Giant-impact hypothesis

Based on Wikipedia: Giant-impact hypothesis

Four and a half billion years ago, something the size of Mars slammed into the young Earth so violently that it liquefied both worlds. The debris from that apocalyptic collision eventually coalesced into the Moon — the same Moon that pulls our tides, anchors our planet's tilt, and has shaped human consciousness since the first hominids looked up at the night sky.

This is not science fiction. It is the leading scientific explanation for how Earth got its companion.

The Problem with the Moon

The Moon is strange. Not strange in the way that Jupiter's volcanic moon Io is strange, or that Saturn's moon Titan with its methane lakes is strange. The Moon is strange because it exists at all.

Our Moon is enormous relative to Earth — about one-quarter of our planet's diameter. No other rocky planet in our solar system has anything comparable. Mercury and Venus have no moons whatsoever. Mars has two tiny captured asteroids, Phobos and Deimos, each only a few kilometers across. But Earth, a middling rocky planet in a middling solar system, somehow has this massive companion locked in orbital embrace.

And the weirdness doesn't stop at size. The Moon orbits Earth in almost the same plane as Earth's equator and spins in the same direction Earth rotates. When scientists examined rocks that Apollo astronauts brought back from the lunar surface, they found something remarkable: the isotopic signatures — essentially the chemical fingerprints that reveal where matter came from — were nearly identical to Earth rocks. This is extraordinarily unusual. Most objects in the solar system have distinct isotopic signatures, like accent markers revealing their different birthplaces in the primordial solar nebula.

The Moon also has an unusually small iron core for its size, giving it a much lower density than Earth. It lacks the volatile substances — elements and compounds that vaporize easily — that should be present if it formed the same way the inner planets did. And Earth itself spins with an anomalously high angular momentum, meaning our planet-Moon system contains far more rotational energy than it should compared to Mercury, Venus, or Mars.

For over a century, scientists struggled to explain these peculiarities.

Early Theories and Dead Ends

In 1898, the astronomer George Darwin — son of Charles Darwin — proposed that the Moon had literally spun off from a rapidly rotating early Earth, torn away by centrifugal forces like a glob of water flung from a spinning wet ball. This "fission hypothesis" had an elegant simplicity. It explained why the Moon's composition resembled Earth's so closely: they had once been the same body.

Darwin even calculated that the Moon had once orbited much closer to Earth and was slowly drifting away. He was right about that. We now know the Moon recedes from Earth by about 3.8 centimeters per year — roughly the rate your fingernails grow. We've confirmed this with extraordinary precision using laser reflectors that Apollo astronauts placed on the lunar surface; scientists bounce laser beams off these mirrors and measure the round-trip time to calculate the Moon's distance to within millimeters.

But Darwin's fission theory had a fatal flaw. When physicists worked through the mechanics, they found that no plausible spinning Earth could generate enough centrifugal force to throw off something the size of the Moon without the whole system flying apart. The math simply didn't work.

Two alternative hypotheses competed for attention. The "capture hypothesis" suggested that the Moon formed elsewhere in the solar system and was later captured by Earth's gravity. But capturing an object that large without it either crashing into Earth or flying off into space requires an implausibly precise set of conditions. The "co-formation hypothesis" proposed that Earth and Moon formed together from the same spinning disk of material. But this couldn't explain the Moon's missing iron or the isotopic similarities.

Each theory explained some observations while failing catastrophically on others.

The Canadian Geologist Who Changed Everything

In 1946, a Harvard geologist named Reginald Aldworth Daly published a paper that almost nobody noticed. Daly suggested that Darwin had been partly right — the Moon had indeed originated from Earth material — but wrong about the mechanism. What if, Daly proposed, something had hit Earth hard enough to blast that material into space?

The idea languished in obscurity for nearly three decades. Then, in 1974, during a conference on planetary satellites, two scientists named William Hartmann and Donald Davis resurrected it. They pointed out that the chaotic final stages of planet formation involved numerous collisions between large bodies. What if one of these collisions had been particularly dramatic?

