The origin of the Moon has been a subject of research for many years and theories about its creation abound. Hypotheses vary from non-scientific proposals, such as that the Moon is a spaceship, to the currently favored idea that it was formed more than 4.5 billion years ago from a dense cloud of debris created when another planet struck the Earth in its early stages of development. Scientists even have a name for this hypothesized astronomical body, which they believe contained about 10 percent of the mass of the Earth—they call it Theia, after the Greek goddess mother of the Moon goddess Selene.

The initial evidence in favor of this theory was obtained during the Apollo Moon missions, nearly five decades ago. Astronauts brought back about half a tonne of Moon rocks from these missions, and analysis has revealed a striking similarity between the chemical compounds found on the Moon and those found on Earth. Other discoveries also support this theory, including evidence that shows a match between water samples collected from crystalline structures in lunar rocks and water collected on Earth.

The Earth-Theia collision hypothesis, also known as the Giant Impact Theory, has been around for awhile. It reflects the current scientific consensus on this question. But it has not gone unchallenged, by either planetary scientists or by the latest available data.

Shooting a Giant Hole in the Giant Impact Theory

At a 2013 scientific meeting sponsored by the Royal Society of London, astronomical researchers openly challenged the conventional collision theory. Relying on complex simulations, they proposed new scenarios suggesting that things might not be so simple. In an article entitled “ Impact Theory Gets Whacked ,” which appeared in an October 2013 edition of the journal Science, physicist and science reporter Daniel Clery discussed the arguments made by dissenting planetary scientists at this meeting, who asserted that a giant impact event couldn’t have created the Moon as it is seen today.

According to the computer models used by these scientists, a new moon emerging from such an event should be comprised largely of matter harvested from the collider (Theia). Our Moon should be easily identifiable as a remnant of another pulverized astronomical body, yet the similarity in chemical composition found when comparing Moon rocks to Earth rocks suggested greater mixing of materials than the Giant Impact collision theory would allow. And if that is the case, then either there was no collision, or the story of that collision is more complicated than originally believed.

Alternatives to the Giant Impact Theory

To explain the apparent homogeneity between the Earth and the Moon, planetary scientists proposed two modified versions of the collision theory, known as the Fast-Spinning Earth and the Half-Impact Earth . Both were discussed in separate articles appearing in the November 23, 2012 edition of Science.

Based on their simulations, Harvard University astrophysicists Matija Cuk and Sarah Stewart proposed that an impact with a planet just 1/200th the size of Earth could have been led to the creation of the Moon. This theory assumes that our home planet was in a proto stage and therefore spinning at a much faster rate than at present. As a result of this collision, enough material would have been expelled from the fast-spinning Earth’s mantle to account for the mass of the Moon, and for the homogeneity that exists between these two bodies.

Working from opposing assumptions, astrophysicist Robin Canup from the Southwest Research Institute in Boulder, Colorado proposed a different kind of collision. Under the Half-Impact scenario, Earth would have been hit 4.5 billion years ago by a slow-moving planetary object of similar size. The level of destruction of both planets would have been profound and about equal, allowing them to mix together to create a mass of debris with entirely different characteristics than the two planets possessed before they collided.

In the past, theories like these were considered untenable. Existing ideas about orbital mechanics seemed to rule them out, since they suggested that both bodies should be spinning and moving through space much faster than they actually are today.

However, new ideas about how the Sun interacts with the Earth and the Moon gravitationally has changed the picture.

Applying a principle called evection resonance , which would allow the Sun to put a brake on the movements of bodies it holds in its gravitational grasp, some planetary scientists hypothesize that the Earth and Moon could have lost considerable speed (angular momentum) over the past 4.5 billion years. If this is true, there might be no conflict between the current value of the Earth-Moon system’s angular momentum and the predictions of the Fast-Spinning Earth and Half-Impact Earth theories.

Giant Impact Theory Advocates Strike Back

Our current understanding of the solar system and its underlying dynamics could be completely wrong and might need to be reconsidered, if the claims made at the 2013 Royal Society meeting are in fact correct.

