Origins of the Moon

The co-accretion or sister-world hypothesis suggests that the Earth and the Moon grew concurrently as a binary system from a shared feeding zone within the solar nebula. In this scenario, planetesimals and dust grains within the ancestral accretion disc were drawn simultaneously towards two distinct gravitational centres, yielding a secondary circumterrestrial debris disc from which the Moon coalesced.

While co-accretion intuitively accounts for the similarities in orbital plane alignment and gross positional stability, it struggles severely to explain the stark compositional divergence between the two bodies. The bulk density of the Moon is notably lower than that of the Earth, reflecting a severe deficiency in metallic iron. Earth possesses a massive metallic core constituting roughly one-third of its total mass, whereas the lunar core represents less than two to three percent of its mass.

If both bodies accumulated unselectively from the same local reservoir of nebular solids, they should have acquired comparable proportions of metallic and silicate fractions. To salvage the co-accretion framework, theorists have suggested complex aerodynamic sorting mechanisms, wherein fragile silicate aggregates were preferentially captured by the proto-lunar swarm while denser iron-rich planetesimals plunged into the growing Earth. However, hydrodynamical simulations demonstrate that gas drag alone is insufficient to produce the pronounced level of compositional differentiation observed between the two bodies.

Proposed in the late nineteenth century, the centrifugal fission model argued that the Moon originated from the rapid rotational shedding of the young Earth's outer mantle. According to this concept, the rapidly spinning, molten proto-Earth became dynamically unstable as its core differentiated, flattening into an oblate spheroid until a massive bulge detached from the equatorial region, ultimately forming the Moon.

While this mechanism offered an elegant explanation for the Moon's predominantly silicate composition and low iron content, it faces intractable mathematical hurdles regarding angular momentum. For centrifugal forces to overcome the self-gravity of a molten planetary body, the proto-Earth would have needed to complete a full rotation every two to three hours.

Such extreme rotational velocity implies a total system angular momentum vastly greater than the combined angular momentum observed in the modern Earth-Moon system today. Although tidal friction steadily transfers angular momentum from Earth's rotation to lunar orbital expansion, the total angular momentum of an isolated system must remain conserved. To account for this discrepancy, proponents must invoke external gravitational resonances, such as solar-evoked evection resonances, to drain the excess rotational energy. However, detailed numerical models indicate that such resonances cannot remove sufficient angular momentum within the brief thermal timescales available before the planet solidified, rendering classic fission physically implausible.

Rather than invoking a single colossal collision, the multiple-impact hypothesis proposes that the Moon was assembled from the debris of roughly twenty smaller planetesimal strikes. In this model, early protoplanets with masses comparable to the Moon or Mars repeatedly struck the nascent Earth over tens of millions of years. Each individual collision ejected a fraction of terrestrial and impactor mantle into orbit, forming a transient circumterrestrial debris ring.

Over relatively short timescales of hundreds of years, each debris ring cooled, condensed, and accreted into a distinct moonlet. As subsequent impacts generated new debris rings further inward or outward, tidal interactions drove the older moonlets outward, where they gravitationally perturbed and eventually merged with newly formed moonlets. Through a continuous sequence of collisions, orbital migrations, and coalescences, a single large satellite was gradually assembled.

This multi-stage scenario offers distinct statistical advantages over single-event models. Under a single giant impact, reproducing the near-identical isotopic signatures of Earth and Moon requires very specific impactor trajectories or extraordinary mixing regimes. In contrast, multiple independent impacts naturally average out the distinct chemical signatures of various impactors. Because each collision draws substantial material from the Earth's mantle, the cumulative debris reservoir becomes progressively dominated by terrestrial composition, mitigating the need to rely on an isotopically unique impactor.

During the earliest phase of an oblique planetary collision, an extreme physical phenomenon known as hydrodynamic impact jetting occurs. When the leading margins of the impactor and the proto-Earth collide, the contact zone experiences shock pressures exceeding hundreds of gigapascals. This sudden compression heats the interfacial materials far beyond their boiling points, expelling a high-velocity spray of vaporised and molten rock before the main bulk of the impactor is decelerated.

Because jetting occurs exclusively at the immediate contact interface between the colliding surfaces, it samples specific geographical and structural layers of the colliding bodies. In an oblique impact, the outer silicate mantles collide first, meaning the initial jet is composed almost purely of silicate vapour and melt, completely devoid of the denser iron cores situated deeper within the bodies. The velocity of this jetted material often exceeds the escape velocity of the local shock zone, allowing it to enter stable orbit without incorporating core metal.

This hydrodynamic ejection mechanism provides an essential explanation for why lunar material lacks dense metallic components while retaining volatile-depleted silicates. Subsequent condensation of this high-temperature plume in orbit allows silicate droplets to aggregate into clumps while highly volatile elements remain in the vapour phase and escape into space. Consequently, jetting simultaneously explains the absence of a large lunar metallic core and the characteristic depletion of volatile elements in lunar rocks.