Around the same time, Canadian astronomer Alastair Cameron and American astronomer William Ward developed a more detailed version of the hypothesis. They proposed that a body roughly the size of Mars had struck Earth at an oblique angle. The physics of such a collision would vaporize the outer rocky layers of both bodies while causing the iron cores to merge. This would send silicon-rich debris into orbit around Earth — debris that was depleted in iron because the iron had sunk into the combined core — which would eventually coalesce into a Moon.

The hypothesis explained, for the first time, nearly all the Moon's peculiarities at once.

The 1984 Conference

Science often advances through gradual accumulation of evidence, but occasionally a field reaches a tipping point. For lunar science, that moment came in October 1984, in Kona, Hawaii.

Eighteen months before the conference, organizers Bill Hartmann, Roger Phillips, and Jeff Taylor had issued a challenge to the lunar science community. "You have eighteen months," they declared. "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."

Before Kona, the scientific community was divided into warring camps, each defending their preferred hypothesis, plus a large middle ground of researchers who suspected the question might never be resolved.

After Kona, essentially two groups remained: those who accepted the giant impact hypothesis and those who weren't yet convinced but couldn't offer a better alternative. The scientific community had reached consensus with unusual speed.

Naming the Impactor

In 2000, English geochemist Alex Halliday gave the hypothetical impactor a name: Theia.

In Greek mythology, Theia was a Titan goddess who gave birth to Selene, the goddess of the Moon. The name was poetically perfect. Just as the mythical Theia gave birth to the Moon goddess, so the planetary body Theia gave birth to our actual Moon.

But where did Theia come from? According to current models, Theia was one of perhaps a dozen Mars-sized bodies that existed in the inner solar system during its early history. The leading hypothesis places Theia at one of Earth's Lagrange points — specifically the L4 or L5 points.

Lagrange points are positions in space where the gravitational pulls of two large bodies, in this case the Sun and Earth, combine in a way that allows a smaller object to maintain a stable position relative to both. The L4 and L5 points form equilateral triangles with Earth and the Sun, one leading Earth in its orbit by 60 degrees and one trailing by the same amount. Objects at these points tend to stay there, gently oscillating around the equilibrium position.

Jupiter's L4 and L5 points contain thousands of asteroids called Trojans. The L4 and L5 points of the Earth-Sun system contain at least one known asteroid. Four and a half billion years ago, they may have harbored something much larger.

Theia could have grown at one of these stable points for millions of years, slowly accumulating mass, until gravitational perturbations from other bodies destabilized its orbit. Once knocked loose, Theia would have begun a chaotic dance with Earth — an encounter that could only end one way.

The Collision

The impact happened approximately 4.4 to 4.45 billion years ago, roughly 100 million years after the solar system began to form. In cosmic terms, Earth was still a newborn.

Computer simulations suggest Theia struck Earth at an oblique angle of about 45 degrees, traveling somewhere around 9 kilometers per second at the moment of impact — that's about 20,000 miles per hour. For comparison, a bullet leaves a rifle at roughly 1 kilometer per second.

What happened next defies human imagination.

The collision released energy equivalent to billions of nuclear weapons detonating simultaneously. Both bodies effectively liquefied. Theia's iron core, denser than the surrounding rock, plunged through Earth's mantle and merged with Earth's own core, like a drop of honey sinking through water. The outer layers of both worlds — the rocky mantles and crusts — vaporized into an incandescent cloud of silicate gas and molten droplets.

A significant fraction of this material achieved escape velocity and was lost to space forever. But roughly 20 percent of Theia's original mass ended up in orbit around the newly enlarged Earth, forming a thick disk of debris that glowed white-hot against the blackness of space.

