, ejecting volatile-poor dust that later coalesced into the Moon.

  Shortly before the field gathered formally, Alastair G. W. Cameron and William R. Ward contributed a complementary idea: that a Mars-sized body had struck Earth at a tangential angle, vaporizing its outer silicates while its metallic core did not vaporize. That distinction would matter enormously for understanding why the Moon contains so little iron.

  Eighteen months before an October 1984 conference on lunar origins, organizers Bill Hartmann, Roger Phillips, and Jeff Taylor issued a pointed challenge to colleagues: "You have eighteen months. Go back to your Apollo data, go back to your computer, and do whatever you have to, but make up your mind. Don't come to our conference unless you have something to say about the Moon's birth." At the Kona gathering, the giant-impact hypothesis emerged as the frontrunner. Before the conference, the field contained committed partisans of three older theories plus a large undecided middle. Afterward, essentially two camps remained: those who accepted the giant-impact model and those who simply called themselves agnostic.

• 3. Theia, the Wandering Planet

  ›

  Alex N. Halliday, an English geochemist, proposed the name Theia in 2000, and it has since been accepted across the scientific community. Theia is thought to have been part of a population of Mars-sized bodies present in the Solar System roughly 4.5 billion years ago. Its most likely home was the L4 or L5 Lagrange point of the Earth-Sun system, positions ahead of or behind Earth in its orbit where gravitational forces can keep an object relatively stable, much like the trojan asteroids that accompany Jupiter.

  In 2004, Princeton University mathematician Edward Belbruno and astrophysicist J. Richard Gott III proposed that Theia's trojan orbit would have become unstable once its growing mass exceeded roughly ten percent of Earth's mass, which is comparable to the mass of Mars. At that threshold, gravitational tugs from other small bodies would have nudged Theia off its stable path and set it on a collision course with proto-Earth.

  Theia's final approach was far from a simple straight-line crash. Computer simulations place the initial impactor velocity below 4 km/s at a large distance where gravity is not yet significant, accelerating to over 9.3 km/s at the moment of contact, at an impact angle of roughly 45 degrees. Some oxygen isotope data in lunar rock, however, hints at a steeper angle, suggesting "vigorous mixing" of the two bodies that a more glancing blow would not easily produce.

  A 2019 study from the University of Munster added a further wrinkle: the molybdenum isotopic composition found in Earth's primitive mantle points toward an origin in the outer Solar System. One reading of that data is that Theia itself formed far from the inner planets, potentially carrying water from the outer Solar System as part of its makeup.

• 4. How a Moon Is Made

  ›

  Astronomers estimate the collision with Theia occurred at roughly 4.4 to 4.45 billion years ago, approximately 0.1 billion years after the Solar System began forming. When Theia struck, its iron core most likely sank deep into the young Earth and merged with Earth's own core. Most of Theia's mantle then accreted onto Earth's mantle. Yet a significant portion of mantle material from both bodies was launched into orbit.

  Models suggest this orbiting debris then built the Moon in three consecutive phases. Material beyond Earth's Roche limit, the distance inside which tidal forces would tear apart a loosely bound object, began accreting immediately. Material inside the Roche limit slowly spread outward over time, pushing along outer bodies through resonant gravitational interactions. After several tens of years, the inner disk extended beyond the Roche limit and began producing new objects. The disk was depleted in mass after several hundreds of years. The whole process was rapid by geological standards.

  Computer estimates suggest roughly twenty percent of Theia's original mass ended up as an orbiting ring around Earth, and about half of that matter coalesced into the Moon. Earth itself gained substantial angular momentum and mass from the impact. Whatever Earth's rotation had been before, the collision left it spinning with a day only about five hours long. Earth's equator and the Moon's orbit would have aligned as a consequence.

  A 2022 study found that at sufficiently high simulation resolution, a giant impact can place a satellite with mass and iron content similar to the Moon directly into orbit well beyond Earth's Roche limit in a single stage. Even objects that initially pass inside the Roche limit can survive by being partially stripped and then gravitationally nudged onto wider, stable orbits. The outer layers of these directly formed satellites, the models show, are molten over cooler interiors, composed of around sixty percent proto-Earth material. That result may help reconcile the Moon's Earth-like chemical fingerprint with what would be expected from an impactor of different origin.

