Where Did the Moon Come From?
How did our Moon form? Where did it come from? Finally, and perhaps most importantly, what clues does the Moon hold in regard to the formation of our solar system? These are just a few of the questions that both present and past astronomers have struggled to understand throughout the course of human history. This article addresses these questions through an analysis of four theories pertaining to the Moon’s formation. Although these theories remain to be unproven by the scientific community, they offer a unique perspective to our Moon’s formative years that is both plausible and credible given our current understanding of the solar system at large.
The most prominent theory pertaining to the Moon’s formation is known as the “Impact Theory.” This hypothesis argues that the Moon likely formed from a massive object hitting the Earth during its early years. Scientists believe that the early solar system was full of drifting debris that was leftover from the cloud of dust (and gas) that surrounded our early Sun. As a result, scientists believe that an impact between our future Earth and a massive object was not only plausible, but inevitable given the chaotic conditions surrounding our planet at the time.
According to scientists, the object that struck Earth (known as “Theia”) was likely the size of Mars. After colliding with the Earth, the massive collision threw large chunks of Earth’s vaporized crust into space, which then became bound to one another through the effects of gravity. This hypothesis helps to explain why the Moon is composed of lighter elements, as its materials came solely from the Earth’s crust rather than its inner core.
According to this theory, scientists also believe that the core of “Theia” remained largely intact from the impact, and served as the gravitational basis for the crust-like debris to form around its center. Scientific models indicate that the impact between Theia and Earth was nearly 100 million times stronger than the latter event that is believed to have destroyed the dinosaurs.
The impact theory remains full of contradictions and problems, however. If the impact theory was completely true, for example, then current models suggest that the Moon should be composed of primarily sixty percent of the material that originated from Theia. However, rock samples from the Apollo missions indicate that the Earth and Moon are nearly identical in their composition; differing in composition by only a few parts per million. As a result, researchers in Israel have recently proposed that multiple impacts may have resulted in the formation of the Moon, rather than a single “Giant Impact” as previously argued.
Another theory pertaining to the Moon’s formation is the “co-formation” hypothesis. This theory suggests that our Moon may have formed at the same time as Earth. According to researcher, Robin Canup (an advocate of the co-formation theory), the Moon and Earth likely formed after the collision of two similar-sized bodies, both of which were approximately five times the size of Mars. After colliding and re-colliding with one another, this theory argues that the Earth would have been “surrounded by a disk of material that [later] combined to form the moon” (space.com). By colliding and partially merging with one another, this theory helps to explain the similarity of the Earth and Moon’s chemical compositions.
One major problem with this theory, however, is that the Moon’s overall density is quite different from the Earth. This, in turn, calls into question the idea that both the Earth and Moon formed from the same pre-planetary material. This hypothesis, which was once favored by many astronomers, therefore, is difficult to follow and has been relegated by the scientific community in more recent years.
"You cannot look up at the night sky on the Planet Earth and not wonder what it's like to be up there amongst the stars. And I always look up at the moon and see it as the single most romantic place within the cosmos."— Tom Hanks
Another scientific theory for the Moon’s formation is the “Capture Theory” which suggests that the Moon may have been snagged by the gravitational pull of the Earth at one point in its early history. Similar to the moons “Phobos and Deimos” that surround Mars, this theory suggests that the Moon may have formed outside of the solar system and eventually drifted towards Earth, where it was then drawn into the planet’s orbit. Other scientists have also hypothesized that the Moon may have been snagged from Venus’s orbit, which would explain the absence of moons around Venus. Such theories remain only speculative at this time.
A major problem with this theory, however, is that captured moons often exhibit highly elliptical orbits. Moreover, captured moons are often oddly shaped (such as Phobos and Deimos) rather than the spherical dimensions of our current moon. According to other mathematical models, the capture of such a large moon (relative to the Earth’s size and mass) is also implausible, if not impossible. For such an event to occur, mathematical models demonstrate that the capture would have only had a small window of opportunity, requiring an extraordinarily precise location for the capture to occur. Given the similarities between the Moon and Earth’s mantle, it is also unlikely that the two bodies formed independently from one another.
Which theory do you believe is most reasonable for the Moon's formation/origins?
A fourth and final theory pertaining to the Moon’s formation is known as the “Daughter Theory.” This theory, which is far older and less-accepted by the scientific community posits that the Moon developed from the Earth itself. Advocates of this hypothesis suggest that the Moon may have originated from the Pacific Ocean basin. Scientists suggest that such a scenario would have occurred during the early years of Earth’s formation, when it was still a molten world and locked in a rapid rotational cycle. This rapid rotation, they argue, may have resulted in the ejection of a massive object from the current Pacific Ocean basin, resulting in our current Moon.
Problems with this theory are numerous, as scientists remain unsure as to how the Earth could have been spinning so fast that a Moon-sized object was ejected from its exterior. Moreover, the possibility of a Moon-sized object being ejected from the Earth and following a stable orbit, afterwards, is also unlikely given that current mathematical models simply do no support the probabilities.
In closing, scientists continue to debate the origins of the Moon as no single model can account, entirely, for its overall formation. As with any scientific study, additional information will eventually shed greater light on the Moon’s formation. Although lunar expeditions from the Sixties and Seventies provided vital clues to the composition of the Moon’s surface and interior, further investigation of its surface is needed as the Moon’s chemical and physical composition is still poorly understood by the scientific community. With advances in technology, future expeditions to the lunar surface may be extremely beneficial to understanding the Moon’s formation. Only time will tell what new information arises about Earth’s closest neighbor
Suggestions For Further Reading:
Aderin-Pocock, Maggie. The Book of the Moon: A Guide To Our Closest Neighbor. New York, New York: Harry N. Abrams, 2019.
Redd, Nola Taylor. "How Was the Moon Formed?" Space.com. November 16, 2017. Accessed April 25, 2019. https://www.space.com/19275-moon-formation.html.
Wikipedia contributors, "Moon," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Moon&oldid=893709795 (accessed April 25, 2019).
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© 2019 Larry Slawson