Introduction

Ever looked up at the night sky and wondered, “Where did the Moon come from?” The Moon, that familiar companion in our nocturnal view, has intrigued people for centuries, dating back to a time long before modern humans roamed the Earth.

Throughout history, the Moon has held a special place in various cultures, often seen as a deity. It inspired tales of poetic beauty, magic, and power. Stories spoke of its alleged role in transforming individuals into werewolves and other mystical beings, even having the potential to push people to the brink of insanity.

Fast forward to today, and we’ve not only gazed at the Moon from afar but have actually set foot on its surface. With plans to return by 2025 under NASA’s Artemis space program, our relationship with the Moon has evolved. We’ve conducted studies on its rocks and observed its impact on our tides. Yet, despite our advancements, the question of how the Moon came into existence remains a puzzle with several theories floating around.

In the following exploration, we’ll delve into humankind’s best guesses about the Moon’s origin, adding another layer to the age-old fascination with our celestial neighbor.

Theory of Coaccretion

Let’s take a closer look at the theory of Coaccretion, one of the earliest explanations for how the Moon came to be. According to this theory, the Moon and Earth didn’t follow a solo act but formed together in a celestial partnership.

Imagine a primordial accretion disk—a disk-shaped flow of gas, plasma, dust, or particles swirling around a celestial body, slowly collapsing inward. In this scenario, both the Moon and Earth emerged simultaneously from this cosmic disk. The gas in the cloud condensed into material and debris, drawn towards either Earth or the Moon. Earth, however, ended up attracting more material, boosting its mass in the process.

Due to Earth’s superior mass, it naturally developed a stronger gravitational pull. As a result, the Moon found itself in orbit around Earth. This coaccretion process potentially explains the geological similarities observed between the two celestial bodies.

Despite its initial appeal, critics of the Coaccretion theory pointed out a significant flaw—it struggled to account for the Moon’s existing angular momentum around Earth. In essence, while this theory offers an early glimpse into the Moon’s formation, it faces challenges in explaining certain aspects of the Moon’s current relationship with Earth.

The concept of Earth’s Fission

Let’s dive into another early theory regarding the Moon’s formation—the concept of Earth’s Fission. Sir George Darwin, an English astronomer and the son of Charles Darwin, proposed this idea. According to this theory, Earth was once spinning so rapidly that chunks of material were ejected from its surface, later condensing into what we now know as the Moon.

The appeal of fission theories lay in the similarity of Earth’s mantle composition to that of the Moon. However, as time passed, these theories lost their luster. The stumbling block was the inability to identify the right combination of properties for a spinning proto-Earth that could produce the specific type of proto-Moon we observe.

Scientists faced skepticism about Earth spinning rapidly enough to shed parts of itself. Doubts arose, and the fission theory faded because no conclusive evidence supported the notion of such a swift spinning event on Earth or the Moon. In essence, despite initial intrigue, the idea that the Moon resulted from the fission of Earth lost ground over time due to a lack of supporting evidence.

The concept of Capture

Now, let’s explore another set of older theories regarding the Moon’s formation—the concept of Capture. According to these theories, the Moon might have originated from a location within the solar system but outside the gravitational influence of Earth. Some scientists even entertain the idea that the Moon could have initially been under the gravitational sway of another planet before breaking free.

The narrative unfolds as the Moon, having its own celestial journey, eventually came close to Earth. In this close encounter, Earth’s gravitational pull was potent enough to capture the Moon within its orbit. It’s akin to a cosmic dance where the Moon, once elsewhere, found itself ensnared by Earth’s gravitational embrace.

However, as captivating as this theory may seem, scientists faced challenges in explaining the mechanics of how Earth could have successfully captured the Moon. The specific processes that would enable Earth to pull the Moon into its orbit, slowing down its velocity enough for a stable orbit, remained elusive.

Furthermore, the popularity of capture theories dwindled when scientists discovered the geological similarities between Earth and the Moon. Despite the initial appeal of the idea, the capture theory faced scrutiny and eventually fell out of favor due to these unresolved scientific puzzles.

