Origins of the Moon

Analysis of lunar highland samples revealed a crust dominated by anorthosite, a low-density rock rich in plagioclase feldspar. Geochemists interpret this widespread uniform lithology as evidence that the young Moon was once enveloped by a global magma ocean tens of kilometres deep. As this silicate melt cooled and crystallised, heavier minerals such as olivine and pyroxene settled towards the lunar interior, forming the mantle. In contrast, buoyant plagioclase crystals floated upwards, accumulating at the surface to create an initial primordial crust. This differentiation process confirms that intense initial heating accompanied the Moon's early accretion history.

A persistent conundrum in lunar science is the isotopic similarity between terrestrial and lunar rocks. Oxygen isotope ratios typically serve as a distinct planetary fingerprint, varying markedly across different regions of the early solar system. Standard collision simulations suggest that the Moon should have formed predominantly from the impactor's mantle, meaning its isotopic profile ought to diverge substantially from Earth's. Yet, highly precise measurements reveal identical oxygen isotope signatures. Researchers infer that either the collision triggered vigorous turbulent mixing that thoroughly homogenised the post-impact disk, or the colliding bodies originated from the same localised orbital reservoir.

To resolve discrepancies in compositional mixing, planetary scientists proposed that the lunar-forming impact produced a synestia: a rapidly spinning, doughnut-shaped structure of vaporised rock extending far beyond conventional planetary boundaries. Unlike a discrete planet encircled by a thin debris ring, a synestia represents a continuous, highly turbulent vapour cloud wherein the proto-Earth and impactor thoroughly mingled. As the outer envelope radiated heat into space, droplets of molten silicate condensed and collected into a central moonlet, which orbited within the vapour atmosphere before emerging. This novel framework provides a robust mechanism for the observed isotopic equilibrium between Earth and Moon.

Before the Apollo missions, three primary hypotheses competed to explain lunar origins: capture, co-accretion, and fission. The capture theory posited that Earth gravitationally ensnared a fully formed Moon passing nearby, but orbital mechanics render such an event statistically improbable without drastic energy dissipation. The co-accretion model suggested simultaneous growth from a shared gas cloud, yet failed to clarify the stark difference in iron abundance between the bodies. The fission hypothesis proposed that a rapidly spinning proto-Earth cast off a molten blob that became the Moon, though the system lacks sufficient angular momentum to support this mechanism.