Top Theories on How the Moon Formed

Giant-impact Hypothesis
Image Credit: Wiki Commons

The Moon has been a constant source of wonder and fascination throughout mankind’s’ history, but despite being located a mere 238,900 miles or 1.28 light-seconds away, we know less about it and how it came to be than we do about objects that are hundreds, or even thousands of millions of light years distant. So while investigations into the Moon’s origin are continuing, let us look at some of the leading theories about how the Earth’s natural satellite came to be.

Giant-impact Hypothesis

1) Collison of Mars-size Object with Earth

The leading theory on the Moon’s formation holds that a large impactor about the size of Mars, called Theia, collided with the young Earth, and in the process tore out a large amount of material which is said to have settled in an orbit around Earth. Eventually, it condensed to form the Moon as we know it today, with this putative collision also credited with having tilted Earth’s axis by 23.5 degrees, thus giving us the seasons. However, one major problem with this approach is that the Earth and the Moon are practically identical in terms of composition.

For instance, when the Apollo missions returned some Moon rocks to Earth, the oxygen isotopes and their ratios in these rocks proved to be essentially identical to rocks that are known to have formed on Earth. This raised quite a few eyebrows at the time, since all solar system bodies are known to have different and unique oxygen isotopic ratios.

Theia Explanation Drawbacks

Thus, if the giant impact theory was to have any validity, Theia had to have had a composition that was essentially identical to that of Earth, a possibility that has subsequently turned out to be less than 1%. Moreover, the facts that the impactor would have been all but vaporized, and that the debris blasted out of Earth would have mixed with the remains of the impactor, seem to rule out an impact of this nature, since no trace of the impactor has ever been found in lunar material. Moreover, measurements of the Moon’s titanium isotope ratio is within 4 parts per million of the same isotopes found in Earth rocks, which further suggests that no solar system body of the required/suggested mass and diameter had ever collided with Earth.

Nonetheless, and despite the obvious problems the Giant Impact Theory was facing, two further studies produced results that supported the impact scenario. One study, performed in 2012, found that the scarcity of zinc isotopes is consistent with the theory, while another study performed in 2013 found that the water in lunar magma is for all practical purposes identical to water in carbonaceous chondrites, and essentially the same as water found on Earth, in terms of the water’s isotopic composition.

The Giant Impact Theory Revisited

2) Two 5 times Mars Sized Objects

In an attempt to explain the glaring inconsistencies in the Impact Theory, Robin Canup, an American astrophysicist, published a paper in 2012 called “Forming a Moon with an Earth-like composition via a Giant Impact”, in which she posited an alternative view. Essentially, Canup states that both the Moon and Earth were formed when two bodies, each five times the size of Mars, collided repeatedly to produce a mass of debris, out of which Earth and the Moon eventually formed.
Astute readers will immediately notice that this theory neatly removes all of the inconsistencies in the main Giant Impact Theory; however, the origin of the two colliding bodies is not explained. At least, not to the satisfaction of the majority of planetary scientists that are working on the Moon’s origins.

3) Collision with Existing Moons

Another hypothesis holds that while the Giant Impact may have happened, it happened after two or more “normal” moons had already formed. This hypothesis argues that if the existing moons shared Earth’s composition, the body created by the Giant Impact could have collided with the existing moons as it spiraled away from Earth, thereby creating one body that largely shared Earth’s composition.

4) Earth Ejecta following Asteroid Impacts

Yet another hypothesis, which was devised in early 2017 by planetary scientists at the Weizmann Institute of Science in Israel, holds that instead of one giant impact, Earth was pummeled by a series of smaller impacts over a period of several hundred million years. According to the theory, each impact was energetic enough to blast relatively large amounts of material into space, and over time, the material ejected from each impact coalesced into a single body, i.e., the Moon.

Other Possibilities

While the Giant Impact Theory in all its iterations is the most widely accepted theory of how the Moon came to be, there have been other hypotheses and theories that do not involve impacts at all. Below is a brief description of the more pertinent alternatives.

5) The Moon was Captured

Tie theory holds that the Moon was captured by Earth as it drifted through the solar system. However, observations have revealed that close encounters with Earth results either in an altered orbit, or a violent collision. The biggest issue with the capture theory is that it cannot explain the capture mechanism. For instance, for this theory to work, the nascent Earth must have had a dense and extensive atmosphere around it at the time of the supposed capture of the Moon, and it must have been dense enough for it to slow the Moon down sufficiently to prevent either a collision, altered orbit, or the escape of the Moon. No evidence of such an atmosphere has ever been found, and as a result, the capture theory was discarded after the mid 1980’s.

6) Earth Expelled the Moon

Officially known as the Fission Theory, this theory devised by George Darwin (1845-1912), the son of the famous biologist Charles Darwin, holds that because the Earth was spinning much faster in its youth than it does today, centrifugal force caused a large part of the Earth’s crust to separate from the mantle.

In fact this theory was widely accepted up and to the Apollo era, when investigations into the lunar material returned to Earth showed that this material does not consist of oceanic crust, but of material formed in the Earth’s mantle during the Precambrian epoch. Interestingly, the Fission Theory holds that the Pacific Ocean Basin represents the scar of the fission event, and even though the Pacific Ocean floor is only 200 million years or so old, this fact alone does not disprove the theory, since it can account for the similar compositions of the Moon and Earth. Nonetheless, the fission theory cannot explain the Earth–Moon system’s angular momentum.

7) The Moon formed alongside Earth

Known as the Accretion Hypothesis, this hypothesis holds that the Moon and Earth had both accreted (or condensed), out of the same primordial dust disc that gave birth to the Sun and the rest of the solar system. However, while this has not been disproven conclusively, this hypothesis cannot explain the Earth-Moon system’s angular momentum, or the significant difference in the diameter of the Earth’s metal core, as compared to the core of the Moon. By way of comparison, Earth’s core comprises 50% of its radius, while the Moon’s core accounts for only 25% or so of its radius.

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