The collision fundamentally altered Earth. Our planet gained substantial mass and angular momentum. Whatever rotation Earth had before the impact was erased, replaced by a new spin that produced a day roughly five hours long. Earth's equator and the plane of the debris disk aligned, which is why the Moon's orbit today lies close to Earth's equatorial plane.

From Debris to Moon

The debris disk didn't become a Moon overnight. The process took centuries — maybe longer — and happened in stages.

First, material in the outer part of the disk, beyond what scientists call the Roche limit, began to clump together. The Roche limit is the distance within which a large satellite would be torn apart by tidal forces — the same forces that raise our ocean tides, but vastly more powerful when objects are closer together. Inside this limit, the debris remained a disk. Outside it, gravity could pull the material together into larger and larger bodies.

As the inner disk slowly spread outward due to internal friction and viscous forces, it pushed material beyond the Roche limit, feeding the growing Moon. Computer models suggest the Moon reached roughly its current mass within a few hundred years — a blink of an eye in geological terms.

But here's a fascinating twist: there may have been two Moons.

The Moon's far side — the hemisphere that permanently faces away from Earth — has a noticeably thicker crust than the near side. One explanation is that a second, smaller Moon, perhaps a thousand kilometers across, formed in one of the Moon's own Lagrange points and remained there for tens of millions of years. As both Moons migrated outward from Earth, solar gravity eventually destabilized the smaller companion's orbit, causing it to collide with the larger Moon in a slow-motion impact that "pancaked" its material onto what we now call the far side.

This would explain why the near side has thin crust punctuated by vast dark plains — the lunar maria, those "seas" visible to the naked eye — while the far side has thick highlands but almost no maria. Magma could punch through the thin crust of the near side to flood its giant impact basins, but couldn't penetrate the far side's thicker shell.

The Isotope Puzzle

The giant impact hypothesis elegantly explained most of the Moon's oddities. But by the early 2000s, researchers noticed a problem.

Remember those identical isotopic signatures? The chemical fingerprints showing that Earth and Moon rocks came from the same source? That's precisely what you'd expect if the Moon formed from Earth material. But the thing is, Theia wasn't Earth. Theia was a separate world that formed elsewhere in the solar system. Its isotopic signature should have been different.

Even if Theia formed nearby, computer simulations consistently showed that roughly 70 to 80 percent of the Moon should have come from the impactor, not from Earth. Yet the Apollo samples showed isotopic ratios identical to Earth rocks to an extraordinary degree of precision.

In 2007, researchers at the California Institute of Technology calculated that the probability of Theia randomly having the same isotopic signature as Earth was less than one percent. Something was wrong with the models.

Refinements and Revisions

Scientists proposed several solutions.

One possibility: the violence of the impact was so extreme that it completely homogenized the two bodies. The Caltech team suggested that in the aftermath of collision, while both Earth and the debris disk were still molten and partially vaporized, they may have been connected by a common atmosphere of silicate vapor. Over roughly a century, convective mixing might have erased the isotopic differences, blending Earth and proto-Moon material together like cream stirred into coffee.

Another possibility came from physicist Andreas Reufer and colleagues at the University of Bern in 2012. They proposed that Theia didn't strike Earth at an oblique angle but rather hit it head-on at higher velocity than previous models assumed. A direct hit would have pulverized Theia so completely that both bodies thoroughly mixed. This modification also allows for a wider range of compositions for Theia — including, intriguingly, up to 50 percent water ice.

A 2018 study proposed an even more dramatic scenario. What if the pre-impact Earth had been spinning extremely fast — so fast that it formed a bizarre, donut-shaped object that scientists named a "synestia"? This unstable structure, a kind of bloated, spinning disk of vaporized rock, would have existed for perhaps a century before cooling and condensing into the Earth-Moon system we know today. More material from the original Earth would end up in the Moon under this scenario, explaining the isotopic match.

