From the time we were kids, the moon has been an object of fascination. But have you ever wondered how it was created? Scientists believe the moon was born of a dramatic collision between the Earth and an incipient planet known as theia. This violent collision created a dusty debris cloud that formed our natural satellite. In this article, we’ll be taking a deeper dive into the formation of the moon and examining the theories of how it may have occurred.
Origin of the Formation Theories
Early Theories
The modern view of moon formation began more than two centuries ago with observations by Scottish philosopher Lord Kelvin. Kelvin theorized that meteorite impacts caused the Moon to form from pre-existing material, believing this process to be the only explanation for the development of the moon.
Two-Body Collision Theory
In the 1970s, two theorists, William Hartman and Jon Allsop suggested a new moon-formation theory. This two-body collision theory suggests that the Moon was the result of a head-on, or nearly head-on, impact between the Earth and a mars-sized protoplanet known as theia. This theory suggests that this impact created a debris cloud around the Earth and Theia, which mixed together and coalesced over time to form the Moon.
Giant Impact Hypothesis
In the 1980s, a new moon-formation hypothesis was proposed based on the two-body collision theory: the Giant Impact Hypothesis. This hypothesis suggests that the Moon was born of a collision between the Earth and theia approximately 4.5 billion years ago, and that the debris that resulted from this impact coalesced to form the moon.
Evidence for the Giant Impact Hypothesis
The modern theory of the Moon’s formation is based on the evidence for the Giant Impact Hypothesis. Here are some of the types of evidence that support this model:
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Similar Composition. The Earth and the Moon share a similar composition with nearly identical isotopic ratios between their different elements. This suggests that the two must have formed from the same material.
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Contribution from Theia. The giant impact hypothesis suggests that the Earth and Theia were the two primary contributors to the formation of the Moon, and this is evidenced in the lower than normal titanium-isotope ratios found in the Earth-Moon system.
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Existence of The Late Heavy Bombardment. The near-surface features of the moon , as well as, isotopic measurements suggest that the moon was hit by a number of large impacts during its formation by an event hypothesised as the ‘Late Heavy Bombardment’.
Current Theories on Moon Formation
The current theoretical models of moon formation are based upon the Giant Impact Hypothesis and generally fall into two main categories: Synestia Model and the Big Splat Theory.
Synestia Model
The Synestia Model suggests that the Earth and Theia collided at high velocity which caused energies to heat and melt the material. This created a rapidly spinning object known as a ‘synestia’ which was composed of a mixture of vaporized and molten rock that encircled the Earth. Over time, the synestia cooled and condensed to form the Moon from the material in its core.
Big Splat Theory
The Big Splat Theory suggests that the material from the moon-forming impact was ejected so far out that it formed a disk around the Earth. This disk then slowly cooled and condensed over time to slowly form the Moon in a ‘Big Splat’. This model is bolstered by evidence showing the presence of a ‘hemisphere asymmetry’ on the Moon which suggests that the Moon formed from a cloud of material.
The most accepted hypothesis today is based on the Giant Impact Hypothesis which suggests that the Moon was formed from the debris of a head-on collision between the Earth and a Mars-sized protoplanet called Theia. This collision would have created a rapidly spinning ‘synestia’ which would eventually cool and condense to form the moon at a later when it was hurled out of orbit by the Earth’s gravity. Evidence for this impact-formation theory comes from the similarity in isotopic ratios between the Earth and Moon, the lower-than-normal titanium-isotope ratios in both bodies, and the presence of a ‘hemisphere asymmetry’ on the Moon. Regardless of the exact details, it is remarkable to think that our moon—one of the most captivating and significant objects in our universe—was formed by a collision between two celestial bodies billions of years ago.
