Origin of the Moon

Over the years, there have been various hypotheses about the origin of the Moon. Several of them were formed before scientists had a chance to study Moon samples, and the eventual analysis (as well as calculations regarding things like energy and angular momentum) of such samples spoke against many of these old approaches.

The "Big Three"

Prior to the lunar landings, there had been three major hypotheses that all had been considered valid explanations for the origin of the Moon.

Hypothesis 1: Fission

Developed in 1878, this hypothesis by George Howard Darwin (son of Charles Darwin) argued that the Moon had been a part of a rapidly spinning Earth. This rapid spin, enhanced by the Sun's gravitational influence, led to a piece of the Earth breaking free and forming the Moon. Osmond Fisher held that the Pacific ocean basin is actually the scar of this process.

Hypothesis 2: Capture

In 1909, Thomas Jefferson Jackson See proposed that the Moon was actually a planet that had been created elsewhere and that had been caught by Earth's gravity after being slowed down by debris.

Hypothesis 3: Coaccretion

Many scientists, including Édouard Roche, supported the hypothesis that the Earth and the Moon formed side by side from the material that formed all planets of this solar system. The name comes from the term "accretion disc", which is a disc formed by diffuse material that orbits a central body.

Observations and calculations

For a long time, these three models had been accepted as describing possible lunar origins. However, later research was to produce evidence that worked against them. After the Apollo program, rock samples taken from the Moon revealed two important things:

1. The Moon lacks iron. This is important to note because the Earth, in comparison, has a lot of it (in the core).
2. The Moon and the Earth have exactly the same oxygen isotope composition.

Along with calculations in the field of energy and angular momentum, these two points spoke against the "Big Three" hypotheses as follows:

Consequences for the Fission Hypothesis

Since the material that spun off the Earth would have come from the mantle, the lack of iron would not have been a problem (Earth's iron drained towards the core early on, leaving a depleted mantle). This would also explain why the Moon has such a small core. For obvious reasons, the theory also covers the issue of the oxygen isotope composition.

However, the total angular momentum today is too small to back this hypothesis.

Consequences for the Capture Hypothesis

Even ignoring the very small chance of a planet easing into an Earth orbit like our Moon did, this hypothesis fails to explain the oxygen isotope similarity and also leaves open the issue of the Moon's small core.

Consequences for the Coaccretion Hypothesis

This model does not require extreme chances like the Capture theory and explains nicely why Earth and Moon share their oxygen isotope composition. However, there are open questions regarding the angular momentum, and the lack of iron spoke against it, too (It should be mentioned that this issue had been addressed recently, see Current Status.).

Giant Impact Hypothesis

Development

In 1974, William Hartmann and Donald Davis suggested the hypothesis that a planetoid (early on estimed to be the size of Mars) impacted with a relatively young Earth and blasted mantle material into on orbit, where it would eventually form the Moon. This model also became known as the "Big Whack" hypothesis. Around the same time, Alastair Cameron and William Ward had come to a similar conclusion. However, their research had been motivated by studies of the angular momentum.

Initially, the the hypothesis was rejected with the claim that a catastrophic event of such a dimension and with such results seemed extremely unlikely. Still, it explained the lack of iron, the oxygen isotope issue and the angular momentum.

Ten years later, the Giant Impact Hypothesis emerged as the leading model after a conference about the Moon origin in Kona, Hawaii. In the 1990s, Robin Canup picked up the research and created simulations to determine possible scenarios that might explain the formation of the Moon.

Canup initially came to the conclusion that much of the material blasted off by a "Big Whack" would either fall back to Earth or fly off into space, thus requiring a very large impactor (one with a factor of two or three in mass, compared to the initial assumptions). However, later on, improved computer simulations suggested that the size of the impactor may have been smaller after all, thus leaving all calculations regarding mass and momentum intact.

Criticism

The hypothesis is far from complete, though. Despite addressing the issues raised in the light of the first lunar landings, the Giant Impact Hypothesis leaves quite a few issues currently not fully addressed:

• One study suggested three tests of the hypothesis, based on things that should have occurred after such an impact. The result of these tests did not prove the Giant Impact Hypothesis, but it also did not disprove it. Still, the paper raised a few points that have to be factored into the model.
• A study analysing the bulk composition has come to the conclusion that "the high bulk FeO content of the Moon rules out the derivation of the proto-lunar material from any but a small fraction of the terrestrial mantle".
• Addressing the above concern, the Giant Impact Hypothesis raised the point that the impactor also contributed a large share of the base material that formed the Moon. But as another paper pointed out, the Moon is lacking siderophilic elements that should be present in such a case.

Current Status

As of now, the Giant Impact Hypothesis still stands. Research on it is not yet complete, and the model is currently neither proven nor disproven. To date, it is the most plausible theory considered by scientists.

Independently of the Giant Impact Hypothesis, Peter Noerdlinger from Saint Mary's University in Halifax, Canada proposed an expansion of the Coaccretion model. He suggested that the Moon formed next to the Earth, complete with an iron core. But this core was then ripped out by the Earth's gravity shortly after the formation, leaving an iron-less Moon behind to orbit the Earth.

Further research will show if either of these hypotheses is correct, or if scientists are going to develop a new theory based on new observations.

The Young Earth Creationist Position

According to Young Earth creationists, the Moon is not as old as any of the above hypotheses suggest. One argument against the its age is that the Moon would have been lost to the pull of Earth's gravity a long time ago Their current estimates put the Moon at approximately 8,000 to 10,000 years old, matching the rough age Genesis suggests.