The NEW SCIENCE of Moon Formation

Earth’s Moon is unusually large relative to Earth, has a surprisingly small iron core, and—most strikingly—shares oxygen isotope ratios with Earth’s crust. Those mismatches between “same building blocks” and “different internal structure” have pushed lunar-formation theories toward a single dominant explanation: a giant impact that both mixed Earth-like material and reshaped the newborn bodies.

Apollo-era measurements provide the clues. Seismometers deployed during Apollo landings recorded moonquakes, letting scientists infer the Moon’s interior. The results point to a small iron inner core—about 20% of the Moon’s diameter—surrounded by a frozen mantle. Meanwhile, Apollo samples brought back roughly half a ton of lunar material, including silicate rocks whose oxygen isotopes match Earth’s to within a few parts per million. In other words, the Moon’s chemistry looks Earthlike in isotopic fingerprints, yet its bulk structure—especially its low iron fraction—doesn’t.

Surface geology adds a timeline. Dark basaltic “mare” fill lowland craters, formed when magma erupted and froze. Lighter anorthosite rocks, rich in calcium, silicon, and oxygen, formed differently: they crystallized inside a long-lived magma ocean and floated upward as they solidified. Together, these layers imply the Moon spent tens to hundreds of millions of years covered by liquid magma, later punctuated by basalt eruptions and then billions of years of impacts.

Several formation routes struggle with the full set of constraints. Building the Moon in a circumplanetary disk alongside Earth would naturally produce shared isotopic ratios, but it would also tend to yield more similar overall elemental proportions and align Earth’s spin axis with the Moon’s orbital plane—yet those axes are misaligned. Capturing a pre-formed moon during close flybys is also difficult because slowing down an incoming body without it escaping requires conditions Earth simply doesn’t have, and capture via binary “exchange” is more plausible for other moons than for one as massive and distinctive as ours.

The giant impact hypothesis fits best. Two proto-planets—proto-Earth and a Mars-sized body called Theia—likely formed near the L4 or L5 Lagrange points. Around 100 million years into solar system history, Theia’s orbit destabilized and it drifted into Earth. The collision would have liquefied rock, sprayed lighter material into space, and left Earth with much of the iron—accounting for the Moon’s tiny core. Mixing during the impact explains why the Moon and Earth share the same oxygen isotope signature.

Recent high-resolution simulations sharpen the picture. A new set of hydrodynamic calculations by Jacob Kegerreis and collaborators at NASA Ames and Durham University used up to 100 million particles—about a thousand times more detailed than typical models. In a scenario with a near-escape-velocity collision at roughly a 45° angle, Theia is largely obliterated and debris strips Earth’s mantle. Crucially, the simulation briefly produces two moons for a few hours; one falls back while the other is pushed into a wider, stable orbit. Within under two days, the surviving satellite resembles the real Moon, with about 1% of Earth’s mass and an outer layer heated to roughly 4000 K—enough to sustain a magma ocean.

Uncertainty remains: the true masses, impact speed, and geometry are unknown, and even improved simulations rely on approximations. But as resolution rises and models add physics such as magnetic fields, researchers can narrow the range of viable impacts and test predictions—potentially even about the Moon’s interior—until the “cosmic cataclysm” story becomes more than a best fit.