Given that the Earth and Moon are often said in the literature to have very similar geochemistry, what is the current theory as to how the Earth's Moon formed?


2 Answers 2


The current accepted theory is known as the Giant Impact Hypothesis, where according to this NASA webpage "Origin of the Earth and Moon" (Taylor) a Mars sized object collided into the early Earth.

This theory allows explanations of (from the link above):

The chemical makeup:

The giant impact hypothesis is consistent with our ideas for how planets were assembled and explains some important features of the Earth-Moon system, such as why the Moon has only a tiny metallic core.

and in terms of orbits:

To account for the amount of angular momentum in the Earth-Moon system, Cameron estimated that the object would need to be about 10% the mass of Earth, about the size of Mars. (Angular momentum is the measure of motion of objects in curved paths, including both rotation and orbital motion. For the Earth and Moon this means the spin of each planet plus the orbital motion of the Moon around the Earth.)


Image source

However, recently, there has been some revisions to this theory.

According to NASA's page "NASA Lunar Scientists Develop New Theory on Earth and Moon Formation", based on concerns that the Mars-sized would likely to have had a different composition to the Earth (inconsistent with the current similarities in geochemistry).

The new hypothesis is (from the link above):

After colliding, the two similar-sized bodies then re-collided, forming an early Earth surrounded by a disk of material that combined to form the moon. The re-collision and subsequent merger left the two bodies with the similar chemical compositions seen today.

This is also discussed in "Huge Moon-Forming Collision Theory Gets New Spin" (Wall, 2012), basing the revised hypothesis on the rotation rate, which is theorised to have been very fast, from the article:

Earth's day had been just two to three hours long at the time of the impact, Cuk and Stewart calculate, the planet could well have thrown off enough material to form the moon (which is 1.2 percent as massive as Earth).

Further discussion, based on the geochemical makeup is discussed in "Geochemical Constraints on the Origin of the Earth and Moon" (Jones and Palme) conclude that:

Although none of these observations actually disproves the giant impact hypothesis, we find it disquieting that the obvious consequences expected of a giant impact are not observed to be fulfilled.

So, there is still some question as to precisely how the Moon was formed.


A recent study reported in the Phys.org article "Study crashes main Moon-formation theory" , based on the Nature Geoscience paper "A multiple-impact origin for the Moon" performed numerical modelling that suggests that

the Moon could instead be the product of a succession of a variety of smaller collisions.

The specific strength of such a theory is that "Numerous "impactors" would have excavated more Earth material than a single one" according to the authors, allowing for a stronger composition-based correlation with the Earth.

The authors also contend that

sub-lunar moonlets are a common result of impacts expected onto the proto-Earth in the early Solar System and find that the planetary rotation is limited by impact angular momentum drain.

The process is simplified as follows (from the article):

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An interesting consequence of this model, according to the authors is that:

"Building the Moon in this way takes many millions of years, implying that the Moon's formation overlapped with a considerable portion of Earth's growth,"


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