The Moon was formed through a giant impact between early Earth and an astronomical body approximately the size of Mars.

Some have argued that the angular momentum constraints of the Earth-Moon System and the chemical composition of the Moon can be explained by a single giant impact event.
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The moon was formed through alternative mechanisms and not by a giant impact between early Earth and an astronomical body approximately the size of Mars.

Others have argued that the angular momentum constraints of the Earth-Moon system and the chemical composition of the Moon are better explained by alternative lunar origin hypotheses.
Created at: 
2020-04-15
  Updated at: 
2020-04-20
Curator:
Katie Koube   Subscribe ...

Background

The presence of volatiles on the Moon

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The presence of volatiles, such as water on the Moon is highly improbable if the Moon did undergo a giant impact event.

eWater is an extremely volatile element which is present and indigenous to the lunar interior.

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When comparing basalts, the Moon’s volatile content is more similar to the howardite-eucrite-diogenite (HED) class of asteroids than it is to Earth’s mantle.

eThe Moon and the HED class of asteroids experience similar volatile element depletion and fractionation of metallic and mafic phases.

Isotopic compositions on the Moon

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Heavy isotopes found on the Moon imply a history of evaporation, consistent with environments hypothesized for a giant impact.

eThe 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.

eHeavy 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.

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The oxygen isotopic compositions on the Moon and in the Earth’s mantle are very similar

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

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In order for the Moon to be isotopically similar to Earth after a giant impact, the Moon would have had to retain the majority of its original material during the impact event.

eThe titanium isotopic ratios between the Moon and Earth are virtually identical.

Angular Momentum Constraints of the Earth-Moon System

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Simulations show that the angular momentum of the Earth-Moon system can be generated by a giant impact.

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

eThe 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.

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Angular momentum of the Earth-Moon System can be adequately explained by alternative theories.

eThe origin of the Moon is better simulated by multiple medium sized impacts rather than one giant impact.