A 2019 model suggested that the crucial factor was the state of matter. If early Earth was covered in a global ocean of magma when Theia struck — entirely plausible given the heat of planetary formation — then the liquid magma would have been heated far more efficiently than the solid rock of the impactor. This would eject proportionally more Earth material into orbit, potentially explaining why up to 80 percent of the Moon's material could have Earth's isotopic signature rather than Theia's.

Most recently, a 2022 study using high-resolution computer simulations found something surprising: under the right conditions, a giant impact can immediately form a satellite in a stable orbit beyond the Roche limit. The Moon might not have assembled gradually from a debris disk at all. It might have formed almost instantly, blasted into existence in a single cataclysmic moment, with its outer layers molten over a cooler interior and composed primarily of Earth material.

Evidence from Other Worlds

The giant impact hypothesis doesn't only apply to our Moon. Astronomers have found evidence of similar collisions around other stars.

Some young stellar systems show infrared signatures of debris disks — exactly what you'd expect to see in the aftermath of planetary collisions. These observations suggest that giant impacts were common during the chaotic period when rocky planets were still forming. Earth's collision with Theia wasn't a freak accident; it was a natural outcome of the violent process by which planets assemble themselves.

The leading theory of solar system formation holds that the inner rocky planets grew through a process of repeated collisions between smaller bodies called planetesimals. These smaller bodies merged into larger protoplanets, which continued to collide until only a handful of survivors remained: Mercury, Venus, Earth, and Mars. Earth is thought to have experienced dozens of significant collisions during its formation. The impact with Theia was merely the last major one.

What the Moon Made Possible

The Moon does far more than light up our nights. Its gravitational influence has shaped Earth in ways we're only beginning to fully appreciate.

The Moon stabilizes Earth's axial tilt at approximately 23.5 degrees. Without this stabilization, Earth's tilt would vary chaotically over millions of years, causing wild climate swings that might have prevented complex life from evolving. Mars, with its tiny moons unable to provide such stabilization, has experienced exactly this kind of chaotic axial wandering.

The Moon's gravity drives Earth's ocean tides, which create the intertidal zones — those unique ecosystems where life first crawled from sea to land. Some researchers have argued that tidal pools were crucial nurseries for the earliest life on Earth.

The Moon has been slowing Earth's rotation since the moment of its formation. Four billion years ago, Earth's day was only six hours long. The Moon's tidal drag has been gradually braking our planet ever since. In the distant future, Earth's day will continue to lengthen until our planet is tidally locked to the Moon, with one side permanently facing our companion — just as the Moon is already tidally locked to Earth.

And of course, the Moon has shaped human consciousness. Every culture has stories about the Moon. The word "month" derives from "Moon." Our earliest calendars tracked its phases. The rhythms of religious observance across countless traditions follow the lunar cycle. We measured time by the Moon long before we understood what it was.

What We Still Don't Know

Despite decades of research, no completely self-consistent model exists that starts with the giant impact and follows all the debris through to a single Moon. The basic story is almost certainly correct — the evidence is overwhelming — but the details remain contentious.

We don't know exactly how fast Earth was spinning before the impact, or precisely what angle Theia struck at, or how long the debris disk persisted before coalescing. We don't know for certain whether there was a second Moon that later merged with the first. We can't yet fully explain why the isotopic signatures match so precisely.

Future lunar missions may help resolve these questions. Samples from the Moon's far side, or from deeper below the surface than the Apollo missions reached, could reveal composition differences that constrain the models. Better computer simulations, running on more powerful hardware, will explore parameter spaces that previous studies couldn't reach.

But the fundamental insight stands. The Moon is not a captured wanderer or a sibling formed alongside Earth. It is a child of catastrophe — the offspring of a collision so violent it reshaped two worlds into one system. We live on a planet that was literally broken and remade, and we circle a Sun accompanied by the scar of that ancient trauma, transformed into a silver companion that has watched over life on Earth since before there was life to watch.

The next time you look up at the Moon, you're looking at what's left of Theia — and at a piece of the proto-Earth that was flung into space 4.4 billion years ago and never came back down.