Evidence

The Moon is enriched with heavy isotopes of potassium compared to the Earth and chondritic meteorites, implying the Moon formed in a high-pressure high temperature environment where light isotopes evaporate faster than heavy ones (Rayleigh Distillation) during giant impact.

The Moon is substantially depleted in volatile elements such as potassium compared to the Earth, chondritic meteorites, and the bulk solar composition. The potassium that is present on the Moon is isotopically heavier than what is found on chondrites and on Earth. The coupled nature of potassium's chemical depletion and isotopic fractionation provides constraints on the nature of such depletion and supports the giant impact theory for lunar formation. Fractionation of potassium isotopes is typically observed in high temperature environments and is caused by preferential evaporation during Rayleigh distillation. However, the mass difference between the two isotopes of potassium (41K and 39K) would imply extreme enrichment of δ41K which does not fit observation. A high-pressure environment, such as an impact event, is postulated to suppress isotope fractionation and could lead to the more moderate enrichment observed. The enrichment observed is consistent with the high-pressure environment that would have been present during a giant impact event. If the moon had been generated by several smaller impacts these higher pressures would never have been reached and a higher enrichment of δ41K would be observed.

Authors

Wang, K. & Jacobsen, S. B.

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Evidence

Heavy zinc isotopes exhibit strong isotropic fractionation during volatilization of rocks, such as vaporization through an impact event, but are minimally fractionated during igneous processes, such as volcanism. The Moon is enriched with heavy isotopes of zinc when compared to terrestrial sources and Martian meteorites, implying a ‘planet-scale’ evaporation event consistent with giant impact theory.

Zinc is known to exhibit strong isotropic fractionation during volatilization of rocks but is minimally fractionated during igneous processes, making it an ideal tracer material for the Moon’s volatile history. The Moon is substantially depleted in volatile elements such as zinc compared to the Earth and Martian meteorites. The zinc that is present on the Moon shows isotropic fractionation consistent with preferential evaporation of light isotopes. This process is referred to as Rayleigh distillation. This fractionation is homogenous across several different lunar samples including both high-titanium and low-titanium mare basalts. The combination of zinc depletion, isotropic fractionation, and chemical homogeneity across lunar samples strongly suggest a “planet-scale” evaporation event consistent with giant impact theory, rather than localized evaporation processes (e.g. a volcanic eruption into space).

Authors

Paniello, R. C., Day, J. M. D. & Moynier, F.

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Evidence

The oxygen isotopic composition of lunar samples is similar to those found in the terrestrial mantle.

The study measured the oxygen isotopic composition of 31 lunar samples. δ17O isotopes plot along the Terrestrial Fractionation Line (TFL), a line which predicts the isotopic fractionation oxygen on Earth. Similarly, δ18O isotopes from lunar samples are virtually identical to the terrestrial mantle within measurement error. Both the Moon and Earth are more enriched in δ18O than Mars or chondrites. This implies that the Moon and Earth formed at a similar distance from the sun, meaning that Theia, the impactor involved in the giant impact event, would have had a relatively small impact velocity when it reached proto-Earth. The similar δ18O isotopes imply that Theia and proto-Earth formed at a similar distance from the Sun; thus their chemical components would have been similar, and the composition of other isotope ratios should be similar. Since several volatile elements show isotropic fractionation and depletion, this implies a secondary event, such as the giant impact, must have occurred.

Authors

Wiechert, U. et al.

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Evidence

Simulations show that there are tenable impact scenarios which yield an angular momentum similar to the one observed by the Earth-Moon system.

Simulations are critical to verifying and exploring the parameters for the giant impact theory. Smoothed particle hydrodynamics simulations can be used to predict different impact outcomes for an impactor – proto-Earth collision. The models presented in this publication show that several conditions must be met for a single-impact hypothesis to be plausible via formation of a pre-lunar accretion disc: the relative velocity must be small (>5 km), the angle of impact not too small or large, and the mass ratio of the impactor to proto-Earth must be greater than 0.1. Once a pre-lunar accretion disc is formed, the physics of planetesimals can progress and the Moon can form. These simulations accurately predict the large specific angular momentum of the Earth-Moon system and accurately predict the Moon’s relatively small iron core.

Authors

Benz, W., Slattery, W. L. & Cameron, A. G. W.

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Evidence

The unique angular momentum observed in the Earth-Moon system could be resolved explained if a giant impact between a proto-Earth and Mars sized impactor was responsible for the formation of the Moon.

When the Moon was close to the Earth, most of the angular momentum resided in the spin of the Earth. The giant impact theory adequately describes the angular momentum for the Earth-Moon system. A collision of a Mars-sized object with a proto-Earth would result in an outward transport of angular momentum and an inward transport of mass. Some particles were ejected past the distance where the gradient of gravitational forces from Earth, called tidal forces, would have torn them apart. The boundary which defines the safe distance of a particle from tidal forces is defined as the Roche limit. For those particles outside the Roche Limit, collective gravitational instability would result in the clumping of particles and the formation of the Moon. The composition of the Moon reflects the two-stage processing hypothesized here. First, the Moon underwent chemical differentiation as a colliding body. Second, the Moon underwent volatilization of the mantle and crust material through evaporation which leads to the selective retention of refractory materials. This two-stage process predicts a moon deficient in metallic iron and volatile elements, and moderately enriched in crustal materials like calcium and aluminum.

Authors

Cameron A.G.W, Ward W.R.

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