Unravelling the Chemistry of the Moon

AFor generations, planetary scientists struggled to construct a coherent model explaining how Earth acquired its disproportionately large natural satellite. In the late nineteenth and early twentieth centuries, three competing ideas dominated discussions. The fission hypothesis proposed that a rapidly spinning, molten infant Earth cast off a massive droplet of material from its equatorial region. Conversely, the capture theory held that the Moon formed independently elsewhere in the solar system before wandering too close to Earth and becoming ensnared by its gravitational pull. A third model, co-accretion, suggested that both bodies gathered mass side by side from the same primordial cloud of dust and gas. However, each concept suffered from fundamental dynamical flaws: fission required an unfeasibly fast initial spin, capture demanded an extraordinarily improbable orbital deceleration without collision, and co-accretion failed to clarify why the Moon possesses a significantly lower overall density and lacks a substantial metallic iron core compared to Earth.

BBy the mid-1970s, researchers sought a framework that could reconcile these orbital mechanics with structural differences. This gave rise to the giant impact hypothesis, which postulated that approximately 4.5 billion years ago, a celestial body roughly the size of Mars collided with the young Earth. In this scenario, the colossal impact shattered the impinging protoplanet and stripped away much of Earth's outer silicate crust. The resulting ring of vaporised and molten debris, predominantly composed of silicate material, orbited Earth briefly before coalescing into a single celestial body. Crucially, the metallic cores of both the impactor and proto-Earth were thought to have merged at the centre of our planet, providing a neat explanation for why the newly formed Moon was depleted of dense iron while retaining an orbital momentum that previous models could not account for.

CThis explanatory elegance, however, encountered serious difficulties when laboratory techniques advanced sufficiently to examine rocks retrieved from the lunar surface. Planetary geochemists measure the ratios of stable isotopes—such as those of oxygen, titanium, chromium, and silicon—to determine a celestial body's "fingerprint." Because different regions of the protoplanetary nebula possessed distinct compositions, meteorites and planets across the solar system generally exhibit unique isotopic ratios. Earth and Mars, for instance, display markedly different oxygen isotope profiles. When scientists analysed lunar rock specimens using ultra-precise mass spectrometry, they anticipated identifying the distinct geochemical signature of the vanished impactor. Surprisingly, the isotopic ratios of lunar and terrestrial rocks proved virtually indistinguishable across multiple elements, suggesting an origin from a shared or thoroughly unified reservoir of material.

DThis discovery presented planetary physicists with a formidable challenge known as the isotopic crisis. Classical computer simulations of the impact consistently indicated that between seventy and ninety per cent of the debris forming the lunar disc should have originated from the impactor rather than the target Earth. If the impactor arrived from a distant orbit, it should have carried a noticeably different isotopic composition, yet no such divergence is visible in the samples. To argue that the impactor coincidentally possessed an identical composition to proto-Earth was statistically implausible, given the wide variation found in other inner solar system bodies. Consequently, researchers were forced to reconsider whether their dynamic models of the collision were fundamentally incomplete.

EIn response to the crisis, several novel collision scenarios were formulated to explain how the material could have mixed so thoroughly. One hypothesis suggested that the impact was vastly more energetic than previously assumed, involving a much faster-moving or larger body that struck a rapidly rotating Earth. Such a high-energy event would have vaporised vast quantities of rock, creating a giant, donut-shaped cloud of molten and gaseous silicate known as a synestia. Within this vast, turbulent structure, intense convection could have completely homogenised the vaporised terrestrial and impactor matter over several centuries before the outer regions cooled and condensed into the Moon. This mechanism neatly accounts for the uniform isotopic signatures, although it relies on specific, extreme pre-impact conditions.

FAn alternative approach dispenses with the single cataclysmic strike altogether, proposing instead that Earth was struck by a succession of smaller protoplanetary bodies. Each moderate impact would have ejected enough material to generate a small ring of debris, which rapidly condensed into a "moonlet." Over millions of years, gravitational interactions would cause these successive moonlets to migrate outward and merge with one another, gradually constructing a full-sized Moon. Proponents of this multiple-impact theory argue that it significantly increases the likelihood of achieving isotopic similarity: because each collision ejects a mixture of terrestrial and impactor rock, the cumulative averaging across numerous independent impacts naturally brings the final bulk composition much closer to that of Earth than a single random event would.

GDespite these theoretical advances, the debate remains unresolved because current evidence relies primarily on rock collected from the Moon's outer crust and upper mantle. Some astrophysicists suggest that the isotopic homogeneity observed in Apollo-era samples might be a superficial feature resulting from late-stage surface mixing, rather than an accurate reflection of the deep lunar interior. Future exploration missions targeting deep impact basins, such as those near the lunar south pole where ancient sub-surface mantle material may lie exposed, could finally settle the question. If deep samples reveal subtle isotopic deviations from Earth's mantle, the classical impact model might be revived; if they remain identical, the case for total homogenisation will be substantially strengthened.