• 5. The Evidence in Moon Rocks

  ›

  In 2001, a team at the Carnegie Institution of Washington analyzed rocks returned by the Apollo program and found their isotopic signature was identical to rocks from Earth, distinct from almost all other bodies in the Solar System. That near-perfect match is hard to explain if the Moon formed independently elsewhere.

  In 2014, a team in Germany reported that Apollo samples did carry a slightly different isotopic signature from Earth rocks. The difference was small but statistically significant. One interpretation is that Theia formed in the same general neighborhood as Earth, producing a composition close but not identical to our planet's.

  Zinc offers another line of evidence. Zinc is strongly fractionated when it vaporizes under extreme heat, but it behaves differently during ordinary igneous processes. Moon rocks contain more heavy zinc isotopes and less zinc overall than equivalent Earth or Mars rocks, consistent with zinc being driven off the Moon's surface by evaporation during a high-energy impact.

  The Moon's iron core is small. Evidence from mean density, moment of inertia, rotational characteristics, and magnetic induction response all suggest the core's radius is less than about twenty-five percent of the Moon's radius, compared to roughly fifty percent for most other terrestrial bodies. Giant-impact models predict exactly this: the impactor's iron core would have sunk into Earth rather than remaining in the material that became the Moon.

  Separately, rocks collected during the Apollo landings show that a large portion of the Moon was once molten. The high concentration of anorthosite in the lunar crust and the presence of KREEP-rich samples both point to a widespread magma ocean early in the Moon's history. Earth's gravity alone could not have melted the Moon so thoroughly; an extremely energetic event like a giant impact could supply that heat. Analysis of stony meteorites bearing impact heating signatures from debris that escaped Earth's gravity has been used to date the impact event to 4.47 billion years ago, consistent with estimates from other methods.

• 6. Rival Explanations and Remaining Puzzles

  ›

  The three traditional alternatives to the giant-impact hypothesis each carry a fatal flaw. The centrifugal spin-off model, the gravitational capture theory, and the co-accretion model, in which Earth and the Moon formed simultaneously from the same disk, all struggle to account for the anomalously high angular momentum of the Earth-Moon system. A giant impact can deliver that momentum; none of the alternatives can do so convincingly.

  The giant-impact hypothesis has its own unresolved difficulties. The ratios of volatile elements on the Moon are not explained by current models, and the presence of volatiles such as water trapped in lunar basalts and carbon emissions from the lunar surface are hard to reconcile with a high-temperature impact that should have driven them away. The iron oxide content of the Moon sits at thirteen percent, intermediate between Mars at eighteen percent and Earth's mantle at eight percent, which rules out most scenarios in which the proto-lunar material came primarily from Earth's mantle. If instead most material came from an impactor, the Moon should be enriched in siderophilic elements, those with an affinity for iron, yet it is deficient in them.

  The Moon's oxygen isotope ratios are essentially identical to Earth's, which creates a paradox. Every Solar System body studied so far carries a distinctive oxygen isotopic signature. If Theia had formed far from Earth, it would likely carry a different signature, and the impact ejecta ought to reflect that difference. The titanium isotope ratio of the Moon is similarly close to Earth's, within four parts per million, suggesting very little of the Moon's mass could have come from a body with a different origin.

  Researchers at the California Institute of Technology calculated in 2007 that the probability of Theia having an isotopic signature identical to Earth's was less than one percent. Their proposed resolution is that after the impact, when both Earth and the proto-lunar disk were molten and partly vaporized, a common silicate vapor atmosphere connected the two reservoirs and homogenized their compositions through convective stirring over roughly a hundred years. Whether a proto-lunar disk can survive intact for that long remains an open question.

  A 2012 simulation by physicist Andreas Reufer and colleagues at the University of Bern proposed a direct hit rather than a glancing blow, at higher velocity, possibly destroying Theia entirely and allowing a composition of up to fifty percent water ice. A 2018 model introduced the concept of a "synestia," a rapidly spinning doughnut-shaped body that existed for about a century before cooling into Earth and Moon. A 2019 model suggested Earth was covered by a magma ocean at the time of impact while the impactor was solid, leading to about eighty percent of Moon-forming debris originating from proto-Earth, the reverse of what many earlier models predicted.

  On the 1st of November 2023, scientists reported that computer simulations suggested remnants of Theia may still be detectable inside Earth today as two large anomalies in the deep mantle, physical traces of a collision that happened before any life existed on this planet.