How did the moon actually form and become a satellite for our planet? Although numerous theories have been proposed over the centuries, the leading theory at the moment is called the collision ring theory.
What has always been a problematic fact about modern moon creation theories is the overall composition of it. The moon has a lower mean density than Earth does, 3.3 times that of water compared to Earth’s 5.5 rating in terms of iron composition. That means the materials of the moon have more similarities to the crust of the Earth than the mantle.
That has led to the development of an idea that the moon was created by an impact that blew rocky materials off the forming Earth and the debris eventually shaped into the moon. This idea begins with the assumption that iron cores had already formed and the core materials never made it into Earth orbit.
Over time, that would become the largest moon in the solar system in relation to the planet which hosts it.
History of the Collision Ring Theory
V.S. Safronov, a Russian astrophysicist, is credited with pioneering the study of how spatial materials can aggregate together into planetary bodies. Safronov’s work studied “planetesimals,” or bodies that are similar to asteroids.
Two senior scientists from the Planetary Science Institute, William Hartmann and Donald Davis, used Safronov’s work to develop the first hypothesis of moon development that involved a collision ring. The pair of doctors ran calculations regarding the rate of growth of spatial neighbors to Earth. They recognized that Earth itself was growing as well.
Their idea was simple, but quite profound. If the impact happened late enough and if it occurred in a direction that was relative to the planetary rotation at the time, then enough energy could have been created to throw a ring of materials into orbit. The heavier materials would have then condensed together over time and that would be able to form the moon.
Hartmann, along with several researchers, would refine the idea in 19876 through the publication of Origin of the Moon. Computer modeling was attempting to confirm the theory, with mixed success.
Dr. Robin Canup then offered new modeling regarding the collision ring theory in the 1990s. It is a theory that is generally accepted as being plausible and is a popular choice for moon formation. Lunar accretion modeling is an ongoing process still to refine the theory even more.
How Did a Ring Form Around the Planet?
Imagine Ancient Earth beginning to form itself into the masterpiece it happens to be today. Then, from the middle of nowhere, a giant body the size of Mars (which is often referred to as “Theia” slams into the developing planet. The impact is tremendous because the giant body that slammed into Ancient Earth is about the size of a planet in its own right.
There wouldn’t be an impact like we have when an asteroid comes through the atmosphere and impacts the planet. The two forming planetary bodies would literally merge together, forming a process where two become one planet.
That type of collision is not without consequence. Both developing bodies would spew debris out into space. They would both have core materials that would combine together, but leave evidence of having two separate bodies at one point simultaneously. That is how the moon can have less iron content than Earth, but still come from the same impact event.
Over time, the debris would settle into an orbit around the newly formed planet. As the pull of gravity brought the debris together, it would begin to form a ring around the planet. From that ring, the debris would eventually condense together and that would lead to the formation of the moon.
The energy required for such an event is estimated by the National Aeronautics and Space Administration (NASA) as being 100 million times greater than the event that would have likely wiped off the dinosaurs from the planet.
How Could the Moon Be Created from a Planetary Ring?
One of the most challenging questions that the collision ring theory brings up involves the actual composition of the moon. Models that have Theia impacting Earth to create a ring that eventually turns into a moon suggest that the moon should be a 60/40 mix of materials, the majority from the impacting planet. Yet the rock samples brought back by the Apollo missions, which creates a problem for the theory.
The moon and Earth are “almost twins,” according to astrophysicist Alessandra Mastrobuono-Battisti at the Israel Institute of Technology. “Their compositions differ by, at most, a few parts in a million,” she told Space.com on November 15, 2017.
That observation has led to an alternative theory: that multiple small impacts helped to form the moon instead of one large planetary-scale impact.
In 2014, however, Space.com offered a different report regarding those same moon rocks. Daniel Herwartz, an isotope geochemist at the University of Gottingen led a study that suggests the collision ring theory offers the best explanation for most of the moon’s features. They discovered that the moon rocks contained more oxygen-17 than oxygen-16 when compared to moon rocks found on Earth.
Their suggestion is that the size of Theia may be over-estimated. According to their compositional research, the 60/40 ratio is correct, but the moon is actually 40% Theia instead of being 60% or higher as originally anticipated.
Why Oxygen Isotopes Are Important to Consider
We have “moon meteorites” that come to our planet on a semi-frequent basis. When an object in space impacts the moon, the material that is ejected from the moon can fall to Earth and be recovered if it is large enough. By studying these meteorites, we have been able to determine that the materials have a similar oxygen isotope composition to those found on Earth.
Even though the moon meteorites are contaminated with Earth oxygen isotopes, the similarities are still present. The rocks from the Apollo mission and recovered meteorites tell the same story.
We also know that the rocks and meteorites that come from Mars, asteroids, or bodies from other locations in the solar system have a different isotope composition. That means the large impact theory, forming an eventual collision ring that would condense into a moon, has added credibility.
The fact is, however, that we may never fully know with absolute certainty how the moon was formed. Perhaps it is part of our planet. It may be part of Theia or a combination of both. It may have been captured by the gravity of our planet early on and stuck around. It is that type of mystery, the kind which fuels our curiosity, which provides the drive for future scientific discoveries.