But collision/creation theorists have refused to go down without a fight. Another discovery, announced less than a year after the Royal Society gathering, provided new evidence in support of the Earth-Theia hypothesis.

In a June 2104 edition of Science, Dr. Daniel Herwartz from the University of Goettingen in Germany discussed results he and a team of researchers obtained from studying basalt samples collected during the Apollo Moon landings. Inside these samples, they found oxygen isotopes that had different chemical profiles than isotopes collected from the Earth’s mantle.

“We have now found small differences between the Moon and the Earth,” declared Dr. Herwartz . “This confirms the giant impact hypothesis.”

The existence of such isotopes was predicted by the giant impact theory, and that is why these findings by the German team are significant. At the time of the 2013 Royal Society meeting, these discoveries had yet to be revealed publicly, giving attendees no opportunity to assess their impact.

Assessing the Evidence

So where does all this leave us today? Has the discovery of divergent oxygen isotopes in Moon and Earth rocks conclusively proven the reality of the Giant Impact (Earth-Theia) theory? Or, have new discoveries about the potential dynamics of evection resonance given other theories greater explanatory power?

And what about other potential explanations, including those that might be classified as exotic? Could the Moon be an artificial object, engineered and constructed by an advanced civilization and put in orbit around the Earth to make the planet more hospitable to life, or for some other unknown reason?

In his 1975 underground cult favorite “Our Mysterious Spaceship Moon,” author Don Wilson argued that anomalies related to the Moon’s size, shape, location and physical characteristics were consistent with the theory that it was a gigantic spaceship, created by an advanced alien civilization with capabilities well beyond our own. This theme was further developed by British authors Christopher Knight and Alan Butler, who in their 2006 book “Who Built the Moon?” raised the possibility that such a feat might have been achieved by time-traveling humans from a far-distant future. They based this assertion on their discovery of multiple numerical and geometrical synchronicities between the Earth, Sun and Moon, which to them implied intelligent design of the latter.

It is easy to dismiss ideas like this as excessively speculative at best and downright pseudo-scientific at worst. But uncertainties that shroud the truth about the Moon’s origin and encourage such speculation persist.

While it remains the consensus choice among most astrophysicists who’ve pondered the question, the evidence in favor of the Earth-Theia hypothesis is hardly overwhelming. Small differences found in oxygen isotope ratios work in its favor, but this is not enough to rule out other collision scenarios. In fact, more recent evidence, obtained from a 2016 study carried out by researchers from Harvard University and Washington University in the United States, would seem to contradict the conclusions of the German researchers who believed they’d confirmed the Giant Impact Theory.

Using the latest techniques of chemical analysis, the scientists who sponsored this study could find no discernible differences in Earth and Moon rocks. On the contrary, their research revealed that these materials were even more homogeneous than previously believed. Furthermore, their chemical analysis uncovered evidence suggesting the collision that created the Moon was much more energetic and catastrophic than predicted by the Giant Impact model.

Similarly, many astrophysicists and planetary scientists remain skeptical of the evection resonance hypothesis. Knowledge about the specifics of this effect is still limited, with many doubting that it is significant enough to preserve the viability of the Fast-Spinning Earth and Half-Impact Earth collision theories.

Just because certain hypotheses are more popular than others does not mean one or the other is correct. If scientists spend an inordinate amount of time seeking evidence to confirm their personal pet theories, they may not give a fair hearing to other viable alternatives. This can be a particular problem for scientific conundrums that cannot be tested through experiment or resolved by direct observation, which is obviously the case for the processes that created the Moon more than four billion years ago.

A quote from Irwin I. Shapiro, the former director of the Harvard-Smithsonian Center for Astrophysics, perhaps best sums up the ongoing confusion over the Moon’s origin.

“Looking at all the anomalies and unanswered questions about the Moon,” Shapiro said, “the best explanation for the Moon is observational error. It doesn’t exist.”

This statement was undoubtedly made with tongue firmly implanted in cheek. Nevertheless, it accurately conveys the elusiveness of the Moon’s true history.

Top image: The Earth and Moon. Credit: Sergey Nivens / Adobe Stock

By Nathan Falde