A colossal collision 

Let’s delve into the initial theory among three that hinges on a dramatic event—a colossal collision between Earth and a Mars-sized planet named Theia. In this scenario, it is proposed that Theia was composed of distinct, possibly less robust materials compared to Earth. When Theia collided with Earth, Earth managed to maintain a relatively intact state. Conversely, Theia shattered into fragments, and over time, these remnants came together, forming what we now recognize as the Moon.

The appeal of this theory lay in the idea that the materials making up Theia were potentially different or weaker than those comprising Earth. However, this theory faced a significant setback as further analysis revealed that Earth and the Moon share similar elemental compositions. Both are primarily made up of elements like silicon and oxygen, making it challenging to sustain the argument that the Moon’s origin is the result of Theia’s remnants. Despite its initial allure, this theory encountered scrutiny and limitations due to the observed similarities in the composition of Earth and the Moon.

The concept of synestia

Consider this intriguing possibility: What if Theia, in its collision with the early Earth, delivered such a powerful impact that both celestial bodies were vaporized? Some scientists suggest an unconventional concept—a rotating, doughnut-shaped cloud known as a synestia—could have emerged from this extraordinary event. This synestia, resembling a rotating mixing bowl, is thought to have evenly blended the chemical elements present in both Theia and the proto-Earth.

Picture it as a cosmic kitchen where the ingredients of Theia and Earth were thoroughly mixed within this peculiar structure. As time passed, the material on the outer edges of the synestia began to coalesce, eventually forming what we recognize today as the Moon. Simultaneously, the remaining material within the synestia came together to give rise to Earth.

This theory proposes a unique cosmic dance, where the aftermath of a colossal collision led to the formation of a synestia, acting as a cosmic chef’s tool in creating the Moon and Earth from the blended ingredients of Theia and the early Earth. While it presents a fascinating scenario, it’s important to note that this concept remains a theoretical hypothesis in the ongoing quest to unravel the mysteries of the Moon’s origin.

Theia collides with Earth

Imagine a scenario where Theia collides with Earth, but instead of vaporization, the debris from this impact comes together to form the Moon. What makes this theory unique is that the material composing Theia is identical to that of Earth. No harm, no foul, right? The intriguing aspect here is how Theia itself came into existence.

According to this theory, Theia and Earth initially formed on opposite sides of the same accretion disk, a disk where material was evenly spread throughout. Picture it like two siblings emerging from opposite ends of a cosmic playground. Later on, some cosmic disturbance altered Theia’s orbit around the Sun, causing it to drift away from its original location. This cosmic drift ultimately led to Theia’s fateful collision with Earth.

In essence, this theory paints a picture of a celestial coincidence where two celestial bodies with identical compositions, born from the same cosmic disk, ended up on a collision course due to the cosmic dance of orbital disturbances. It provides a unique perspective on the Moon’s origin, presenting a cosmic tale of like bodies colliding in a grand celestial ballet.

Impact Hypothesis

Consider this intriguing hypothesis: rather than a single catastrophic impact, the early Earth experienced a series of pummeling collisions from extraterrestrial bodies. Each of these strikes generated a debris field, leading to the formation of small moonlets. Over time, these smaller moonlets coalesced and merged, gradually giving rise to the Moon. This hypothesis stands out because it doesn’t rely on a single “smoking gun” event but allows for incremental growth through multiple impacts.

The model proposed by Israeli scientists in early 2017 suggests that after each impact, a disk of material would swiftly form within hours. Over the course of a few hundred years, this material would condense into a single moonlet. The cumulative effect of multiple high-velocity impacts, according to this theory, could have produced enough material to eventually form the Moon.

However, while this hypothesis provides a compelling framework for the Moon’s formation, it acknowledges a crucial gap in our understanding. The mechanisms explaining how these individual moonlets came together to form one larger celestial body remain a puzzle yet to be described. It’s a captivating idea that presents a nuanced perspective, allowing for the Moon’s creation to unfold gradually through the cumulative impact of multiple extraterrestrial events.

Source

Rafferty, J. P.. “Where Did the Moon Come From?.” Encyclopedia Britannica, October 18, 2017. https://www.britannica.com/list/where-did-the-moon-come-from.